What’s the Difference Between Alternate Day Fasting vs. Intermittent Fasting?

Intermittent fasting vs. alternate day fasting

Intermittent fasting is any eating plan that restricts when you do—and don’t—eat. There are many versions and each promises any number of health benefits—ranging from weight loss to a reduced risk of several diseases.

One of the most popular types of intermittent fasting is alternate day fasting. You tend to lose more weight on this plan than some of the other methods, but it can be hard to stick with it.

Alternate day fasting involves fasting every other day, then eating what you want on non-fast days. On fast days, you stick to no more than about 500 calories, says Krista Varady, PhD, a professor of nutrition at the University of Illinois and author of The Every-Other-Day Diet.

laflor/Getty Images

How alternate day fasting works

For strict alternate day fasting diets, you follow a fast/feast plan all week. That means on fasting days you have just your limited calories and as much water and other non-caloric drinks as you want (including lemon water). Then on non-fast days, you can eat anything you want. (Learn more about what you’re allowed to drink while fasting.)

There are modified versions of the plan that some people find easier to follow, says Varady.

For example, the Eat-Stop-Eat approach requires a 24-hour fast once or twice a week. Some alternate day fasters may fast for two or three days a week instead of every other day. The popular 5:2 method calls for fasting two days a week and feasting on five. Learn more about the various alternate day fasting schedules.

How other intermittent fasting techniques work

The time-restricted version of intermittent fasting limits eating to certain hours. One popular approach: the 16:8 method, meaning you fast for 16 hours and feast during an eight-hour window. You might, for example, eat between 7 a.m. and 3 p.m., or between 11:30 a.m. and 7:30 p.m.

It takes about 10 days to adjust to an intermittent fasting regimen whether alternate day fasting or time-restricted feeding, says Varady.

Alternate day fasting and weight loss

The goal of alternate day fasting is to flip your metabolic switch so your body burns fat instead of glucose for energy. This tends to happen within eight to 72 hours. As the body breaks down fat, ketones begin to build up in your bloodstream. This is called ketosis and it shows that your body is burning fat for fuel. These are chemicals your body makes for fuel when you don’t have enough sugar for energy.

“Alternate day fasting produces faster weight loss, about 10 to 15 pounds in three months, compared with time-restricted feeding,” says Varady. “But it’s harder to follow because you have to count calories every other day.”

Time-restricted intermittent fasting and weight loss

With time-restricted fasting, weight loss is about six to seven pounds in three months, Varady says. But “all you need is a clock and you can do time-restricted feeding.”

Weight loss may be even greater when alternative day fasting is combined with a low-carb diet, she adds. “People lose 15 percent more weight when they eat a low-carb diet as part of their alternate day fasting than with alternate day fasting alone.” This means choosing low carb foods on fast days.

If alternate day fasting is too difficult, “5-2 is a doable aggressive approach that may be a good choice if you have a lot of weight to lose,” adds Courtney M. Peterson, PhD, an associate professor in the department of nutrition sciences at the University of Alabama at Birmingham.

Alternate day fasting calories

With alternate day fasting, you can eat few to no calories on fast days, explains Matthew McAllister, PhD, assistant professor in the department of health and human performance at Texas State University in San Marcos, Texas. Typically, on fasting days, people consume no more than about 25 percent of their usual caloric needs (about 500 calories). Unlimited non-caloric beverages are permitted. “You can have coffee if it is black, but not with added milk or sugar,” he says.

Time-restricted intermittent fasting calories

Because you’re only skipping, essentially, one meal out of the day, you can skip calories altogether during the time you’re not eating. However, as you adjust to the diet—and anytime you feel light-headed, have headaches, feel dizzy, or experience any unusual symptoms—you should snack on nuts, veggies, or other healthy foods.

Alternate day fasting: Health risks

There was concern that alternate day fasters may eat more on feast days and negate some of the weight loss benefits, but this has not been the case, Varady says. “We have shown most people only eat 10 percent more on feast days because they get control of hunger.”

You may feel irritable on fast days, she says. In general, all types of fasting can cause headaches, fainting, weakness, and dehydration, says Scott Kahan, MD, the director of the National Center for Weight and Wellness in Washington, D.C. The lack of food triggers steep dips in your blood sugar, which can be dangerous for some people with chronic conditions, such as diabetes, heart disease, or gout; pregnant women, children, and the elderly should also avoid fasting for any length of time.

Time-restricted intermittent fasting: Health risks

Putting off eating for part of the day can lead to some of the same issues as alternate day fasting—headaches, irritability—but they shouldn’t be as severe. Even though the time of fasting is shorter compared to the alternate day method, blood sugar swings can mean trouble if you have underlying health issues. With any diet program, always discuss the plan with your doctor before starting.

Alternate day fasting: Health benefits

Research suggests health benefits outside of weight loss for alternate day fasting. In a study, published in 2019 in Cell Metabolism, participants in the alternate day fasting group had reduced levels of soluble intercellular adhesion molecule-1, a marker of inflammation and age-related disease. What’s more, alternate day fasters showed lower levels of the thyroid hormone triiodothyronine, without any thyroid problems.

Previous research suggests that lower levels of this hormone may be linked with longevity. The alternate day fasting group had lower levels of cholesterol and dangerous belly fat. Belly fat may release inflammatory chemicals and other molecules that can increase the risk of heart disease, liver disease, and diabetes.

Time-restricted intermittent fasting: Health benefits

There are also benefits apart from weight loss that are seen with time-restricted feeding too, McAllister says. “Time-restricted feeding can reduce body fat and blood pressure and increase levels of high-density lipoprotein, or ‘good’ cholesterol, and adiponectin levels, which helps the body burn fat and glucose,” says McAllister, echoing the results of a study published in 2020 in Nutrition Research.

These changes were independent of caloric intake, he says. “You see these benefits even if you are not limiting calories each day.” Still, the healthier the foods that you eat in the window, the greater the benefits.

The bottom line? Whatever fasting method you choose should fit into your lifestyle, McAllister says.

Eva Blanco / EyeEm/Getty Images

Intermittent Fasting Schedule

Andrii Kobryn/shutterstock

7 Intermittent Fasting Benefits

Westend61/Getty Images

Trying Intermittent Fasting