By: Robert Melick
Most diets rely on creating a negative energy balance to lose weight. This means that the body is taking in less than it needs so it uses the energy we have stored such as glycogen, fat and muscle protein. Humans require a certain amount of calories each day just to support necessary functions like regulating heart rate, controlling our breathing, digesting food and other automatic processes. This bare minimum of energy we need to support these life preserving functions is known as our basal metabolic rate. Believe it or not but even as we sleep we are burning as many calories as when we are sitting down watching television.
So, are all calories the same? Is eating a 400 calorie donut the same as eating 400 calories worth of fish? If you are trying to lose weight this would mean you could eat pretty much anything you wanted as long as you stayed within your calorie range. I mean, it would not be healthy, but you would lose weight. Like most other topics I have researched, there is much debate in the scientific community about whether losing weight is strictly about minimizing calories or is the percentage of protein, carbohydrates and fat you eat just as important. Unfortunately, there are few long term randomized studies addressing this issue but I will use what we have thus far.
Some studies have found that a low carbohydrate, high protein diet offered greater weight loss than a low fat diet even when both groups consumed equal calories and started the study at similar weights. These studies would seem to suggest that it is not just calories which induce weight loss and macronutrient intake (protein, carbs, fat) also play a major role. There are a few ways in which different macronutrient intake could promote different amounts of weight loss.
Metabolizable energy is the difference between the energy of food (calories) and the energy that we lose as heat or in feces and urine (1). It is also the amount of energy from the foods that are available for our body to turn into ATP, which is the power source for all of our bodily processes (1). Currently, there are known values for the metabolizable energy of the macronutrients, created by Atwater (1). If a energy source, say protein, was eaten and only a few of those calories were available for our body to turn into ATP, and we compared it to the same calories worth of carbohydrates but which had more calories available for our body to turn into ATP , over time we would lose more weight on the protein diet since our body is not utilizing it as much as the carbs.
There are two main problems with Atwaters factors. First, it is assumed that the amount our intestinal tract absorbs from different macronutrients remains constant. In a vacuum this may be true, but when you eat fiber with a food, the amount of fat, nitrogen and energy that you excrete are all increased (1). This means two people eating the exact same foods as each other but one person is taking a fiber supplement, the person with the added fiber will excrete more and thus reduce the amount of calories absorbed by the body.
The other main problem was the finding that after eating peanuts the fecal excretion of fat was increased by 15% (1). The same finding was discovered when using almonds instead of peanuts. This is explained by some of the lipids passing through our digestive tract without being broken down.
Calorie Sources and Energy Expenditure
A study assigning subjects to either a high protein pork diet, high protein soy diet or a high carbohydrate diet found that despite their lower overall energy intake both of the high protein groups saw a 3% higher 24 hour energy expenditure (2). This shows that total energy consumed is not the only factor which affects energy expenditure. An increase in protein turnover may account for up to 68% of the acute thermogenic effect of protein consumption (2). Protein turnover refers to the balance between the building of and breaking down of bodily proteins. It requires a lot of energy to build protein.
A mixed meal high in protein was shown to cause an 11%-14% increase above the resting metabolic rate post consumption (1, 3). When the effects caused by ingestion of single macronutrients are measured, protein produces a much greater response in energy expenditure than carbohydrates or fat (1, 4, 5).
Thermogenic Effect of Food
The thermogenic response causes release of the metabolizable energy as heat which would leave it unavailable for our body to use as ATP. Although this seems to be the reason as to why high protein diets cause more weight loss, the thermogenic increase would only result in an additional loss of about 42 calories per day (1). This cannot be the only factor involved if high protein diets do in fact cause more weight loss than high carbohydrate or high fat diets.
