How Good a Diet Is Intermittent Fasting?
The popular fasting diet regimen can work well for weight loss, but many other claims about its benefits remain to be proved.
Healthy weight management comes with many perks. Among the proven benefits: a reduced risk of diabetes, less joint pain, lower chances of certain cancers and an overall fitter cardiovascular system. Some regimens, particularly the Mediterranean diet, seem especially well suited to delivering these advantages, though, as with all diets, only to the degree that people can stick with them and avoid overeating. Now research hints that another trendy diet may offer even more extensive health benefits. At least that is the claim by some who study an approach to eating—and not eating—called intermittent fasting.
Intermittent fasting (IF) has its roots in decades of studies showing that if you feed rodents only every other day, they not only remain lean but develop fewer aging-related diseases and live 30 to 40 percent longer. In a 2019 review article in the New England Journal of Medicine, gerontologist Rafael de Cabo of the National Institute on Aging and neuroscientist Mark Mattson of Johns Hopkins University summarized a wealth of findings in animals and a more limited number in people. In rodents and to some degree in monkeys, IF is a veritable fountain of youth, lowering body weight, blood pressure and cholesterol levels, improving glucose control, reducing systemic inflammation, maintaining brain health, and even boosting endurance and coordination. In humans, studies have shown that various forms of IF can be effective ways to lose weight, control blood sugar and lower blood pressure. There are hints that the more stringent forms—those with longer or stricter fasts—offer additional benefits. “But to be honest, a lot of the benefits that we see in animals are not really translating to humans,” says Krista Varady, a professor of nutrition at the University of Illinois at Chicago. “It’s not a magic diet.”
IF comes in three main flavors: alternate-day fasting, when people alternate between feast days (eating normally or a little extra) and fast days with one meager meal of about 500 calories; the 5:2 plan, which means eating normally five days a week but only one scant meal the other two days; and time-restricted eating, when daily dining is confined to a window of eight hours (or, in some versions, six or 10 hours).
Scientists attribute many of the positive effects of IF to something called metabolic switching—after 10 or 12 hours of fasting, the body depletes its supply of glycogen (a stored form of glucose) and starts burning ketones (a fuel made from fat by the liver). This switch affects growth factors, immune signals and other chemicals. “But ketones are not the whole story,” Mattson says. “These periods of fasting-eating-fasting-eating activate genes and signaling pathways that make neurons more resilient,” he says, mainly based on animal research. “It stimulates a process called autophagy: the cells go into a stress-resistance and recycling mode where they get rid of damaged proteins.” Mattson likens cycles of fasting and eating to exercise and rest: “Your muscles don’t get bigger during exercise; they get bigger during the recovery.”
There is good evidence that IF helps people shed pounds. For example, two studies, each with about 100 overweight women, compared the 5:2 regimen with a diet that cut daily calories by 25 percent; both found that the two diets led to the same amount of weight loss over three to six months. The intermittent fasters, however, wound up with better blood sugar control and a greater reduction in body fat. In addition, a 2019 study by Varady’s team with 43 overweight people showed that alternate-day fasting improved the body’s response to insulin by more than twice as much as a typical calorie-cutting diet.
IF may also have an edge in reducing blood pressure, says Courtney Peterson, assistant professor of nutrition at the University of Alabama at Birmingham. In a small but rigorous study with prediabetic men, Peterson’s laboratory showed that restricting meals to a six-hour window that ended at 3 P.M. led to better insulin sensitivity and blood pressure even without weight loss. As for other benefits, dozens of human trials are underway to test IF as a way to slow cancer growth and reduce symptoms of multiple sclerosis, stroke, Crohn’s disease and other illnesses.
In the end, the only successful diets—whatever the goal—involve permanent changes in eating habits. IF can work well over the long haul for meal skippers and people who hate to count calories. But Varady saw a high dropout rate in a yearlong study of alternate-day fasting and is skeptical of time-restriction windows that shut too early: “Nobody wants to skip dinner.”
