Quikfeet509
2/23/2005 10:37am,
this is an interesting article from one of the best exercise/nutrition sites on the web, www.thefactsaboutfitness.com (it is a pay site but worth it). it discusses the interaction between the macronutrient profile of food and the laws of thermodynamics:
A calorie is a calorie... right?
One controversial aspect of low-carbohydrate diets is the so-called metabolic advantage — the idea that more weight is lost calorie for calorie compared with diets higher in carbohydrate.
But not everyone agrees that the metabolic advantage even exists.
"A calorie is a calorie, regardless of where it comes from," says Dr. Elizabeth Pivonka, president of the Produce for Better Health Foundation. There are no foods that increase your metabolic rate, or help you burn calories, she says.
Dr. Pivonka probably felt rather confused after seeing the results of a recent Harvard School of Public Health study.
The findings, which generated national attention, show that low-carbohydrate dieters lost more weight than low-fat dieters despite eating 25,000 extra calories over a 12-week period [3].
Is the concept of the metabolic advantage in conflict with the laws of thermodynamics? Is it a myth? If not, how does it happen? Where does the energy go? Let's take a closer look at the research.
A group of dieters was assigned to one of three different regimens. They included a low-fat group (1800 calories for men; 1500 calories for women), a low-carbohydrate group that ate the same number of calories, and a third group on a similar low-carbohydrate plan that included 300 extra calories a day. Participants in all three groups lost weight, with the low-fat group losing an average of 17 pounds and the low-carbohydrate group that ate the same number of calories losing 23 pounds. The biggest surprise was that the low-carbohydrate dieters eating extra calories lost three pounds more than those on the low-fat diet.
Lead author of the study Penelope Greene cautions that the study was just a pilot for a larger study. The groups were relatively small, containing just seven participants each. However, conventional dietary wisdom dictates that the extra daily calories eaten by those in the third group — totaling 25,000 calories over the 12-week study — should equal about seven pounds, a difference that would have been statistically significant even in such small groups.
The study was also carefully controlled. Rather than giving participants a list of approved foods and quantities and setting them free, Greene had the food prepared daily at a restaurant. First Greene herself, then the restaurant's chefs, prepared and bagged meals so participants could pick them up each day.
Each bag, color-coded and picked up in the early evening, contained that night's dinner, a snack, the next day's breakfast and lunch, and a multi-vitamin and mineral supplement. Greene, along with Juniper Devecis, a registered dietician, spent considerable time before the study began creating the menu, trying out different dishes and different preparations of the same dishes. Her aim was to make the meals tasty and as similar as possible even though people were eating very different diets. The result was that participants had similar meals, though portion sizes and preparation did vary. Only one participant dropped out, and that happened early enough that a replacement was enrolled. Greene herself was extensively involved in the food preparation. Not only did she — with Devecis' help — design the menu, she also worked in the restaurant's kitchen preparing meals and training staff. These lengths were needed, Greene says, to address potential failings, such as faulty memories and inexact portion sizes, in studies that rely on subjects self-reporting their diets.
Greene's interest in low-carbohydrate diets goes back to 1999. She spent months researching previous studies after visiting the New York City clinic of Robert Atkins.
Staff there presented evidence from patient records over an 18-month period. While Greene felt the evidence of weight loss presented by the clinic was compelling, it didn't satisfy her, mainly because the records would naturally be skewed to include those patients who experienced success on the diet and thus stuck with it, rather than any who didn't and might have left the clinic. Though Greene's study didn't allow low-carbohydrate dieters to eat all they wanted, it still provided extra calories for the third group that, somehow, didn't translate into extra weight.
Some have said her results defy the laws of thermodynamics, since more calories should equal more weight, whatever the calories' source.
Thermodynamics
The First Law of Thermodynamics is a conservation law: it says that energy is neither created nor destroyed over time. Although it might change form, the sum total always remains the same.
