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Alternate day fasting: what is it and how does it work?

alternate day fasting meals
(Image credit: Getty Images)

Alternate day fasting is a type of intermittent fasting that can be used for weight loss and other potential health benefits. It may not be the best form of intermittent fasting for everyone, and we encourage you to explore other types if you find it doesn’t suit you. In particular, women and those with underlying health conditions should be careful.

Fasting has numerous health benefits, possibly due to ancestral eating patterns. Early hunter-gatherer humans would go for longer periods of time without eating than we do in the modern day, and the body might have used these periods to ‘clean’ itself of toxins and potentially dangerous mutated cells. As well as this cleaning system, when the body goes into fasting mode, it switches from using food as fuel to using ketones, which the body makes by metabolizing fat stores. We can still trigger these responses through fasting, for results such as weight loss. 

There are many different types of intermittent fasting. Here, we’ll cover everything you need to know about alternate day fasting in particular, including the potential benefits and risks.

What is alternate day fasting?

According to Dr Nirusa Kumaran, medical director and founder of Elemental Health Clinic (opens in new tab), with an alternate day fasting method, you eat nothing on your fasting days. “These alternate during the week,” she says. “For example, if you choose to fast on Monday, Wednesday, Friday and Sunday in one week, then you eat normally on the days in between and, as such, this pattern continues.”

Josie Porter, a Doctify (opens in new tab)-reviewed dietitian, also explains that fasting days don’t necessarily require abstaining from food completely. “Alternate day fasting is a form of intermittent fasting that requires dieters to carry out a fast every other day,” she says. “This means that the restriction is only required half of the time. When we look at the literature, fasting days typically are up to 800 calories per day, with the non-fasting days being eating to satiety – not seeing this as a free day to make up for the fast.”

What are the benefits of alternate day fasting?

A review in Nutrition Reviews (opens in new tab) indicates that intermittent fasting can be effective in reducing body weight in people of every size, with most studies focusing on alternate day fasting or whole-day fasting trials. As such, fasting for weight loss may be better suited to these styles of fasting. 

Porter agrees that alternate day fasting can be useful for weight loss. “A lot of people carry out forms of intermittent fasting like alternate day fasting, for goals of weight loss or improvements to health conditions, such as diabetes and cardiovascular disease,” she says. “Generally evidence appears to suggest that alternate day fasting is as beneficial as more traditional methods of weight loss, such as creating a daily calorie deficit. Interestingly, of all intermittent fasting regimes, such as time-restricted eating, alternate day fasting appears to have a slight advantage.”

A study in the journal Autophagy (opens in new tab) also suggests that short-term fasting, as one might do with alternate day fasts, triggers the process of autophagy. Autophagy is a state of ‘starvation’ the body enters when fasting, where energy usually used to digest food can be used on cellular renewal and clearing debris and free radicals from our bodies. A study in Carcinogenesis (opens in new tab)indicates that autophagy may be protective of cancer and integral in tumor suppression, making it a promising area for cancer research. 

How to do alternate day fasting

When fasting on alternate days, you will want to either stop eating or reduce your calorie intake to below 500-800kcal on fasting days. On non-fasting days you can then eat whatever you want. In comparison with other methods of intermittent fasting, such as the 5:2 method, or time-restricted eating, alternate day fasting is intense, which is why some people choose to eat one meal at the start of their fasting days instead of fasting completely. It is important to stay hydrated on fast days, and you can have an unlimited amount of calorie-free beverages, like black tea or coffee, which can help with your motivation throughout your fast.

low calorie meal on an alternate day fasting plan

(Image credit: Getty Images)

Is alternate day fasting safe?

A study carried out by the Obesity Society (opens in new tab) found that women of a healthy weight began to experience an impaired glucose response with alternate day fasting. Time-restricted eating may be a safer and more suitable option for women than alternate day fasting. 

Kumaran also encourages women to take care with alternate day fasting due to the effect it can have on their menstrual cycle. “Women can be more sensitive to fasting due to the monthly menstrual cycle and subsequent hormonal fluctuations that women experience,” she says. “Women also have a greater sensitivity to changes in energy balance, as fasting can trigger an exaggerated response to stress. This can then have the opposite effects of the intended health benefits.” 

New to intermittent fasting? Check out our guide to intermittent fasting for beginners for a more detailed guide.

If you have a chronic health condition or mental health condition, you will also want to take extra caution when undertaking big dietary changes, such as alternate day fasting. We recommend you don’t try alternate day fasting without the support of your doctor or a nutrition specialist if you have any underlying conditions.

Porter adds: “Those taking medication for their diabetes or blood pressure should always seek advice from their GP or dietitian before trying any form of restrictive diets. This is because eating minimal or no calories may work against these medications.”

This article is for informational purposes only and is not meant to offer medical advice.

Lou Mudge
Lou Mudge

Lou Mudge is a health writer based in Bath, United Kingdom for Future PLC. She holds an undergraduate degree in creative writing from Bath Spa University, and her work has appeared in Live Science, Tom's Guide, Fit & Well, Coach, T3, and Tech Radar, among others. She regularly writes about health and fitness-related topics such as air quality, gut health, diet and nutrition and the impacts these things have on our lives. 

She has worked for the University of Bath on a chemistry research project and produced a short book in collaboration with the department of education at Bath Spa University.