However, the long-term studies of this diet have not been especially promising. Ketogenic diets have yielded no better weight-loss results than other diets, none of which have fared well over sustained periods of time.
The reason for this appears pretty straightforward: Like all restrictive eating regimens, the ketogenic diet is tough to stick to. While it may at first sound mouthwatering to make bacon a keystone of your meal plans, over the long haul, most people cannot limit themselves each day to the carbs you find in a single slice of bread. (Compare this to the daily intake of a typical American, who currently gets roughly half of all calories from carbohydrates.)
Even if a person has the willpower to follow a strict ketogenic eating plan, a diet so high in fat presents many potential obstacles. You must be sure to eat lots of leafy greens, as most other vegetables (other than avocados) and pretty much all fruit are forbidden. Ditto with calcium supplements, as milk, cheese and yogurt are off the table.
You also need to pay careful attention to the types of fat you are eating, as you will be consuming extremely large volumes of something that's been implicated in heart disease and hypertension. But because almost nobody manages to adhere to this diet for years, there's little evidence as to its longer-term impact on people's health.
If your friends are faring well on this diet, more power to them. Going forward, they should be sure to monitor items like their cholesterol and triglyceride levels and blood pressure to ensure the diet is not having an adverse effect.
For you, a vegetarian, this diet is an obvious no-go, unless you wanted to subsist on eggs, olive oil and avocados alone. (For the record, I do not recommend this.) But even if you ate meat, there's another reason you, as a marathoner, would want to avoid it.
A new study looked at how Olympic-caliber long-distance race walkers fared on this diet. While their bodies burned more fat than those of their counterparts on more conventional diets, their times were markedly slower. I'd expect similar results for marathoners or any other endurance athletes, who rely on the easy-to-access fuel found in carbs for optimal performance.
Prescott, a physician, and medical researcher, is president of the Oklahoma Medical Research Foundation. Cohen is a marathoner and OMRF's senior vice president and general counsel.