For example, the chemical energy in gasoline is changed into the same amount of movement energy in a moving car. When you put the brakes on, this movement energy isn't lost. Rather, it's converted into heat energy in the brakes. The same principle holds true for the food you eat. Green plants use carbon dioxide, water, and the energy from the sun to form a type of sugar called glucose. That's where the word "carbohydrate" comes from. Carbo means "carbon" while hydrate means "water." When you eat the plant (or the animal that's eaten the plant), the energy then gets stored in your body, in the form of fat, carbohydrate, or protein. When you exercise, this chemical energy is converted into both movement and heat energy.
The second law says that energy tends to flow from being concentrated in one place to becoming dispersed and spread out. For example, a hot frying pan cools down when you take it off the kitchen stove. The "heat" energy flows out to the cooler room air.
Imagine a brick resting on a high window ledge. As the brick rests on the ledge, it has potential energy. Knock the brick off the ledge, and the potential energy is converted to movement energy as it accelerates toward the ground. When the brick hits the ground, the movement energy is converted to light energy (sparks), sound energy (a bang), and chemical energy (the brick breaks).
So, the "concentrated" potential energy in the brick is dispersed or spread out.
In nutrition, this dispersion of energy is called postprandial thermogenesis. It refers to the rise in metabolic rate that normally happens after you eat. Postprandial refers to the period after a meal. Thermogenesis means the production of heat.
In technical terms, thermogenesis is defined as "the increase in energy expenditure above basal fasting level divided by the energy content of the food ingested and is commonly expressed as a percentage" [16].
Protein
There are several studies to show that your body burns more calories after a meal high in protein compared with a meal high in carbohydrate. In other words, while a piece of steak and a slice of bread may contain an equal number of calories, the amount of energy that actually reaches the cells of your body, to fuel movement or store as fat, is different.
One trial, published in the journal Diabetes Care, shows that a meal containing roughly equal amounts of carbohydrate and protein can increase postprandial thermogenesis by almost 30% compared to a meal high in carbohydrate [13].
Measured over the course of a day, thermogenesis is also greater with a protein-rich diet than one high in carbohydrate.
For example, researchers from Arizona State University examined the effect of two different diets [6]. Thermogenesis was doubled with an 1800 calorie diet high in protein (47% carbohydrate, 29% fat, and 24% protein) compared to a low protein diet (59% carbohydrate, 29% fat, and 12% protein) providing the same number of calories.
In a longer study lasting 30 days, researchers overfed subjects with 1,000 extra calories each day [11]. In one group, most of the calories came from carbohydrate. In the second group, the majority of the extra calories came from protein.
Subjects in the high-carbohydrate group gained approximately six pounds of weight. But those on the high-protein diet gained only 3.9 pounds. The researchers attribute the reduced weight gain to the thermogenic response to protein.
In a group of men given hourly high-protein or high-carbohydrate meals, thermogenesis in the high-protein group (9.6% of calorie intake) was much greater than the high-carbohydrate group (5.7% of calorie intake) [5].
In fact, thermogenesis has been measured at over five times greater following a high-protein meal than a high-carbohydrate meal with the same number of calories [7].
Eating more protein will also boost the number of fat calories you burn each day [14]. Swedish researchers compared two diets over six days. Both diets contained an equal number of calories, but differed in the amount of protein and carbohydrate they contained.
Moderate-protein Low-protein
Protein 21% 8%
Carbohydrate 46% 53%
Fat 34% 39%
At rest, the group on the moderate-protein diet burned 37% more fat than those on the low-protein diet. After exercise, fat burning was also 18% higher in the moderate-protein group.
Subjects following a moderate-protein diet (28% protein, 42% carbohydrate, 28% fat) were able to eat approximately 500 more calories each day than subjects on a low-protein diet (16% protein, 55% carbohydrate, 26% fat) without gaining fat at a faster rate [12].
Biological value
Protein with a high biological value (such as meat, poultry, fish, dairy products and eggs) increases the metabolic rate to a slightly greater extent than protein with a lower biological value (such as beans, wheat and peanuts).
