When a person adopts a low carbohydrate diet, the body continues for several days to draw down glycogen stores to fuel the brain and muscles. Glycogen stores typically amount to about 300 to 400 g in the average subject, 100 g of which can reside in the liver. The body stores glycogen in combination with water in a ratio of about 3 g water for each 1 g of glycogen. Thus, on a low carbohydrate diet, initial depletion of glycogen can alone account for up to 1.2 to 1.5 kg, creating an illusion of rapid change in body composition.
As noted by Flatt, “The body ignores that 1 g of fat contains more than twice the energy present in 1 g of carbohydrate, an element of information needed to determine the energy balance.”(1) The body does not regulate energy balance; it regulates the balances of carbohydrate, protein, and fat, each of which has its own economy.
Ingested carbohydrate, protein, and fat differ markedly in their potential fates:
Utilization in structures of glycomolecules
Utilization for repair and maintenance of tissues
Utilization for repair and maintenance of cells
Oxidation to support body functions
Conversion to glucose; oxidation to support body functions (only in a carbohydrate deficient system)
Oxidation to support body functions
Oxidation to produce heat
Conversion to glucose; oxidation to produce heat (only in a carbohydrate deficient system)
Storage as body fat
Storage as glycogen (500 to 600 g)
Conversion to glucose; storage as glycogen (only in a carbohydrate deficient system)
Conversion to body fat
Conversion to body fat
Numerous studies have shown that the concept of energy balance does not strictly apply to human metabolism because of the different ways the body manages the separate economies of fat, protein, and carbohydrate.
As an example, Miller and Mumford did three experiments involving overfeeding humans with a supplement of 1300-1500 kcal per day, either high in protein (~15%) or low in protein (~2.7%). [2 full text] Fat content of high- and low- protein diets was held constant, so high protein diets were reduced in carbohydrate, and vice versa.
Their data showed large deviations from predictions of the energy balance hypothesis. Subjects overfed a low-protein, high-carbohydrate diet consistently gained less weight than predicted by the increased kcaloric intake; in fact, some subjects on low protein diets lost weight despite consuming an excess of 8-10,000 kcal in a week.
In one experiment, subjects on low protein diets overconsumed an average of 35, 230 kcal but gained only 0.9 kg, compared to the energy balance prediction of 5.9 kg. In contrast, subjects getting the excess kcalories from higher protein foods gained an average of 3.7 kg, two-thirds of the amount predicted by the energy balance equation. Miller and Mumford ruled out significant loss of lean mass by multiple methods of estimating body composition, including whole body potassium, skinfold, nitrogen balance, and urinary creatinine, none of which indicated significant change in lean body mass in these subjects.
These data indicated that, given the same excess “energy” intake, higher protein intake increases body weight gain compared to higher carbohydrate intake. These data clearly contradict the energy balance idea as well as the idea that high protein diets have a metabolic advantage over high carbohydrate diets; on the contrary, they suggest that high carbohydrate diets have the metabolic advantage.
Prewitt et al [3 full text] found that women (black, Hispanic, white, and Asian) assigned to a 60% carbohydrate, 20% fat diet required 14-28% (average 19%) higher caloric intake to maintain weight than when assigned to a 44% carbohydrate, 37% fat diet (protein intake was constant at ~19%). Despite efforts to maintain stable body weight on the lower fat intake by increasing carbohydrate intake, the subjects lost an average of 11% of body fat (2.5 kg) over 24 weeks.
Grams, Not Calories
In the nutritional biochemistry literature, research along these lines has led to the realization that the body deals with substrate balances, not energy balance. That means, the body has a fat balance, protein balance, and carbohydrate balance, the latter two of which it appears to regulate.
Since the body processes each nutrient (protein, fat, carbohydrate) differently, we can’t reduce them to hypothetically equivalent kcalories with equivalent fates. Ironically, this has become the battle cry of advocates of low carbohydrate diets, when the data (some of which I cited above, some below) overwhelmingly supports the conclusion that high carbohydrate diets have the “metabolic advantage” over low carbohydrate diets.
To lose body fat, you must create a metabolic situation in which the body oxidizes more fat than it deposits in stores, which we can call ‘negative fat balance.’ This could occur through increased oxidation of fat relative to deposition, or decreased deposition relative to oxidation, or both.
Hill et al tested nutrient balance in humans using diets high and low in carbohydrate or fat.  Figure 2 shows the nutrient oxidation rates for protein, fat, and carbohydrate at baseline, and three different experimental diets. The high fat diet supplied 20 percent of calories from each protein and carbohydrate, and 60 percent from fat. The high carb diet supplied 20 percent of calories from each protein and fat, and 60 percent from carbohydrate. The mixed diet supplied 20 percent of calories as protein, 35 percent as carbohydrate, and 45 percent as fat.
