Cornell University Cooperative Extension
 
Low-Carbohydrate Diets: Heresy or Hype?

by David A. Levitsky, Ph.D.
http://www.cce.cornell.edu/food/expfiles/topics/levitsky/lev itskyoverview.html

David Levitsky, Ph.D., is an obesity researcher and award-winning Professor of Nutritional Science at Cornell University.

"FORGET THE FIGHT AGAINST FAT! BREAK THE
SUGAR-STARCH HABIT TODAY AND ENJOY STEAK,
EGGS, CHEESE, EVEN WINE AS YOU GET HEALTHY
AND LOSE WEIGHT WITH SUGAR BUSTERS!"

Can this advertisement be true? Has everything that nutritionists and dietitians been telling the public about reducing fat been wrong?

Is reducing the fat content of your diet not a safe and effective way to lose weight?

The quote stated above was taken from a Web page advertising a diet book called Sugar Busters! Cut Sugar to Trim Fat by H. Leighton Steward, Dr. Samuel S. Andrews, Dr. Luis A. Balart and Dr. Morrison C. Bethea. There are many other books that also sell the same idea, namely you can lose weight by eating a low-carbohydrate diets. Perhaps the low-carbohydrate diet that has received the most attention in the press is described in a book written by Dr. Robert C. Atkins, entitled Dr. Atkins' New Diet Revolution, which is actually an evolution from his previous book, Dr Atkins' Diet Revolution; The High Calorie Way to Stay Thin Forever written over 25 years ago. A similar theme is put forth in a book entitled, The Carbohydrate Addict's Diet: The Lifelong Solution to Yo-Yo Dieting" by Dr. Rachael F. Heller and Dr. Richard F. Heller. Another husband and wife team, Michael and Mary Eades wrote Protein Power: The High-Protein/Low Carbohydrate Way to Lose Weight, Feel Fit, and Boost Your Health-in Just Weeks! several years ago, but have co-authored a more recent version of the high fat, low carbohydrate diet with Charles Hunt called Charles Hunt's Diet Evolution : 'Eat Fat and Get Fit!'.

Are Carbohydrates the Culprits?

The basic idea that unites all of these books against the traditional nutrition advice establishment is that excess dietary carbohydrate is the major cause of overweight and obesity today. According to these diet gurus, excessive carbohydrate consumption stimulates an over-production of insulin. Insulin is the hormone responsible for transporting nutrients into fat cells and causing them to synthesize fat. Thus, our increasing prevalence of overweight and obesity is a direct result of eating too many carbohydrates.

If one accepts this simple notion, then the solution for overweight individuals is to decrease their intake of carbohydrates, which leaves foods that are mainly high in protein and/or fat. The only low-carbohydrate sources of protein are meat and meat products. Plant sources of protein typically come packaged with complex carbohydrates. Animal sources of protein typically come packaged with fat, particularly saturated fat. Consequently, as one begins to eat greater amounts of dietary protein, the amount of dietary fat ingested is also increased. Because of these relationships between macronutrients, a low-carbohydrate diet is usually high in both protein and fat.

How much of the argument in favor of a low-carbohydrate diet for weight control is supported by scientific literature? Like so many nutritional scams, there is always a grain of truth in what is said. Do carbohydrates cause overweight and obesity? Figure 1 shows some data that frighten many public health officials and support the claims of the diet gurus: Americans are getting fatter. Figure 1 shows that the percent of Americans who are considered overweight by the Centers for Disease Control and Prevention (95th percentile for Body Mass Index) has been increasing ever since they began collecting health data in 1960 (1). The prevalence of obesity has been increasing at similar rates. It should be noted from this figure, however, that the biggest jump in BMI occurred between 1980 and 1994.

The diet gurus are also correct when they say that Americans are consuming more carbohydrates than in the past. Figure 2 shows the average daily consumption of carbohydrate since 1970 based on USDA's food disappearance data (2).

The absolute numbers are higher than surveys that are based on dietary recalls, but what is important is not the absolute values, but the change in these values over the years. The diet gurus were right: Daily carbohydrate consumption started to increase in the early 1980's roughly corresponding to the time when the surge in obesity became evident from Figure 1.

It's Not Just Carbohydrates

What the diet gurus don't tell you, however, is that along with the increase carbohydrate consumption, there was also an increase in protein consumption as can be seen in Figure 3 (2).

In fact, fat consumption also increased during this time, although the increase was not as great as for the other two macronutrients. So it appears that Americans were not only eating more carbohydrates, but just about everything. Indeed, Figure 4 (2) shows that the daily per capita availability of calories has been increasing ever since the mid 1980's, undoubtedly a major cause of the increase in overweight and a more plausible explanation than the increase in carbohydrate alone.

No Magical Weight Loss

All right, so the diet gurus got a little carried away with the increase in carbohydrate as the only cause for the increasing body weight. What cannot be denied, however, is that when people strictly follow a low-carbohydrate, high-protein diet they do lose weight, and they lose it quite rapidly.