Some evidence for this comes from research published in the American Journal of Clinical Nutrition [15]. The study shows that after four days on a diet where the majority of protein came from pork, the number of calories burned each day was slightly higher than a diet containing mostly vegetable protein in the form of soy.
Biological value (BV) is used to measure the quality of protein. Foods with a high BV contain all nine essential amino acids. Foods with a low BV don't always contain all the essential amino acids. The table below shows the biological value of several common foods. The closer the BV is to 100, the higher the quality of the protein.
Food BV
Whole egg 93.7
Milk 84.5
Fish 76.0
Beef 74.3
Soybeans 72.8
Rice, polished 64.0
Wheat, whole 64.0
Corn 60.0
Beans, dry 58.0
So, why does your body burn more calories when you eat more protein and less carbohydrate?
Certain parts of your body, such as the central nervous system, require glucose (a form of carbohydrate) [8, 9]. If enough carbohydrate isn't supplied by the diet, your body has to make it. On a very low-carbohydrate diet, most of the extra glucose needs are met by the conversion of protein into glucose. This conversion process (called gluconeogenesis) requires extra energy.
When you eat more protein, the turnover of protein (an increase in the rate of protein synthesis and breakdown) is increased. Protein turnover also requires extra energy [6].
Although the effect of protein on the metabolic rate is well established, nobody is sure what the energy cost is. In other words, how many extra calories do you burn when you eat more protein and less carbohydrate?
The general conclusion is that the thermic effect of nutrients is approximately 2–3 % for fat, 6–8 % for carbohydrates, and 25–30% for proteins [4].
Dr. Richard Feinman of the State University of New York and Eugene Fine of Jacobi Medical Center in New York have used these figures to calculate the effective energy yield — the number of calories your body uses to fuel movement or store as fat — for a 2000 calorie diet [1].
Assuming the diet contains 55% carbohydrate, 30% protein and 15% fat, Feinman and Fine estimate that the effective energy yield is 1848 calories.
As carbohydrate is reduced, and the calories removed are distributed equally between fat and protein, the wasted calories due to thermogenesis increase, and lower the effective energy yield of the diet to 1750 calories at 21% carbohydrate.
When carbohydrate intake is reduced still further to 8% carbohydrate (the value for the early phase of the Atkins diet), an extra 40 calories are lost.
Of course, these are theoretical estimates. It's impossible to predict whether the figures will stay the same over a long-term diet. But the calculations show that the possibility of metabolic advantage should not come as a surprise. "Your brain needs glucose to function properly," Feinman says. "There's no argument about that. Now, this glucose can come from dietary carbohydrates, but your body can make glucose from protein and, to a much lesser degree, from fat. However, the process of making glucose from protein is inefficient, and to get the extra energy needed, your body will burn the fat that it has already stored."
The study by Penelope Greene shows a metabolic advantage of approximately 422 calories per day. The low-carbohydrate group given extra calories lost 10,500 calories (three pounds) more than the low-fat group. They also ate an extra 25,000 calories during the study.
That's a total of 35,500 calories over the 12-week study, which works out at 422 calories per day. It's a lot more than the 140-calorie estimate calculated by Feinman and Fine.
Why the discrepancy?
We know that protein has the effect of suppressing appetite [17]. The low-carbohydrate group in Greene's study might not have eaten all the food they were given.
In one study, the metabolic advantage came to more than 1200 calories, which does seem a little too good to be true.
In the study, a group of 39 children, aged 12 to 18, were placed on either a low-carbohydrate ketogenic diet or a moderate-fat (less than 30% fat) diet [10].
Those on the low-carbohydrate diet lost 12.8 pounds more than those on the moderate-fat diet, while consuming many more calories each day. Calculated from the difference in daily calorie intake — 1830 on the low-carbohydrate diet minus 1100 on the moderate-fat diet — there would have been a 730-calorie metabolic advantage if both groups had lost same amount of weight.