After we eat a food our body expends energy to break it down and absorb it, referred to as the thermic effects of food (TEF). This process accounts for 10-15% of our total daily energy expenditure (1). It was found that different macronutrients were able to influence the TEF. High protein meals were found to cause a greater thermogenic response than equal calorie high carbohydrate or high fat equivalents (1, 2, 6). When mixed macronutrient meals were consumed that were high in fat, protein or carbohydrates, the thermic effect of food over a 7 hour period was an average of 261 for protein, 92 for high carbohydrate meal and 97 for a high fat meal (1, 2).
Humans, like other mammals, must keep a constant core temperature of 98.6 no matter cold it is outside. Generating internal heat like shivering when it is cold is the basis of thermogenesis (7). We humans have a much higher resting metabolic rate than reptiles which are cold blooded and adapt to whatever temperature they are in or sit in the sun to warm up. We therefore require a great amount of external energy everyday just to keep warm which comes in the form of calories from food. Reptiles can go long lengths without needing food (crocodiles can go a year without eating). Put simply, thermogenesis is production of heat.
A study involving healthy women compared the thermogenic effect of either a high protein low fat diet or a low fat high carbohydrate diet found that meal induced thermogenesis measured after the meals was twofold higher in the high protein versus the high carbohydrate diet (8). After eating protein, there is a rapid increase in the REE (similar to our resting metabolic rate) which is maintained for up to 4 or 5 hours following the meal (9). On the other hand, high carbohydrate meals have been shown to cause a smaller rise in the REE which is only maintained for 1-2 hours after the meal (9).
Believe it or not but thermogenesis has been shown to be increased just from the mere sight, taste and smell of food (10). The palatability of food also seems to increase thermogenesis. In a study of subjects eating 43 grams of beef protein versus 43 grams of protein from cod found that the beef was rated more palatable and it also increased thermogenesis more than the fish (10).
A reason why protein would increase thermogenesis over carbohydrates is because our body has no long term storage capacities for protein so it would have to be processed right away (10). The synthesis of protein, the high energy cost of making peptide bonds, and the high energy cost of making urea may all contribute to the increase in thermogenesis (10).
Gluconeogenesis is when the body forms glucose (a sugar) from non carbohydrate precursors (11). This may partly explain why high protein diets have been shown to increase energy expenditure in humans. If we are following a low carbohydrate high protein diet our body may be forced to convert the protein into glucose (gluconeogenesis) since our dietary intake of carbohydrates is low (11). It takes a lot of energy to convert protein into glucose so our energy expenditure would obviously rise (11).
One study measured gluconeogenesis in individuals in either a high protein low carbohydrate group or a normal protein group (11). On average, the high protein group produced 26 grams of extra glucose through gluconeogenesis which led to an increase in energy expenditure (11). The increased gluconeogenesis contributed to 42% of the rise in energy expenditure (11).
Protein elicits more of an energy cost (23%) to be digested, absorbed and metabolized than either carbohydrates (6%) or fats (3%). The oxidation of amino acids, especially when present in abundance, can contribute to the thermic effects of protein (12). Our body must oxidize excess amino acids as the body has nowhere to send them and this requires energy.
The studies seem to support the idea that protein causes us to burn more calories than an equal amount of carbohydrates or fat. This is backed up by the facts that protein has no long term storage so it must be used, it is costly in energy to break down and utilize and it increases thermogenesis. The main issue is finding out if this increase in calories burned is high enough to lead to weight loss.
Although a high protein diet may burn additional calories, an overall low calorie diet, whether high in carbohydrates, fats or proteins would still create a negative energy balance and help you lose weight. If you raise the amount of protein you take in you would obviously need to adjust your intake of carbs or fats so be mindful of consuming adequate carbohydrates. I would not advise eating a very high protein diet just to lose weight.
A substantial amount of carbohydrates are needed in the diet to get all of the vitamins, minerals and antioxidants that plants have to offer so don’t skip out on them. While weight loss is great we must also think about long term health so focus most on consuming a low calorie but varied and healthy diet. Eating a high protein and high animal diet may due more long term damage (heart disease, cancer) which overrides any health benefit achieved through weight loss.
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