This article was originally published with the title “Feast and Famine” in Scientific American 323, 3, 23 (September 2020) Click scientificamerican.com to be taken to the article.
Dr. Oz: ‘The Time Restricted Eating Plan I Follow’
This is my favorite — and easiest — version of intermittent fasting. By Dr. Oz
I love food and I love people. There’s nothing better than catching up with my family and friends over a good meal, but working around other people’s schedules can be difficult — especially when I’m on a diet that relies on the clock. The diet I maintain is called time restricted eating, which is an iteration of intermittent fasting — this is also a big component of my new lifestyle plan, System 20. It might sound difficult, but it’s easier to do than you think. Here’s how I make it work and how you can get started today.
It started back in the 1930s when Cornell researcher Clive McKay found that rodents who consumed less calories led them to live longer, healthier lives. Since then, similar experiments of caloric restriction had shown to prolong the lifespan of worms, fleas, and even monkeys.
Research on caloric restriction and periodic fasting came together in 1945 when researchers at the University of Chicago demonstrated that alternate day fasting improved the lifespan of rodents as well. Thus, the concept of intermittent fasting was born.
The idea behind intermittent fasting is that you eat within a window of time and fast between a window of time. This allows your body to eat in sync with your circadian rhythm, which in turn regulates your metabolism, leading to your body burning more energy instead of storing it as fat.
What Is Time Restricted Eating?
The main difference in the time restricted eating plan is the amount of hours you have or “window” to eat your day’s worth of calories. Studies show that restricting the window of time in which you eat can help with weight gain and, in animals, is associated with longevity. The goal here is to that you want to eat for only eight hours a day and fast for the rest of the time. Most of the time you are fasting is overnight, so you barely notice it. Here’s how I plan my meals from the moment I wake up.
What My Day Looks Like:
- Breakfast: I aim to skip breakfast in order to keep my fasting window at 16 hours. Instead of eating when I wake up, I wait until 11 a.m. and then begin the day by eating a healthy snack — like a handful of almonds or a small cup of Greek yogurt with blueberries.
- Lunch: Lunch is my largest meal of the day. For this, I eat a lean protein, like fish or chicken, with a bunch of vegetables, as well as a healthy serving of beans.
- Snack: After I’m done shooting my show, if I feel hungry, I’ll have a light snack to tie me over until dinner.
- Dinner: I have a light dinner, usually around 6 p.m. or as early as I can. I’ll usually have a light salad with a little protein.
How People Get Time Restricted Eating Wrong
My friends, and frequent guests on The Dr. Oz Show, Drs. Michael Crupain and Mike Roizen actually wrote a book that details how time restricted eating works and how it can change your body called, What to Eat When. They’re the ones who got me on board. Time restricted eating is my favorite iteration of intermittent fasting, and to me, it’s the easiest to maintain. I want to make sure everyone has the tools they need to try this effective method.
Most people who try this type of intermittent fasting get it wrong, which is so frustrating to me because it has the potential to really change your body for the better. People usually try to eat smaller portions during the day and turn to big portions at night, which I don’t agree with. Your body is primed to burn carbohydrates in the morning and to burn fat at night. If you add extra food at night instead of giving your body time to burn fat, it then turns that food into more fat.
Your body’s clock essentially changes your metabolism so that it expects you to eat more earlier and eat less later. It’s best to make lunch your largest meal and dinner your smallest. You want to get 80 percent of your calories in ideally before 3 p.m. Then at night, leave a window of at least 12 hours (preferably 16) between your last meal and your first meal the next day. We’ve seen some great benefits from eating this way, including weight loss, sleeping better, and we’ve even seen people with diabetes improve their blood sugar control. And if you’re on the full System 20 plan, this type of eating in combination with stress-reducing habits like meditation, along with exercise and improving personal connections, you may just reduce your risk of heart disease by 20%.