However, both groups did not lose the same weight. The low-carbohydrate group actually lost 12.8 pounds more than the moderate-fat group. These calories need to be accounted for as well. These 12.8 pounds of weight loss mean an additional deficit of 532 calories per day (12.8 pounds x 3500 calories ÷ 84 days = 532 calories). Adding these two differences together (730 plus 532 calories) would mean that the low-carbohydrate diet in this study had a metabolic advantage of 1262 calories a day.
That's roughly three times greater than the metabolic advantage in the Greene study, which was 422 calories per day.
A closer look at the study reveals why.
The mean weight loss in the low-carbohydrate group was so high because three of the 16 children lost more than twice as much weight as the other 13. The median figure for weight loss in both groups was actually very similar — 8.8 pounds in the low-carbohydrate group versus 6.6 pounds in the low-fat group.
The median (or "middle" number) is another way of figuring out the average. It's useful when subjects in a study have vastly different results from everyone else. To work it out, the numbers are arranged in rank order (smallest to largest). Then, you count in from one end until you find the middle.
In other words, the results of 3 of the 16 children in the low-carbohydrate group were not typical and skewed the results of the group. What's more, although the children were told to record what they ate, there's no way of knowing how accurate these records are.
"Although both groups had their diets analyzed by the same technique, and every effort was made to encourage accurate reporting," says lead author of the study Dr. Stephen Sondike. "It is possible that the low-carbohydrate group, told that they could eat as much fat as they wanted beforehand, reported more accurately than the [moderate-fat] group."
The bottom line
The idea that "a calorie is a calorie" comes from a misunderstanding of the laws of thermodynamics. The fact that diets higher in protein and lower in carbohydrate produce greater weight loss does not mean that the laws of thermodynamics need to be re-written.
They work because protein increases your metabolic rate (so you burn more calories) and keeps you feeling fuller for longer (so you eat less).
"Attacking the obesity epidemic will involve giving up many old ideas that have not been productive," says Dr. Richard Feinman. "'A calorie is a calorie' might be a good place to start."
References
1. Feinman, R.D., & Fine, E.J. (2004). "A calorie is a calorie" violates the second law of thermodynamics. Nutrition Journal, 3, 9
2. Feinman, R.D., & Fine, E.J. (2003). Thermodynamics and Metabolic Advantage of Weight Loss Diets. Metabolic Syndrome and Related Disorders, 1, 209-219
3. Greene, P., Willett, W., Devecis, J., & Skaf, A. (2003). Pilot 12-week feeding weight-loss comparison: Low-fat vs. low-carbohydrate (ketogenic) diets. Obesity Research, 11, A23
4. Jequier, E. (2002). Pathways to obesity. International Journal of Obesity and Related Metabolic Disorders 26, S12-S17
5. Robinson, S.M., Jaccard, C., Persaud, C., Jackson, A.A., Jequier, E., & Schutz, Y. (1990). Protein turnover and thermogenesis in response to high-protein and high-carbohydrate feeding in men. American Journal of Clinical Nutrition, 52, 72-80
6. Johnston, C.S., Day, C.S., & Swan, P,D. (2002). Postprandial thermogenesis is increased 100% on a high-protein, low-fat diet versus a high-carbohydrate, low-fat diet in healthy, young women. Journal of the American College of Nutrition, 21, 55-61
7. Karst, H., Steiniger, J., Noack, R., & Steglich, H.D. (1984). Diet-induced thermogenesis in man: thermic effects of single proteins, carbohydrates and fats depending on their energy amount. Annals of Nutrition and Metabolism, 28, 245-252
8. Ackermans, M.T., Pereira Arias, A.M., Bisschop, P.H., Endert, E., Sauerwein, H.P., & Romijn, J.A. (2001). The quantification of gluconeogenesis in healthy men by (2)H2O and [2-(13)C]glycerol yields different results: rates of gluconeogenesis in healthy men measured with (2)H2O are higher than those measured with [2-(13)C]glycerol. Journal of Clinical Endocrinology and Metabolism, 86, 2220-2226
