Fasting for Weight Loss: What the Research Says

First there were the three squares, then calorie-counting, all-day grazing, carb restricting and now, alternate day or intermittent fasting, made popular by The 20-Day Diet and The 5:2 Diet books, the latter which is billed as “the part-time diet with lifetime results.”

Can you really lose weight and keep it off by feasting one day and fasting the next? Some people do have success fasting for weight loss, and very early research indicates that it may have other health benefits as well.

Intermittent fasting can consist of eating about 500 calories or 25 to 35 percent fewer calories every other day (alternate day fasting) or two days out of five. Other people do what used to be forbidden on a diet—skip meals several days a week.

The Facts About Food Addiction

The original interest in occasional fasting came from studies on the effects of calorie restriction on longevity. Animal studies suggest that not eating regularly—like our three squares dictum—can extend lifespan by as much as 30 to 40 percent.

How does it work? Fasting periodically can reduce oxidative stress. That kind of stress is physical—it causes damage to our cells that can lead to cancer, diabetes, cardiovascular disease and other ailments. It can be triggered by toxins in the environment and poor diet, but it’s also the natural byproduct of normal metabolism, the process by which our bodies digest and turn food into fuel. Hence, not eating—at least sometimes—may have benefits, early research suggests.

But to lose weight and gain the health advantages, you don’t have to give up food altogether those few days a week. A small, 2015 study by University of California researchers found that eating a pared down diet—between 34 and 54 percent of recommended daily calories—five days a week and reverting to a normal diet two days a week was as good as total fasting for improving markers of health. (Note: That’s the opposite of the 5:2 Diet.) It helped the people in the study lose weight, lower blood sugar and reduce inflammation, a risk factor for heart disease, cancer and other diseases. It had side effects, of course, but not many. Some people felt fatigued, weak and had headaches. But no one was famished for the entire length of the study, which spanned three months.

The scientist who wrote the book The 2-Day Diet, Michelle Harvie, PhD, of University Hospital of South Manchester in England, conducted a study to find out if intermittent fasting would help overweight women with a history of breast cancer lose weight. Her ultimate goal: To help those women prevent breast cancer. Being overweight, particularly if fat is clustered around the middle, is a risk factor for the disease post-menopause, largely because estrogen is stored in body fat, according to the National Cancer Institute.

In her 2012 study, Harvie reported that women who did intermittent fasting—reducing normal calorie intake by 25 percent on fasting days—lost more weight than those who restricted calories daily. There is no evidence that losing weight can prevent breast cancer in women, but the study was proof that less restrictive “fasting” could achieve good weight loss results.

The big problem with the fasting studies is that so far, those with human subjects are very small and of short duration. And they’re not all in accord. When Dr. Harvie put 106 premenopausal women on two diets—one on which they ate 25 percent less every day and another on which they ate 25 percent less on only two days—the two groups lost the same amount of weight. Bottom line: Either method can work, as long as you stick to it.

The Glycemic Index Explained

There’s also evidence that calorie cutting isn’t the end-all, be-all for intermittent fasters. One human study involving 29 women done by researchers at the University of Illinois at Chicago found that even when you’re fasting, diet composition matters. The women in their research who ate a high-fat diet while fasting had greater concentrations of heart-damaging fatty acids in their system than women who ate a low-fat diet, even though both groups lost weight.

Most experts recommend that you not try intermittent fasting if you have a history of eating disorders, are an emotional eater, or if you tend to binge. Although the very few studies that looked at the diet plan in people didn’t find that fasters gorged themselves on “feast” days, it is a risk.