Editor's note. As we went to press with this issue the controversy over the death of Dr Atkins erupted upon the unauthorized release of documents indicating that he may have died not just of complication of a head injury from a fall, but from being overweight. To date this has not been confirmed, but the cultural conversation surrounding the charge, has reintroduced a new level of doubt about the Atkins diet from nutrition experts. As always, we must turn to the science to answer the fundamental questions about all such low-carb diets. Patrick Johnson does so admirably within these page.
SINCE 1980, THE PERCENTAGE OF THE ADULT population in the U.S. that is obese has risen approximately 8 to 12%, and the prevalence of disease that is often associated with excessive body weight has increased correspondingly. In 1999, Americans spent about $60 billion on products that claim to aid in weight control. Some of the most popular products are diet plans with novel food restrictions. But scientific theory states that if an individual simply takes in more energy than he or she expends, the excess will be stored as fat. (1)
In the late 1990s the idea that obesity is caused by carbohydrates (sugars and starches) rather than overeating became popular and has been put forth in several diet books, such as Dr. Atkins' New Diet Revolution, Protein Power, Enter the Zone, Sugarbusters, and Neanderthin. (2) The specifies of these plans vary, but they all share the common claim that significantly reducing carbohydrates will lead to improved health and a reduction in body fat without an overall reduction in caloric intake. This, according to the authors, is due to the effect that reducing carbohydrates has on insulin levels and the subsequent availability of fat for use as an energy source.
After review of the basics of fat and carbohydrate metabolism, as well as the evidence for the efficacy of some of the common claims made by the respective authors, it becomes clear that, though theoretically plausible, these claims are not well supported. It is also apparent that the methods used by the authors to verify these claims are not scientific. At its simplest, weight loss still appears to involve the difficult task of increasing daily energy expenditure and lowering food intake. The authors who claim otherwise bear the burden of proof and, thus far, have not met that burden. (3)
An Introduction to Diet Science
In the body, carbohydrate breakdown occurs in what is known as the glycolytic pathway. The process, which is known as glycolysis, takes one sugar molecule and splits it into two pyruvate molecules (Figure 1). Pyruvate is further broken down in either an aerobic or an anaerobic fashion.
During aerobic metabolism (slow glycolysis), pyruvate is converted to a molecule called acetyl-coenzyme-A (acetyl-CoA), which then continues on to what is called the tricarboxylic acid (TCA) cycle. In anaerobic metabolism (fast glycolysis) acetyl-CoA is converted to lactate and does not go directly through TCA. (4)
Glycolysis, both fast and slow, is what allows mammals to perform high-intensity activity; when it is operating fat metabolism is slowed. If glycolysis is inhibited-- carbohydrate deprivation or metabolic dysfunction--fat metabolism becomes the prime source of energy, and the intensity level during activity becomes difficult to maintain at higher than about 50-60% of an individual's V[O.sub.2]max (aerobic capacity determined as a function of the volume of [O.sub.2] that one can consume in a given time period), an important physiological consideration for anyone who is trying to include regular moderate exercise in his or her lifestyle.
TCA and Oxidative Phosphorylatlon
The tricarboxylic acid (TCA) cycle, along with oxidative phosphorylation in the electron transport chain (ETC), is the body's primary means of adenosine triphosphate (ATP) generation. …
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