9. Bisschop, P.H., Pereira Arias, A.M., Ackermans, M.T., Endert, E., Pijl, H., Kuipers, F., Meijer, A.J., Sauerwein, H.P., & Romijn, J.A. (2000). The effects of carbohydrate variation in isocaloric diets on glycogenolysis and gluconeogenesis in healthy men. Journal of Clinical Endocrinology and Metabolism, 85, 1963-1967
10. Sondike, S.B., Copperman, N., & Jacobson, M.S. (2003). Effects of a low-carbohydrate diet on weight loss and cardiovascular risk factor in overweight adolescents. Journal of Pediatrics, 142, 253-258
11. Webb, P., & Annis, J.F. (1983). Adaptation to overeating in lean and overweight men and women. Human Nutrition Clinical Nutrition, 37, 117-131
12. Parker, B., Noakes, M., Luscombe, N., & Clifton, P. (2002). Effect of a high-protein, high-monounsaturated fat weight loss diet on glycemic control and lipid levels in type 2 diabetes. Diabetes Care, 25, 425-430
13. Luscombe, N.D., Clifton, P.M., Noakes, M., Parker, B., & Wittert, G. (2002). Effects of energy-restricted diets containing increased protein on weight loss, resting energy expenditure, and the thermic effect of feeding in type 2 diabetes. Diabetes Care, 25, 652-657
14. Forslund, A.H., El-Khoury, A.E., & Olsson, R.M., Sjodin, A.M., Hambraeus, L., & Young, V.R. (1999). Effect of protein intake and physical activity on 24-h pattern and rate of macronutrient utilization. American Journal of Physiology, E39, E964-E976
15. Mikkelsen, P.B., Toubro, S., & Astrup, A. (2000). Effect of fat-reduced diets on 24-h energy expenditure: comparisons between animal protein, vegetable protein, and carbohydrate. American Journal of Clinical Nutrition, 72, 1135-1141
16. Westerterp, K.R. (2004). Diet induced thermogenesis. Nutrition and Metabolism, 1, 5
17. Barkeling, B., Rossner, S., & Bjorvell, H. (1990). Effects of a high-protein meal (meat) and a high-carbohydrate meal (vegetarian) on satiety measured by automated computerized monitoring of subsequent food intake, motivation to eat and food preferences. International Journal of Obesity, 14, 743-751
A calorie is a calorie... right?
One controversial aspect of low-carbohydrate diets is the so-called metabolic advantage — the idea that more weight is lost calorie for calorie compared with diets higher in carbohydrate.
But not everyone agrees that the metabolic advantage even exists.
"A calorie is a calorie, regardless of where it comes from," says Dr. Elizabeth Pivonka, president of the Produce for Better Health Foundation. There are no foods that increase your metabolic rate, or help you burn calories, she says.
Dr. Pivonka probably felt rather confused after seeing the results of a recent Harvard School of Public Health study.
The findings, which generated national attention, show that low-carbohydrate dieters lost more weight than low-fat dieters despite eating 25,000 extra calories over a 12-week period [3].
Is the concept of the metabolic advantage in conflict with the laws of thermodynamics? Is it a myth? If not, how does it happen? Where does the energy go? Let's take a closer look at the research.
A group of dieters was assigned to one of three different regimens. They included a low-fat group (1800 calories for men; 1500 calories for women), a low-carbohydrate group that ate the same number of calories, and a third group on a similar low-carbohydrate plan that included 300 extra calories a day. Participants in all three groups lost weight, with the low-fat group losing an average of 17 pounds and the low-carbohydrate group that ate the same number of calories losing 23 pounds. The biggest surprise was that the low-carbohydrate dieters eating extra calories lost three pounds more than those on the low-fat diet.
Lead author of the study Penelope Greene cautions that the study was just a pilot for a larger study. The groups were relatively small, containing just seven participants each. However, conventional dietary wisdom dictates that the extra daily calories eaten by those in the third group — totaling 25,000 calories over the 12-week study — should equal about seven pounds, a difference that would have been statistically significant even in such small groups.