While is it is true that most people who follow this diet lose weight, several questions must be answered before nutritionist and dietitians change their advice for the public. The first question is, how does the diet promote weight loss? According to a study by Golay et. al. (3), it is clear that the rate of weight loss as well as the composition of the weight loss does not differ between a low-or high-carbohydrate diet. The authors state that "The results of this study showed that it was energy intake, not nutrient composition, that determined weight loss in response to low-energy diets."

If it really is the caloric intake that is the determining factor in the weight loss, why does it seem like low-carbohydrate diets are more effective than conventional low-calorie diets? There are several explanations. The first concerns the initial rate of weight loss. Weight loss during the first week on a low-carbohydrate, high-protein diet is impressive. The numbers on the scale seem to drop much faster on a low-carbohydrate diet than on a typical low-calorie diet. But the numbers on the scale are deceiving and not a good indication of loss of body fat. Much of the initial weight loss on a low-carbohydrate diet is due to a loss of liver glycogen, a storage form of carbohydrate. The glycogen is lost because the low-carbohydrate diets do not provide enough glucose to maintain blood sugar and the liver glycogen is used to maintain normal blood sugar. Glycogen is heavy because it includes a large numbers of water molecules. When the liver converts the glycogen to glucose, this water is lost from the body. Therefore, much of the initial weight is due to water loss, not loss of body fat.

The second explanation for the apparent ease of losing weight on a low-carbohydrate diet is the loss of appetite. It is easier to restrict your intake when you are not hungry and a high-protein diet will definitely curb your appetite. The reason rests with the liver. There is a considerable amount of evidence from animal studies that feeding a high protein diet inhibits appetite. Hannah et. al. (4) concluded that "feeding a high-protein diet results in a physiological appetite suppression, possibly mediated through branched-chain amino acids." Thus, the low-carbohydrate, high-protein diet suppresses appetite, making it easier to restrict calories. The weight loss that ensues is merely a consequence of the reduced caloric intake. There's nothing magical about that.

You Can't Stay on the Diet Forever

Usually, people are so thrilled with the initial weight loss that they do not realize that they can't stay on the diet forever. As soon as people begin to increase the amount of carbohydrate in their diets and, therefore, decrease the amount of protein, appetite returns, caloric intake increases, glycogen stores get replenished, and body weight quickly returns to its pre-diet levels

But why can't one remain on this diet forever? The main reason is monotony. One fundamental characteristic of human eating behavior is that we crave different foods. If the number of different foods is limited, the amount of the monotonous food consumed is decreased and the desire to eat different foods is enhanced. This aversion to monotony in the diet serves a very useful function of preventing us from getting nutrient deficiencies. If our diet is varied, it is unlikely that a deficiency in a particular nutrient will occur. A low-carbohydrate, high-protein diet is actually quite limiting in the selection of foods that one can consume. The longer one remains on this diet, the greater the tendency to drift from the diet. When the drift involves eating some carbohydrates, there goes the diet and the weight loss.

Are Low-Carbohydrate Diets Safe?

Are low-carbohydrate, high-protein diets safe? In actuality, we don't know because no one has carried out a long-term study of people on low-carbohydrate diets, but we can make an educated guess from studies of people who eat high-protein diets. Perhaps one of the most consistent relationships observed in the nutrition literature is the positive relationship between the consumption of animal protein and the development of chronic disease. It is not clear, however, whether the effects are due to the consumption of large amounts of protein, large amounts of fat associated with eating high-protein diets, or due to the lack of consuming enough protective "phytochemicals" in plant based foods. Nevertheless, we know that it is healthier to eat plant-based foods than animal-based foods.

The Bottom Line on Weight Control

Despite the seduction of the low carbohydrate diets for weight control, there is no evidence that it is effective as a long-term weight management technique. There are no long-term follow-up studies. The best advice is still the same that nutritionists and dietitians have been giving the public for years: 1) eat only when hungry, 2) eat lower fat foods, 3) and exercise as much as possible. It is as true today, as it ever was.

References

1. Centers for Disease Control and Prevention, National Center for Health Statistics, Division of Health Examination Statistics. 1996.
2. America's Eating Habits: Changes and Consequences. Edited by Elizabeth Frazão, Food and Rural Economics Division, Economic Research Service, U.S. Department of Agriculture. Agriculture Information Bulletin No. 750 (AIB-750). May 1999.
3. Golay A; Allis AF; Morel Y; de Tonic N; Tankova S; Reaven G, Similar weight loss with low- or high-carbohydrate diets. Am J Clin Nutr, 1996; 63 (2):174-8
4. Hannah JS; Dubey AK; Hansen BC. Postingestional effects of a high-protein diet on the regulation of food intake in monkeys. Am J Clin Nutr 1990;52 (2):320-5.


 
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