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The carbohydrate-insulin model of obesity, the underlying theory that ketogenic diets have some sort of metabolic advantage, has been experimentally falsified. The keto diet’s proponents’ own studies showed the exact opposite: ketogenic diets actually put you at a metabolic disadvantage and slow the loss of body fat. How much does fat loss slow down on a low-carb diet?

If you cut about 800 calories a day of carbohydrates from your diet, you lose 53 grams of body fat a day, but if you cut the same number of fat calories, you lose 89 grams a day. Same number of calories, but nine pats of butter worth of extra fat melting off your body every day on a low-fat diet, compared to a low-carb diet. Same number of calories, but about 80 percent more fat loss when you cut down on fat instead of carbs. Here’s the graph. Cut 800 calories from your diet by cutting carbs, and you do lose body fat. But cut out the same number of calories by cutting fat, and you lose even more body fat—80 percent more body fat lost. The title of the study speaks for itself: “Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity.”

But just looking at the bathroom scale, though, would mislead you into thinking the opposite. After six days on the low-carb diet, study subjects lost four pounds. On the low-fat diet, they lost less than three. So, step on the scale and it looks like the low-carb diet wins, hands down. So, you can see why low-carb diets are so popular.

But what was happening inside their bodies tells the real story. The low-carb group was losing mostly lean mass—water and protein. This loss of water weight helps explain why low-carb diets have been “such ‘cash cows’ for publishers” over the last 150 years. That’s their secret. As one weight loss expert noted, “Rapid water loss is the $33-billion diet gimmick.”

When you eat carbohydrates, your body bulks your muscles up with glycogen for quick energy. Eat a high-carbohydrate diet for three days, and you may add about three pounds of muscle mass onto your arms and legs. Those glycogen stores drain away on a low-carb diet, and pull water out with it. (And, the ketones also need to be flushed out of the kidneys, pulling out even more water.) On the scale, that can manifest as four more pounds coming off within 10 days, but that all can be accounted for in water loss.

The bottom line: keto diets just don’t hold water.

But the thrill of seeing the pounds come off so quickly on the scale keeps many coming back. When the diet fails, the dieters often blame themselves. But the intoxication of that initial rapid weight loss may tempt them back, like getting drunk again after forgetting how terrible the last hangover was. This has been dubbed “the false hope syndrome.” The diet industry thrives off of “repeat customers,” something low-carb diets were built for, given that rapid initial water loss.

But what we care about is body fat. In six days, the low-fat diet extracted a total of 80 percent more fat from the body than the low-carb diet. And it’s not just one study. If you look at all the controlled feeding trials where researchers compared low-carb diets to low-fat diets, swapping the same number of carb calories for fat calories or vice versa. If a calorie is just a calorie, then all the studies should have crossed this zero line in the middle, and indeed, six did. One study showed more fat loss on a low-carb diet, but every other study favored the low-fat diet: more loss of body fat eating the same number of calories. Put all the studies together…and we’re talking 16 grams more daily body fat loss on the low-fat diets. That’s four more pats of butter melting off your body on a daily basis. Less fat in the mouth means less fat on the hips, even taking in the same number of calories.

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