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As shown, on the high fat and “mixed” diets, the subjects oxidized (“burned”) more fat than either carbohydrate or protein, and on the high carbohydrate diet, they oxidized more carbohydrate than either fat or protein. Notice that protein oxidation was essentially the same regardless of diet, because all diets had equivalent proportion of protein. This confirmed the long-known fact that carbohydrate spares fat oxidation. From this figure, you might naturally conclude that eating a low carbohydrate diet will lead to fat loss by increasing fat oxidation.
The next figure shows the balance (intake minus oxidation) of protein, fat, and carbohydrate on each of the diets.
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On the 60% fat diet, the subjects were in slight negative fat balance (burning more fat than consumed) on day three, but by day seven, they were in positive fat balance (burning less fat than consumed)--hence, on day seven they were storing dietary fat in adipose. This happened despite the fact that they had increased fat oxidation. Decreasing dietary carbohydrate forced the body to burn more fat, but because they were consuming a high fat diet, they were consuming more fat than they could burn in a day, resulting in a positive fat balance....increasing adipose.
In contrast, the subjects on the 45% fat and 20% fat diets were in negative fat balance—burning more fat than consumed—on all measured days, with those on the 20% fat diets in the greatest negative fat balance—losing body fat at the greatest rate.
Note that this means that on the same kcaloric intake, when the subjects ate a 60% fat diet, they were accumulating body fat, but when they ate a 45% or 20% fat diet, they were losing body fat...and they lost fat faster on the 20% fat diet than on the 45% fat diet.
Notice also that when the subjects were on the 60% carbohydrate diet they had positive protein balance at both measured days, but when on either the 60% or 45% fat diets, by day seven they were in slight negative protein balance—burning more protein than consumed. That means they were burning up lean mass, despite a high (20% of calories) protein, kcalorically adequate diet. This happened because carbohydrate is protein-sparing. In other words, eating adequate carbohydrate prevents the use of body protein to produce glucose or glycogen; eating too little carbohydrate leads to the body breaking down lean mass to generate carbohydrate. This is why many people find it difficult to maintain and especially to build lean mass on a low carbohydrate diet.
What about carbohydrate? All diets showed positive carbohydrate balance. This means they were storing carbohydrate, in the form of glycogen, at time of measurement. Humans are continuosly oxidizing carbohydrate, but intermittently feeding on carbohydrate. Given adequate dietary carbohydrate, our body's will maintain a positive carbohydrate balance during the day, because we don’t eat at night. This carbohydrate gets burned at night during sleep, when not eating. Hill et al comment:
As noted above, the body does not regulate energy balance, it regulates balance of nutrients. The body has different ways of handling each nutrient, in general it avoids converting glucose into fat since it needs glucose and can store it as glycogen.
In the case of carbohydrate, we know that when carbohydrate intake increases, the body increases carbohydrate usage and converts some (about 10%) to heat (thermogenesis), and it stores any excess carbohydrate as glycogen. It appears that the body regulates carbohydrate stores (glycogen) by increasing carbohydrate oxidation when carbohydrate is abundant and reduces glucose oxidation when dietary carbohydrate is scarce (to conserve glycogen). Experiments involving overfeeding 500 grams of carbohydrate daily have shown that the body converts very little of this to fat, and only after prolonged overfeeding; after seven days of such overfeeding people produce only about 5-10 g of fat via conversion of glucose.[6 , 7] Furthermore, conversion of glucose to fatty acids consumes about 25% of the energy in the glucose.
In the case of dietary fat, when you eat more grams of fat than you burn, you will store those grams of fat in fat stores; the body has no other way to store them, and most research shows most fats do not have a thermogenic action. The high prevalence of obesity shows that the body does not regulate fat storage effectively. Since it avidly stores fat without regulation, this suggests that evolutionary diets did not have much fat (avid storage would evolve as a response to scarcity).
So, if you consume 10 g excess fat daily, you will directly store those grams of fat in fat stores. Over a month, those 10 g of fat add up to 300 g, or about three quarters of a pound. The body does not calculate the kcaloric value of those grams of fat; the kcaloric value is simply irrelevant to the body. I repeat, energy balance is irrelevant. The body has no means of regulating "energy," a theoretical entity; it only shuffles grams of substances like fat, carbohydrate, and protein.
When you eat fewer grams of fat than you burn, you will release fat from fat stores. Eat 10 g less fat daily than you burn, and you will lose 300 g of fat per month; to lose one pound of fat weekly, you need to create a fat deficit of about 65 g daily (i.e. consume 65 g less fat than you burn).
You might be able to achieve this on a low carbohydrate diet, and you might not. If eating a low carb diet allows you to eat less fat than you burn daily, you will lose fat, and if it doesn’t you will not. On the other hand, regardless of theoretical "energy" intake, if eating a low carbohydrate diet results in your consuming more fat than your body burns daily, you will increase your body fat day by day.
In my experience, many people increase fat intake well beyond fat oxidation when eating low carbohydrate diets, in spite of reduced kcalorie intake.
The body does not store "energy," it stores fat or carbohydrate, gram by gram. A gram of fat is a gram of fat; if you don’t burn it, you will store it.