The study was also carefully controlled. Rather than giving participants a list of approved foods and quantities and setting them free, Greene had the food prepared daily at a restaurant. First Greene herself, then the restaurant's chefs, prepared and bagged meals so participants could pick them up each day.
Each bag, color-coded and picked up in the early evening, contained that night's dinner, a snack, the next day's breakfast and lunch, and a multi-vitamin and mineral supplement. Greene, along with Juniper Devecis, a registered dietician, spent considerable time before the study began creating the menu, trying out different dishes and different preparations of the same dishes. Her aim was to make the meals tasty and as similar as possible even though people were eating very different diets. The result was that participants had similar meals, though portion sizes and preparation did vary. Only one participant dropped out, and that happened early enough that a replacement was enrolled. Greene herself was extensively involved in the food preparation. Not only did she — with Devecis' help — design the menu, she also worked in the restaurant's kitchen preparing meals and training staff. These lengths were needed, Greene says, to address potential failings, such as faulty memories and inexact portion sizes, in studies that rely on subjects self-reporting their diets.
Greene's interest in low-carbohydrate diets goes back to 1999. She spent months researching previous studies after visiting the New York City clinic of Robert Atkins.
Staff there presented evidence from patient records over an 18-month period. While Greene felt the evidence of weight loss presented by the clinic was compelling, it didn't satisfy her, mainly because the records would naturally be skewed to include those patients who experienced success on the diet and thus stuck with it, rather than any who didn't and might have left the clinic. Though Greene's study didn't allow low-carbohydrate dieters to eat all they wanted, it still provided extra calories for the third group that, somehow, didn't translate into extra weight.
Some have said her results defy the laws of thermodynamics, since more calories should equal more weight, whatever the calories' source.
Thermodynamics
The First Law of Thermodynamics is a conservation law: it says that energy is neither created nor destroyed over time. Although it might change form, the sum total always remains the same.
For example, the chemical energy in gasoline is changed into the same amount of movement energy in a moving car. When you put the brakes on, this movement energy isn't lost. Rather, it's converted into heat energy in the brakes. The same principle holds true for the food you eat. Green plants use carbon dioxide, water, and the energy from the sun to form a type of sugar called glucose. That's where the word "carbohydrate" comes from. Carbo means "carbon" while hydrate means "water." When you eat the plant (or the animal that's eaten the plant), the energy then gets stored in your body, in the form of fat, carbohydrate, or protein. When you exercise, this chemical energy is converted into both movement and heat energy.
The second law says that energy tends to flow from being concentrated in one place to becoming dispersed and spread out. For example, a hot frying pan cools down when you take it off the kitchen stove. The "heat" energy flows out to the cooler room air.
Imagine a brick resting on a high window ledge. As the brick rests on the ledge, it has potential energy. Knock the brick off the ledge, and the potential energy is converted to movement energy as it accelerates toward the ground. When the brick hits the ground, the movement energy is converted to light energy (sparks), sound energy (a bang), and chemical energy (the brick breaks).
So, the "concentrated" potential energy in the brick is dispersed or spread out.
In nutrition, this dispersion of energy is called postprandial thermogenesis. It refers to the rise in metabolic rate that normally happens after you eat. Postprandial refers to the period after a meal. Thermogenesis means the production of heat.
In technical terms, thermogenesis is defined as "the increase in energy expenditure above basal fasting level divided by the energy content of the food ingested and is commonly expressed as a percentage" [16].
Protein
There are several studies to show that your body burns more calories after a meal high in protein compared with a meal high in carbohydrate. In other words, while a piece of steak and a slice of bread may contain an equal number of calories, the amount of energy that actually reaches the cells of your body, to fuel movement or store as fat, is different.
One trial, published in the journal Diabetes Care, shows that a meal containing roughly equal amounts of carbohydrate and protein can increase postprandial thermogenesis by almost 30% compared to a meal high in carbohydrate [13].
