Intermittent fasting: The latest research

Jennifer Aniston says it changed her life. No, we’re not talking about a sitcom. We’re talking about the current trending diet of intermittent fasting. But before you dismiss it as just a fad, Jeffrey Hooper, MD, says for some people intermittent fasting has real benefits that may go beyond weight loss.

“It’s not new,” says Dr. Hooper, medical director for the MultiCare Center for Weight Loss & Wellness. “Like so many things in weight loss, the pendulum swings, and intermittent fasting is getting popular again.”

And because it’s been around for a while, intermittent fasting “actually has science behind it,” he adds.

In late December, the New England Journal of Medicine published an article reviewing previous intermittent fasting studies, giving the method another big publicity boost.

The review highlighted findings that, along with promoting weight loss, intermittent fasting decreases insulin resistance and the chances of developing diabetes, reduces risk for some cancers and forms of dementia and increases cardiovascular health and stress resistance. Studies in rats showed that intermittent fasting greatly increased their lifespans and slowed the aging process.

“A lot of these things are connected to weight loss, but others are less related,” Dr. Hooper says. “There’s benefit from doing this that goes beyond just the weight loss effect.”

Researchers know it works, but not necessarily how, he says. The New England Journal review theorizes that the key is something called “metabolic switching.” The body always needs energy, and when you’ve eaten, that energy comes from the glucose in your food. But when you haven’t eaten for a few hours, your body turns to glucose stored in your liver. When you still don’t eat, your body runs out of its stored glucose, then turns to burning ketones from your fat cells.

This process is also the basis for the ketogenic diet, Dr. Hooper says. The difference between intermittent fasting and a strict keto diet is that you don’t have to restrict any foods. In other words, you can still eat carbs, just not all day.

“For some people, that’s a really attractive extra,” Dr. Hooper says.

There are two versions of intermittent fasting. The first, which has been studied a bit more, is alternate-day eating or the 5:2 plan. In this version, you eat only 300 to 500 calories either every other day or two non-consecutive days each week.

“It’s hard to get patients to do it,” Dr. Hooper says of those daylong fasts. “It’s tough to do and in my experience, they don’t stick to it.”

Instead, Dr. Hooper recommends patients try the 18:6 method of intermittent fasting. That means you limit your calorie consumption to just six hours a day. That usually means skipping breakfast and starting with a late lunch for most people, although you can move your meals to the first part of the day if you’d prefer. In any case, eat two meals, one at the beginning of your six-hour window and one near the end.

Whichever kind of intermittent fasting you take up — day long or 16-hour long — it won’t help you lose weight if you feel so deprived that you eat far more than you normally would, so it’s not for everyone, Dr. Hooper says.

Fasting is most difficult when you initially start, as your body expects food throughout the day.

“Are you going to be hungry? Yes, you are,” he admits. “But it’s less than you might think.”

You’ll typically feel hunger at the times you normally eat, but as the day progresses, those hunger pangs go away. They’ll return at the next meal time, probably more intensely, but they should ease with a regular, healthy meal.

For many, the idea of skipping breakfast goes against years of messaging that it’s the most important meal of the day. Dr. Hooper says there’s no evidence that is true — except perhaps in children.

“Studies show that kids, when you give them a protein-based breakfast, they think better and last to lunch better,” he says.

Some adults also seem to do better when they eat soon after waking.

“For many, starting the day with a good breakfast gets them going on a good path,” Dr. Hooper says.

If skipping breakfast leads you to more snacking and poor choices at lunch, you need to eat breakfast. But people who simply aren’t hungry in the morning shouldn’t feel like they have to eat breakfast to be healthy.

“A lot of what we do these days is to let people choose what’s right for them,” Dr. Hooper says.

Another myth that intermittent fasting busts is that your metabolism slows and your body goes into a “starvation” state if you don’t have a steady amount of calories throughout the day. Even many dietitians mistakenly believe this, Dr. Hooper says. It’s not the lack of calories, but the weight loss that triggers your metabolism to slow.

“It’s not fair, but it’s reality,” he says. “Yes, your metabolism will slow, but it’s slowing because your body is sensing weight loss. However you lose the weight, your metabolism is going to slow.”

If you want to lose weight, Dr. Hooper recommends reaching out to a weight loss specialist, who will have more training than a primary care doctor in analyzing and recommending a program that will work for you. Because whatever diet you use, it has to be one you can stick with permanently.

“Intermittent fasting is something that’s popular right now and isn’t too hard to employ,” he says. “Primarily it helps to give folks a little structure, and structure to your eating pattern is useful.”

But working with a dietitian and diet specialist can give you the extra pieces you need to get past metabolic set points and make whatever weight loss method you use more effective.

“Whatever we do, we have to keep doing it,” Dr. Hooper says. “We can’t do it for just a month.”