Claim 3. Low-carbohydrate diets are more effective for weight loss.
If one's goal is simply to lose as much weight as possible without regard to composition of weight loss, knocking out carbs may be the way to go. A study comparing short-term weight loss on a ketogenic (very low CHO and high in fat, leading to ketosis) and nonketogenic diet illustrate this point quite clearly. Even though total weight loss was greater on the ketogenic diet, fat loss was essentially equal, water loss was 177 percent greater (due to decreased muscle glycogen and muscle water loss), and protein loss was 88 percent greater on the keto~enic diet. So, for the goal of fat loss, there is no benefit to the ketogenic diet.20
The disadvantages, however, would likely be a decrease in 24-hour energy expenditure due to dehydration and loss of lean body mass. Additionally, for most athletes participating in high-intensity exercise, the decreased muscle glycogen stores would impair performance significantly and high-protein diets would decrease testosterone levels when compared with appropriately mixed food intakes, thus having a negative impact on recovery.21 Finally, there is increasing evidence that a high-fat diet may actually promote body-fat storage in genetically predisposed obese and post obese individuals.22,23,24,25
Claim 4. A ketogenic diet offers a "metabolic advantage".
Dr Atkins, in his book Dr. Atkins' New Diet Revolution, states that following his ketogenic diet will allow one to lose weight on a number of calories that once led to weight gain.26 The state of ketosis that Atkins seeks can be measured by testing for ketones in the urine. This leaves us with the knowledge that in the production and use of ketones for energy, some is excreted (wasted) in the urine. Essentially, calories are just eliminated. For those interested in losing fat while gaining muscle, eating a ketogenic, isocaloric diet sounds very appealing (i.e., eat the calories your body requires but have some wasted in the urine, creating a calorie deficit and, therefore, fat loss). However, this excretion of ketones most likely amounts to only 50 to 60 calories a day, hardly what would be considered a metabolic advantage. The low glycogen stores that are an inevitable result of a ketogenic diet would more likely have negative effects on exercise intensity and appetite, yielding a metabolic disadvantage.27
Another flaw in the "metabolic advantage" theory is related to the thermic effect of food (TEF). Thermic effect of food measures the increase in metabolic rate in response to the ingestion of food. Studies put this contribution at 5-15 percent of basal metabolic rate (BMR), when consuming a mixed diet. The low end of the range is seen in those eating a high fat diet, and the high end is seen in those eating a high complex-carbohydrate diet.3 If a person had an absurdly low BMR of 1000 calories, this would translate into a TEF of 50 calories on the high fat diet versus a TEF of 150 calories for a high complex CHO diet. So much for "metabolic advantage"!
