Alternate-day fasting: a beginner’s guide

Evidence based

By Jada Rankin, RD, medical review by Dr. Bret Scher, MD Evidence based

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Interest in intermittent fasting, or intentionally going without food and caloric beverages for a short period of time, has grown tremendously in the past few years. There are several different forms of intermittent fasting regimens, and a popular option is alternate-day fasting. 

This guide will provide an overview of alternate-day fasting and explore what the scientific literature says about how it affects weight loss, metabolic health, and cardiovascular blood markers. It’ll also address alternate-day fasting’s safety and provide some practical tips for fasting successfully.

What is alternate-day fasting?

Alternate-day fasting is exactly as its name implies: you alternate fasting days, when you restrict calorie intake, with feasting days when you eat whatever you want, in any amount.¹

Fasting days can either be the strict version, when you don’t take in any calories at all, or the modified version, which allows you to consume just 25% of your energy needs, or about 500 calories. Most of the studies done on alternate-day fasting use the modified method, and this is the type I’ll refer to in this guide.²

On fasting days, food can be eaten all at once or as small meals throughout the day.³ Meals can be either high or low in fat, as long as they are less than 500 calories.⁴ You can make your food from scratch or buy packaged meals that have the calorie content already measured for you. Water, black coffee, unsweetened tea, and sugar-free gum are also allowed in most trials.

When doing a modified fast, foods high in protein and fiber will help you feel less hungry, and soups can fill you up without adding many calories.⁵

Although there are no rules on what to eat on a feast day — remember that alternate-day fasting is about when to eat, not what or how much — it is always important to choose high-quality foods with plenty of nutrients for overall health.

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Health benefits of alternate-day fasting

Most research on alternate-day fasting in humans has focused on its role in helping weight loss, reducing the risk of metabolic diseases, and affecting cardiovascular indicators. Let’s explore each in detail.

Weight loss

To lose weight, our bodies need to burn more energy than they take in. Often, the first thing people try when they want to lose weight is to cut back on how much they eat at most meals. This is known as caloric restriction.⁶

The problem with caloric restriction is that people find it hard to stick to because it requires reducing energy intake every day, which can stimulate hunger and cravings.⁷ In a world filled with around-the-clock availability of delicious, high-calorie foods, it is easy to see why people often fail when trying to diet by eating less every day, day after day.

The part of the weight loss conversation that gets left out is how to create a calorie deficit in a way that feels easier and is sustainable in the long term. Alternate-day fasting can do just that. The idea is that eating significantly less every other day will create enough of a calorie deficit over time to cause weight loss. Even better, you won’t have to eat foods you don’t like, buy expensive supplements, or weigh, track, and measure every bite of food you eat.

Studies report that alternate-day fasting causes moderate weight loss in a relatively short amount of time, resulting in 4% to 8% reductions in body weight in 6 to 12 weeks of alternate-day fasting and a high-carbohydrate diet.⁸

If you combine alternate-day fasting with a low-carbohydrate diet, even more weight may be lost.⁹ The rate of weight loss is often similar to what is seen in traditional caloric restriction. It’s significant because as little as a 5% weight reduction can greatly improve health outcomes.¹⁰

Furthermore, when research subjects undergoing alternate-day fasting engaged in endurance exercise, they lost more weight — up to twice as much — than alternate-day fasting or exercise alone.¹¹

Losing weight is sometimes the easy part, while keeping the weight off for good can be much trickier. Researchers have tested how much weight is regained when people doing alternate-day fasting no longer receive food and nutritional counseling.

They found that when people were allowed a higher calorie intake on fasting days or fewer fasting days per week, participants experienced mild weight regain of 1% to 2%, which is close to that seen in traditional caloric restriction.¹² The study authors noted that people may need to keep up a more rigorous alternate-day fasting protocol to maintain their weight loss.

Body composition is another important factor to consider when losing weight. Ideally, you want to lose body fat, not muscle or lean tissue. Lean tissue helps maintain your resting metabolic rate, meaning that you burn more energy, even at rest.

