Although it first appeared in the 1930s, the grapefruit diet reached its zenith of popularity in the 1970s, when it was renamed "The Mayo Clinic diet." This association with the famed Minnesota medical clinic lent an air of scientific credibility to the diet, despite the fact that the Mayo Clinic neither devised nor advocated use of the diet. Most forms of the diet encourage consumption of meat, but limit the amounts of fruits and vegetables consumed, which makes it a low-carb plan. Along with this, it's often advised that you take two days off the diet.
Most scientists have long held that the grapefruit diets in all their various incarnations are nothing but a fad diet that lacks essential nutrients and could prove hazardous to long-term health. But in 2004, in a study that was funded by the Florida Citrus Department, subjects lost an average of 3-4 pounds over 12 weeks by consuming half a grapefruit or drinking a glass of grapefruit juice with each meal. The subjects also exercised regularly. Some of the study participants lost as much as ten pounds. Researchers conducting the study suggested that grapefruit may reduce insulin levels, which would have the effect of promoting body fat loss.
A new study,however, suggests that the active ingredient in grapefruit may be something else. The study identified a substance in grapefruit called nootkatone that is a natural stimulator of a enzyme called AMP-activated protein kinase or AMPK. AMPK plays a pivotal role in fuel usage by muscles. When muscles are low in glycogen, AMPK is activated, and the muscle begins to use fat as a major fuel source. AMPK is also activated by aerobic exercise. In the study, a dose of 200 milligrams per kilogram of bodyweight of nookatone was provided to mice. The study found that long-term intake of this substance led to a significantly reduced body fat gain in the mice even after the rodents consumed diets high in sugar and fat. It also blunted abdominal fat accumulation, and the development of elevated levels of insulin, blood glucose, and leptin in the mice. It even increased exercise endurance, as evidenced by a 21% increased swimming time in mice provided with nookatone compared to mice not provided with the substance. All this occurred because of activation of AMPK, which opened the metabolic door to increased fat oxidation.
Would nookatone work as well in humans? That remains to be seen. On the other hand, the presence of nootakone in grapefruit could explain the recurring popularity of grapefruit as a weight-loss aid. And there is no argument about the metabolic effects of AMPK in humans, so there might be something to eating grapefruit for fat loss after all.
Murase, T et al. Nookatone, a characteristic constituent of grapefruit, stimulates energy metabolism and prevents diet-induced obesity by activating AMPK.Am J Physiol Endocrinol Metab 2010;299:E266-E275.