Measured over the course of a day, thermogenesis is also greater with a protein-rich diet than one high in carbohydrate.
For example, researchers from Arizona State University examined the effect of two different diets [6]. Thermogenesis was doubled with an 1800 calorie diet high in protein (47% carbohydrate, 29% fat, and 24% protein) compared to a low protein diet (59% carbohydrate, 29% fat, and 12% protein) providing the same number of calories.
In a longer study lasting 30 days, researchers overfed subjects with 1,000 extra calories each day [11]. In one group, most of the calories came from carbohydrate. In the second group, the majority of the extra calories came from protein.
Subjects in the high-carbohydrate group gained approximately six pounds of weight. But those on the high-protein diet gained only 3.9 pounds. The researchers attribute the reduced weight gain to the thermogenic response to protein.
In a group of men given hourly high-protein or high-carbohydrate meals, thermogenesis in the high-protein group (9.6% of calorie intake) was much greater than the high-carbohydrate group (5.7% of calorie intake) [5].
In fact, thermogenesis has been measured at over five times greater following a high-protein meal than a high-carbohydrate meal with the same number of calories [7].
Eating more protein will also boost the number of fat calories you burn each day [14]. Swedish researchers compared two diets over six days. Both diets contained an equal number of calories, but differed in the amount of protein and carbohydrate they contained.
Moderate-protein Low-protein
Protein 21% 8%
Carbohydrate 46% 53%
Fat 34% 39%
At rest, the group on the moderate-protein diet burned 37% more fat than those on the low-protein diet. After exercise, fat burning was also 18% higher in the moderate-protein group.
Subjects following a moderate-protein diet (28% protein, 42% carbohydrate, 28% fat) were able to eat approximately 500 more calories each day than subjects on a low-protein diet (16% protein, 55% carbohydrate, 26% fat) without gaining fat at a faster rate [12].
Biological value
Protein with a high biological value (such as meat, poultry, fish, dairy products and eggs) increases the metabolic rate to a slightly greater extent than protein with a lower biological value (such as beans, wheat and peanuts).
Some evidence for this comes from research published in the American Journal of Clinical Nutrition [15]. The study shows that after four days on a diet where the majority of protein came from pork, the number of calories burned each day was slightly higher than a diet containing mostly vegetable protein in the form of soy.
Biological value (BV) is used to measure the quality of protein. Foods with a high BV contain all nine essential amino acids. Foods with a low BV don't always contain all the essential amino acids. The table below shows the biological value of several common foods. The closer the BV is to 100, the higher the quality of the protein.
Food BV
Whole egg 93.7
Milk 84.5
Fish 76.0
Beef 74.3
Soybeans 72.8
Rice, polished 64.0
Wheat, whole 64.0
Corn 60.0
Beans, dry 58.0
So, why does your body burn more calories when you eat more protein and less carbohydrate?
Certain parts of your body, such as the central nervous system, require glucose (a form of carbohydrate) [8, 9]. If enough carbohydrate isn't supplied by the diet, your body has to make it. On a very low-carbohydrate diet, most of the extra glucose needs are met by the conversion of protein into glucose. This conversion process (called gluconeogenesis) requires extra energy.
When you eat more protein, the turnover of protein (an increase in the rate of protein synthesis and breakdown) is increased. Protein turnover also requires extra energy [6].
Although the effect of protein on the metabolic rate is well established, nobody is sure what the energy cost is. In other words, how many extra calories do you burn when you eat more protein and less carbohydrate?
The general conclusion is that the thermic effect of nutrients is approximately 2–3 % for fat, 6–8 % for carbohydrates, and 25–30% for proteins [4].
Dr. Richard Feinman of the State University of New York and Eugene Fine of Jacobi Medical Center in New York have used these figures to calculate the effective energy yield — the number of calories your body uses to fuel movement or store as fat — for a 2000 calorie diet [1].
Assuming the diet contains 55% carbohydrate, 30% protein and 15% fat, Feinman and Fine estimate that the effective energy yield is 1848 calories.