Research has shown dieters who are alternate-day fasting lose most of their weight as fat and lose just a few ounces of muscle.¹³ Furthermore, fat reduction occurs mainly in the belly area, where the dangerous internal fat surrounding our organs is most often stored.¹⁴ However, the same may not apply to healthy, lean individuals, as one study suggested they lose more lean tissue than fat.¹⁵

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Metabolic health

Studies suggest that alternate-day fasting may have either minimal, or a slight beneficial effect on fasting glucose and hemoglobin A1c (HbA1c) levels.¹⁶ However, studies consistently report that alternate-day fasting reduces fasting insulin and HOMA-IR, a measure of insulin resistance, especially in people with the highest levels of insulin resistance.

Researchers have concluded that alternate-day fasting is more effective than caloric restriction at improving metabolic disease markers in people most at risk of developing diabetes. However, in metabolically healthy adults with obesity, no significant difference was found in metabolic disease markers between subjects doing alternate-day fasting compared to caloric restriction.¹⁷

One potential hypothesis from the literature is that the effect of alternate-day fasting on metabolic health may be more pronounced in those with more significant metabolic dysfunction.

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Cardiovascular markers

Studies report that alternate-day fasting decreases total cholesterol, low-density lipoprotein cholesterol (LDL-C), triglycerides, and systolic and diastolic blood pressure.¹⁸

Researchers have also noted positive impacts on LDL-C particle size. Subjects who exercised while following an alternate-day fasting diet and subjects who did the diet alone had increased LDL particle size after 12 weeks of intervention.¹⁹ This is significant because larger LDL particles are associated with less risk of coronary heart disease compared to smaller particles.²⁰²¹

Is alternate-day fasting safe?

Fasting often gets confused with starvation, but they’re two very different things. Fasting is something you choose to do, with a defined goal in mind, while starvation is an unfortunate situation due to conditions out of your control, such as war or famine.

As long as you aren’t malnourished or underweight, alternate-day fasting appears to be safe, both short and long term. Hunger, irritability, constipation, bad breath, occasional sleep problems, dizziness, and weakness are the most common side effects. Reported symptoms are mild and often go away a couple of weeks after beginning alternate-day fasting.²²

Some studies of caloric restriction report that it may weaken immune function in animals²³ and decrease bone mineral density in humans, although these are not universal findings.²⁴ In people who had been doing alternate-day fasting on their own for more than six months, no difference was found in bone mass, bone mineral density, white cell counts, or other markers of immune function when compared to people who had never fasted.

Furthermore, there was no evidence that thyroid function had been negatively affected in people doing long-term alternate-day fasting, even though some anecdotal reports suggest this may be an issue with frequent, extended fasting, especially for women.

Even so, certain groups of people likely should not attempt fasting.

As with any dietary change, it’s a good idea to seek medical supervision if you decide to start alternate-day fasting.

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4 practical tips for fasting

Try these practical tips to help you successfully implement alternate-day fasting into your health routine.²⁵

1. Have a plan
   Knowing what and when you are going to eat will help keep you on track when life gets messy. This is especially important for a fasting day: having your food planned and ready to go makes it less likely that you’ll swing by a fast food restaurant on your way home after a busy day. As the saying goes, failing to plan is planning to fail.
2. Don’t fear salt, and supplement with electrolytes
   Sodium, magnesium, potassium, and calcium can be depleted by fasting, regardless of which protocol you choose. You lose even more electrolytes if you are physically active or eat a diet low in carbohydrates and processed foods. Liberally salting your food and supplementing with electrolyte drops, powders, or pills can help make sure you don’t become deficient in these important nutrients. Or try Diet Doctor’s electrolyte elixir, which can be drunk on fasting or eating days.
3. Become fat adapted
   When you’re metabolically flexible, your body can tap into its stored fat for energy, meaning you can go for longer periods without feeling hungry. While the act of fasting itself improves fat adaptation, following a low-carbohydrate, ketogenic-style diet is a great way to prepare your body for fasting.
4. Stay busy
   Fasting is easier when you have something productive to do to occupy your mind and body. It’s tempting to break your fast if you’re bored and hungry. This would be a great time to take a long walk, tackle a challenging puzzle, or do whatever hobby keeps you mentally and physically occupied.

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Summary

Alternate-day fasting is an alternative to traditional caloric restriction that shows promise to help you lose weight, improve your metabolic health, and positively affect your cardiovascular blood markers. It can be easier to do than the usual recommendation of eating less all the time because you only need to reduce your energy intake every other day. 

Other than a few minor side effects, alternate-day fasting is safe for most people. It is certainly worth a try if you’ve struggled with different ways of dieting in the past.

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