The Kilojoule Backlash

So you want to lose five kilos? Science has a few different variations on how to go about it (and the Internet has hundreds more!). Amid the debate, though, one thing is growing increasingly clear: the once-all-important kilojoule is busted.

- Bill Gifford

COUNT KILOJOULES, SKIP THE FAT: For 40 years, these have been the twin pillars of most mainstream weight-loss advice. But according to Dr. David Ludwig, author of the 2016 bestseller Always Hungry?, the best way to lose weight is to do neither. Fat is not the villain, he writes; processed carbohydrates are. And put down the low-fat snacks! “For 40 years we gobbled up low-fat foods,” he says, “but they were actually making us fatter, because they were filled with processed carbs instead of fat.”

Epidemiological data backs him up: The rise in obesity rates began in the late 1970s, around the same time the U.S. Department of Agriculture began recommending that Americans eat a low-fat diet. After 30 years of scientific research and watching his patients struggle with their weight, Dr. Ludwig decided there had to be a better way to eat.

Perhaps because he's also a professor at Harvard Medical School, Dr. Ludwig can’t resist including pop quizzes in Always Hungry? to see if you’re with him so far. “What is the minimum amount of carbohydrate required for long-term survival?” he asks in Mini Quiz #1. The answer, on page 73: Zero. We must have fat and we must have protein, but we can survive without carbs, he says.

As an endocrinologist, Dr. Ludwig has long been focused on what happens when the carbs we eat turn into sugars in our bloodstream, spiking levels of the hormone insulin. This is why he’s more concerned with starches than butter: all the simple carbs we consume are making us gain weight, he says.

Here’s Mini Quiz #2: “Which of the following raises your blood glucose and insulin the most after consumption, kilojoule for kilojoule?” Your choices: (1) white potato (baked); (2) ice cream; or (3) pure table sugar.