As carbohydrate is reduced, and the calories removed are distributed equally between fat and protein, the wasted calories due to thermogenesis increase, and lower the effective energy yield of the diet to 1750 calories at 21% carbohydrate.
When carbohydrate intake is reduced still further to 8% carbohydrate (the value for the early phase of the Atkins diet), an extra 40 calories are lost.
Of course, these are theoretical estimates. It's impossible to predict whether the figures will stay the same over a long-term diet. But the calculations show that the possibility of metabolic advantage should not come as a surprise. "Your brain needs glucose to function properly," Feinman says. "There's no argument about that. Now, this glucose can come from dietary carbohydrates, but your body can make glucose from protein and, to a much lesser degree, from fat. However, the process of making glucose from protein is inefficient, and to get the extra energy needed, your body will burn the fat that it has already stored."
The study by Penelope Greene shows a metabolic advantage of approximately 422 calories per day. The low-carbohydrate group given extra calories lost 10,500 calories (three pounds) more than the low-fat group. They also ate an extra 25,000 calories during the study.
That's a total of 35,500 calories over the 12-week study, which works out at 422 calories per day. It's a lot more than the 140-calorie estimate calculated by Feinman and Fine.
Why the discrepancy?
We know that protein has the effect of suppressing appetite [17]. The low-carbohydrate group in Greene's study might not have eaten all the food they were given.
In one study, the metabolic advantage came to more than 1200 calories, which does seem a little too good to be true.
In the study, a group of 39 children, aged 12 to 18, were placed on either a low-carbohydrate ketogenic diet or a moderate-fat (less than 30% fat) diet [10].
Those on the low-carbohydrate diet lost 12.8 pounds more than those on the moderate-fat diet, while consuming many more calories each day. Calculated from the difference in daily calorie intake — 1830 on the low-carbohydrate diet minus 1100 on the moderate-fat diet — there would have been a 730-calorie metabolic advantage if both groups had lost same amount of weight.
However, both groups did not lose the same weight. The low-carbohydrate group actually lost 12.8 pounds more than the moderate-fat group. These calories need to be accounted for as well. These 12.8 pounds of weight loss mean an additional deficit of 532 calories per day (12.8 pounds x 3500 calories ÷ 84 days = 532 calories). Adding these two differences together (730 plus 532 calories) would mean that the low-carbohydrate diet in this study had a metabolic advantage of 1262 calories a day.
That's roughly three times greater than the metabolic advantage in the Greene study, which was 422 calories per day.
A closer look at the study reveals why.
The mean weight loss in the low-carbohydrate group was so high because three of the 16 children lost more than twice as much weight as the other 13. The median figure for weight loss in both groups was actually very similar — 8.8 pounds in the low-carbohydrate group versus 6.6 pounds in the low-fat group.
The median (or "middle" number) is another way of figuring out the average. It's useful when subjects in a study have vastly different results from everyone else. To work it out, the numbers are arranged in rank order (smallest to largest). Then, you count in from one end until you find the middle.
In other words, the results of 3 of the 16 children in the low-carbohydrate group were not typical and skewed the results of the group. What's more, although the children were told to record what they ate, there's no way of knowing how accurate these records are.
"Although both groups had their diets analyzed by the same technique, and every effort was made to encourage accurate reporting," says lead author of the study Dr. Stephen Sondike. "It is possible that the low-carbohydrate group, told that they could eat as much fat as they wanted beforehand, reported more accurately than the [moderate-fat] group."
The bottom line
The idea that "a calorie is a calorie" comes from a misunderstanding of the laws of thermodynamics. The fact that diets higher in protein and lower in carbohydrate produce greater weight loss does not mean that the laws of thermodynamics need to be re-written.
They work because protein increases your metabolic rate (so you burn more calories) and keeps you feeling fuller for longer (so you eat less).
"Attacking the obesity epidemic will involve giving up many old ideas that have not been productive," says Dr. Richard Feinman. "'A calorie is a calorie' might be a good place to start."
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