Alternate-day fasting involves switching between a day of fasting (not eating) and a day of normal eating. Research suggests this approach may support weight loss and improve heart health. However, it may not be suitable for everyone.
Alternate-day fasting (ADF) is a form of intermittent fasting designed to reduce total calorie intake over time, which can lead to weight loss. The diet may also help improve metabolism.
Normally, when you eat, your body stores energy from carbs and fat in your liver, muscles, and fat tissue. When you fast, your body first uses stored glucose (glycogen) for energy. Once glycogen stores are depleted, your body starts burning fat for fuel.
The cycle of storing nutrients after eating and burning them during fasting is thought to improve metabolic flexibility. This may help improve insulin sensitivity (how well your cells respond to insulin), boost fat burn, and aid weight loss.
Guidelines
When following an ADF eating plan, you’ll fast for 24 hours and eat what you want the following 24 hours, essentially eating every other day. There are no strict rules about macronutrient ratios or forbidden foods on non-fasting days.
The ADF diet doesn’t have a set duration, but studies have shown benefits when it’s followed for several weeks or longer.
There are two main types of ADF:
- Modified alternate-day fasting (MADF): This is the most common approach to ADF. On fasting days, you consume about 25% of your usual calorie intake (around 500 calories), and follow your usual eating pattern on non-fasting days. This method is generally more sustainable and has been shown to offer similar health benefits to stricter versions.
- Zero-calorie alternate-day fasting: This version allows only zero-calorie beverages on fasting days, with normal eating on non-fasting days. It can be more challenging to maintain over time.
When following a modified ADF diet, your total calorie intake should be about 20%–30% of your usual daily needs. For example, if you normally eat 2,000 calories a day, you would limit yourself to about 400–600 calories on fasting days.
There are no official guidelines on what to eat during fasting days, but it’s generally recommended to stick with zero-calorie beverages and high-protein, fiber-rich foods to help keep you full.
Examples include:
- Broth-based soups
- Leafy greens and non-starchy vegetables (kale, cabbage, spinach)
- Eggs or egg whites
- Greek yogurt
- Small amounts of healthy fats, like avocado, nuts, and seeds
- Lean meats like chicken or turkey
- Tofu or tempeh
- Berries or a small piece of fruit
- Black coffee, tea, or zero-calorie sparkling water
During your eating window (the days when you aren’t fasting), you don’t have to count calories or restrict foods. However, choosing whole foods, including lean proteins, healthy fats, fruits, vegetables, and whole grains, can help support overall health.
During your eating window (the days when you aren’t fasting), you don’t need to count calories or restrict specific foods. However, many eating plans recommend limiting certain foods that can contribute to high blood sugar, cholesterol, and other health factors that increase your risk for disease. Consider limiting foods high in:
- Saturated fat (processed and high-fat meats, heavy cream, fried foods)
- Added sugar (sweet snacks, sugary beverages, candy)
- Salt (prepackaged foods, cured meats, deli meats)
Here’s what a day of eating might look like for both fasting and non-fasting days on a modified ADF diet:
Fasting Day (MADF)
- Breakfast: One scrambled egg with spinach
- Snack: One small apple
- Lunch: One cup of low-calorie vegetable soup
- Snack: Ten almonds or low-fat plain Greek yogurt
- Dinner: Steamed broccoli and grilled tofu, chicken, or fish
Tracking your intake can help you stay within your calorie limit on fasting days.
Non-Fasting Day
- Breakfast: Scrambled eggs with spinach and tomatoes, served with a slice of whole grain toast topped with avocado, and a small orange
- Lunch: Grilled chicken breast with a side of quinoa or brown rice, steamed broccoli, and roasted carrots drizzled with olive oil
- Snack: Plain Greek yogurt topped with a handful of berries, plus a small handful of almonds
- Dinner: Baked salmon with roasted sweet potatoes, steamed asparagus, and a side salad made with mixed greens and light dressing
Both traditional and modified types of ADF may offer several health benefits. Some of these benefits come from eating fewer calories, while others may be from fasting itself.
ADF may be easier to follow than traditional calorie counting because you only need to monitor your intake on fasting days.
Possible benefits include:
- Weight loss: Several studies show that ADF can lead to moderate weight loss. One study found it might work better than other types of intermittent fasting plans and traditional calorie restriction for weight loss. However, more research is needed to determine if it is more effective over the long term.
- Type 2 diabetes: ADF has been shown to improve insulin sensitivity and glucose levels in some people. When combined with weight loss, this may help prevent or improve symptoms of type 2 diabetes.
- Heart health: Some evidence suggests ADF can reduce low-density lipoprotein (LDL) cholesterol and blood pressure, two risk factors for heart disease.
- Liver health: A study found that ADF, with or without exercise, may help reduce liver fat and improve liver health in people with non-alcoholic fatty liver disease (NAFLD).
Alternate-day fasting, especially modified ADF, seems to be safe for most adults when followed over several months. However, it may not be appropriate for everyone. Talk to your healthcare provider before trying AFD if you fall into one of these groups:
- People who are pregnant or breastfeeding
- Children and teens
- People with a history of eating disorders
- People with diabetes who are dependent on insulin (due to the risk of low blood sugar)
- Anyone with a history of preexisting health conditions
When starting ADF, some people may experience mild side effects, including:
- Fatigue
- Irritability
- Headaches
- Difficulty concentrating
These side effects often improve over time as your body adjusts. If symptoms continue or worsen, stop fasting and talk with a healthcare provider.
Drawbacks
Alternate-day fasting can be challenging to maintain, especially at first, because you might feel hungry or tired on fasting days. It may also make it harder to participate in social activities, such as eating out with friends.
If not carefully planned, ADF may increase the risk of nutrient deficiencies. Eating a variety of whole foods on your non-fasting days can help ensure you get the nutrients you need. Getting support from a healthcare provider and a registered dietitian nutritionist can help make the diet safer and easier to follow.
Alternate-day fasting is a form of intermittent fasting that alternates fasting and non-fasting days. While more research is needed, it may support weight loss, blood sugar control, and heart health.
AFD is safe for many people, but fasting is not appropriate for everyone. If you’re considering AFD, work with a registered dietitian or healthcare provider who can help you find an approach that fits your lifestyle and nutritional needs.
