Eating Only One Food to Lose Weight Is a Terrible Idea

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Photo: James Ross/Getty Images

Comedian and magician Penn Jillette has a new book about how he lost 100 pounds last year, and he kicked off his diet by eating nothing but potatoes for two weeks. (Sounds extreme, but his doctor suggested gastric bypass after he had surgery for a heart blockage. Jillette was not so into that.) He told the New York Post that he ate about five naked potatoes a day and lost 18 pounds. Eating just one food is what’s known as a monotrophic or mono diet.

Jillette didn’t lose all of the weight by eating potatoes: After the two-week mono diet, he ate only vegetables for three months before following a “nutritarian” diet, a.k.a. no animal products, processed grains, or added sugar or salt. It’s basically lots of vegetables, plus fruits, beans, nuts, seeds, and whole grains, and occasional meat, fish, and dairy. He’s now kept the weight off for 17 months.

The initial mono-diet phase was suggested to Jillette by Ray Cronise, a former materials scientist at NASA, whom he met backstage in Vegas. Cronise asked Jillette to “interrupt his current relationship with food” via a major lifestyle transformation. Jillette says the one food consumed for 14 days could have been something else plant-based, like corn or beans, but he settled on potatoes, and he could have as many of them as he wanted. He said he had about five a day.

While it is theoretically possible that a temporary diet of a bland, single food would reduce salt and sugar cravings as well as uncouple the act of eating from both a mindless activity and sociable fun, there is practically no way you can get enough vitamins and nutrients from eating the same fruit, vegetable, or legume all day long. Chief among said nutrients, at least from a weight-loss perspective, is protein.

One medium potato has about 4 grams of protein, so five per day would yield a little more than 20 grams — not nearly enough for a man of Jillette’s size. Doctors have pointed out that, at his starting weight, he would have needed about 100 more grams of protein per day (about 30 potatoes total) to maintain muscle mass. But that would be nearly 5,000 calories; there are much more efficient vehicles for protein. Five potatoes’ worth isn’t even enough for a 150-pound-woman, who needs about 54 grams daily.

Weight-loss experts agree that people should lose as little muscle as possible when slimming down because it burns more calories than fat. Muscle mass is part of what determines your resting metabolic rate (RMR), which already slows down when you lose weight as an evolutionary survival mechanism. Losing muscle would only slow it down more and make dropping pounds that much harder.

A mono diet is also often a crash diet: Jillette’s five potatoes totals about 815 calories. No sane dietitian would recommend eating this little. And, not for nothing, but some of the top search results for “mono diet” are on a pro-anorexia site. Like the weeklong banana diet before it, even a short mono diet could encourage a restrictive, unhealthy relationship with food.

In reality, there is nothing special about eating just one food. Potatoes are not a magic weight-loss secret. Jillette likely lost his first 18 pounds as a result of caloric deprivation. He previously admitted that the approach wasn’t the most sensible — doctors wouldn’t advise losing 100 pounds in just a few months — but it worked for him because he’s not good with small changes. The mono diet was the preface to changing his entire way of eating. That’s what really worked.