There are always new weight-loss diets out there—or old ones dressed up as new—and one of the biggest recent fads is “calorie-shifting” plans. Proponents promise rapid and easy weight loss, which is always a warning sign, and cite theories and research that may sound convincing. Here’s a look behind the hype.
The problem with crash diets
A fundamental problem with crash diets is that when you drastically cut calories for more than a few days, your body compensates by lowering the rate at which you burn calories when at rest (its basal metabolic rate), which makes it hard to keep losing weight and then even harder to keep the lost pounds from returning. Calorie-shifting diets alternate calorie intakes—and sometimes types of food—from day to day. Their advocates claim this tricks the body into not adapting to the reduced calorie intake so that your metabolic rate won’t drop.
The diets offer a variety of calorie roller coasters. The Every Other Day Diet and the QOD Diet (a medical acronym for “every other day”) alternate days of normal eating and days of nearly fasting—just 300 to 500 calories a day. The UpDayDownDay Diet calls for eating no more than 500 calories on “down” days and all you want on the “up” days during the induction phase. The latest entry is a British import called The Fast Diet, which involves eating just one-quarter of your normal daily calories (500 to 600 calories) two days a week.
Some calorie-shifting diets involve complicated patterns—say, 1,300 calories on Monday, 2,000 on Tuesday, 800 on Wednesday, 1,800 on Thursday and so on. Fat Loss 4 Idiots maps out your up and down days via a software program. Some, like the Intermittent Fasting Diet, require total fasting on some days.
The explanations presented for the diets vary as much as the eating patterns. Some say the diet works by affecting hormones (such as insulin) involved with fat storage, or by activating genes that boost fat burning. Others say the secret is to limit hunger to single days, rather than prolong it for weeks. Some claim the diet won’t cause loss of muscle—another problem with crash diets—and will actually speed up metabolism. Proponents make general health claims, too, that the diets can help prevent chronic diseases by reducing inflammation, blood cholesterol and free radicals. Still others say the fasting days “cleanse” your body of toxins, which is nonsensical. Or that semi-starvation diets can prolong your life.
So far, there is no convincing evidence to support most of the theories, and none demonstrating any of the diets’ long-term safety and effectiveness.
Not so fast
There has been little good human research on the diets. Mouse studies, including one done at UC Berkeley in 2007, found that alternate-day fasting does not produce weight loss (the mice simply ate twice as much on the feasting days), but may have some potentially beneficial effects, such as improved insulin and glucose metabolism and shrinkage of fat cells. Some widely publicized animal studies have found that very-low-calorie (semi-starvation) diets can lead to increased longevity, but they did not use calorie-shifting diets or alternate day fasting, and these results have never been demonstrated in humans.
A small human study in the American Journal of Clinical Nutrition in 2005 found that alternate-day fasting/feasting for three weeks resulted in a few lost pounds and lower insulin levels. But many people reported feeling irritable and hungry on the fasting days, and some experienced constipation, leading the researchers to conclude that most people could not stick to this eating pattern for long.
The most positive results came from another very small study in the same journal in 2009. It found that obese people who went on near-fasts on alternate days lost about 12 pounds after eight weeks. Cholesterol levels and blood pressure improved, as they would with any weight loss. But the study was poorly designed, and didn’t assess how the diet affected mood and quality of life.
Neither human study had a control group, so we don’t know how the results would compare to, say, a standard reduced-calorie, heart-healthy diet.
Bottom line: Obviously, if you eat little or no food several days a week and don’t eat like a ravenous mouse on the others, you’ll lose weight. And if you are in good health, there’s no harm in an occasional 24-hour fast or near-fast. Like any crash diet, such regimens may feel empowering the first week or two, especially as the pounds come off, largely thanks to water loss. But these diets are not long-term solutions to obesity and are not healthy ways to relate to food.
Despite the testimonials on the diets’ websites and book jackets, most people won’t feel good on calorie roller coasters, and most couldn’t stick to the crazy regimens. Very-low-calorie diets can cause fatigue, headaches, irritability and heart rhythm problems. Skipping meals can sometimes result in malnutrition and is ill advised for people with diabetes. Alternating fasting with gorging is especially risky if you have an eating disorder. And if you don’t have one, you’ll be eating as if you did.
Originally published June 2012. Updated April 2013.