Is intermittent fasting safe for older adults?

Learn the risks before diving into this experimental eating style.

Intermittent fasting is a popular eating strategy being studied in labs and practiced in kitchens across America. And it's more than a fad. Restricting your calories or mealtimes may have the potential for many benefits, such as weight loss and reduced risk of various diseases. We don't have much evidence, however, about intermittent fasting's effect on the health of older adults.

What is intermittent fasting?

Intermittent fasting restricts when or how much you eat — and sometimes both. There are several approaches.

In alternate-day fasting, you eat normally every other day. On days in between, you eat just 25% of your daily calorie needs, in one meal. So if you consume 1,800 calories on Monday, Wednesday, and Friday, you'd eat a 450-calorie meal (and nothing else) on Tuesday, Thursday, and Saturday.

In the 5:2 approach, you eat normally for five days in a row; then for two days in a row, you eat just 400 to 500 calories per day.

In the 16:8 approach, each day is the same: you fast 16 hours in a row, and then eat normally within an eight-hour period, like between 9 a.m. and 5 p.m.

What are the benefits?

The benefits of intermittent fasting seem to affect many aspects of health. These effects could theoretically result from the flipping of a metabolic switch.

"Fasting leads to lower levels of glucose [blood sugar]. In response, the body uses fat instead of glucose as a source of energy, after turning the fat into ketones," explains registered dietitian Kathy McManus, director of the Department of Nutrition at Harvard-affiliated Brigham and Women's Hospital. This shift from glucose to ketones as a source of energy also changes body chemistry in healthy ways.

Regular fasting in animals is associated not only with weight loss but also with lower blood pressure and heart rate, reduced insulin resistance, lower "bad" LDL cholesterol levels, higher "good" HDL cholesterol levels, and less inflammation. Some studies also have found improved memory.

Intermittent fasting is also associated with a longer life span, at least in animals. Why? Recent Harvard research suggests that intermittent fasting may allow the energy-producing engines (mitochondria) of each cell to produce energy more efficiently and remain in a more youthful state.

"By eating in the day, you're not challenging the mitochondria at night, when they're supposed to be doing other things," explains Dr. William Mair, a researcher and associate professor of genetics and complex diseases at the Harvard T.H. Chan School of Public Health. "But we have many unanswered questions."

Potential risks

While intermittent fasting shows promise, we don't have solid evidence about the benefits or how fasting might affect older adults. Human studies have looked mostly at small groups of young or middle-aged adults, for only short periods of time.

But we do know intermittent fasting could be risky in some cases. "If you're already marginal as far as body weight goes, I'd be concerned that you'd lose too much weight, which can affect your bones, overall immune system, and energy level," McManus says.

Dr. Suzanne Salamon, associate chief of gerontology at Harvard-affiliated Beth Israel Deaconess Medical Center, expresses another concern: "People who need to take their medications with food — to avoid nausea or stomach irritation — may not do well with fasting. Also, people who take heart or blood pressure medications may be more likely to suffer dangerous imbalances in potassium and sodium when they're fasting."

Intermittent fasting may also be harmful if you have diabetes and need food at certain times or take medication that affects your blood sugar.

Still want to try it?

If you're thinking of trying intermittent fasting, especially if you already have health conditions such as diabetes or heart disease, talk to your physician.

McManus advises easing into the diet. "Slowly reduce the time window for eating, over a period of several months," she advises.

Also: continue your medication regimen as recommended by your doctor. "Taking medications doesn't break the fast, and neither does having calorie-free drinks like water or black coffee," says Dr. Alexander Soukas, an endocrinologist and molecular geneticist with Harvard-affiliated Massachusetts General Hospital.

What if you do need food with medication? "Perhaps you can try a modified fast. I suspect it would still do a lot of good for people who are overweight," Dr. Salamon says. "Just work with your doctor on a plan that will benefit your health without risking it."

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