Testing the Keto Diet Theory

Do low-carb and ketogenic diets have a metabolic advantage for weight loss? 

When you don’t eat enough carbohydrates, you force your body to burn more fat. “However, this rise in fat oxidation [burning] is often misconstrued as a greater rate of net FM [fat-mass] reduction” in the body, ignoring the fact that, on a ketogenic diet, your fat intake shoots up, too. What happens to your overall body fat balance? You can’t empty a tub by widening the drain if you’re opening the faucet at the same time. Low-carb advocates had a theory, though, the “carbohydrate–insulin model of obesity,” which I discuss in my video Keto Diet Theory Put to the Test.  

Proponents of low-carb diets, whether a ketogenic diet or a more relaxed form of carbohydrate restriction, suggested that decreased insulin secretion would lead to less fat storage, so even if you were eating more fat, less of it would stick to your frame. We’d burn more and store less, the perfect combination for fat loss—or so the theory went. To their credit, instead of just speculating about it, they decided to put it to the test. 

Gary Taubes formed the Nutrition Science Initiative (NuSI) to sponsor research to validate the carbohydrate–insulin model. He’s the journalist who wrote the controversial 2002 New York Times Magazine article “What If It’s All Been a Big Fat Lie?” which attempted to turn nutrition dogma on its head by arguing in favor of the Atkins diet with its bunless bacon cheeseburgers based on the carbohydrate–insulin model. (Much of Nina Teicholz’s book The Big Fat Surprise is simply recycled from Taubes’ earlier work.)  

In response, some of the very researchers Taubes cited to support his thesis accused him of twisting their words. One said, “The article was incredibly misleading…I was horrified.” Said another, “He took this weird little idea and blew it up, and people believed him…What a disaster.” It doesn’t matter what people say, though. All that matters is the science. 

Taubes attracted $40 million in committed funding for his Nutrition Science Initiative to prove to the world that you could lose more body fat on a ketogenic diet. NuSI contracted noted researcher Kevin Hall from the National Institutes of Health to perform the study. Seventeen overweight or obese men were effectively locked in what’s called a metabolic ward for two months to allow researchers total control over their diets. For the first month, they were placed on a typical high-carbohydrate diet (50 percent carbs, 35 percent fat, 15 percent protein), then were switched to a low-carb ketogenic diet (only 5 percent of calories from carbohydrates and 80 percent fat) for the second month. Both diets had the same number of daily calories. So, if a calorie is a calorie when it comes to weight loss, there should be no difference in body fat loss on the regular diet versus the ketogenic diet. If Taubes was right, though, if fat calories were somehow less fattening, then body fat loss would become accelerated on a keto diet. Instead, in the very study funded by the Nutrition Science Initiative, researchers found that body fat loss slowed during the ketogenic diet. 

Why do people think the keto diet works if it actually slows fat loss? Well, as you can see in the graph below and at 3:40 in my video, if you looked only at the readings on bathroom scales, the ketogenic diet would seem like a smashing success. Participants went from losing less than a pound a week on the regular diet during the first two weeks of the study to losing three and a half pounds within seven days after switching to the ketogenic diet. What was happening inside their bodies, however, told a totally different story: Their rate of body fat loss was slowed by more than half. So, most of what they were losing was just water weight. It’s presumed the reason they started burning less fat on a ketogenic diet was because, without the preferred fuel of carbohydrates, their bodies started burning more of their own protein—and that’s exactly what happened. Switching to a ketogenic diet made them lose less fat mass and more fat-free mass. Indeed, they lost more lean mass. That may help explain why the leg muscles of CrossFit trainees placed on a ketogenic diet may shrink as much as 8 percent. The vast lateralis, the biggest quad muscle in your leg, shrunk in thickness by 8 percent on a ketogenic diet.