Historically, the ritual of abstaining from food for a said period, known as fasting, has been linked to religious practices. However, fasting comes in all shapes and sizes, and this behavior is no stranger to those partaking in current dietary trends. Over the last few years, the popularity of intermittent fasting (IF) and time-restricted eating (TRE) has skyrocketed — and likely for good reason. Studies show that adhering to a daily fasting period can help people manage their weight, stabilize their blood glucose, support brain health, and lead to a reduction in risk factors for cardiovascular disease. But how long should you fast to get these benefits? And is fasting a safe and evidenced-based practice? Let’s dive into the details.

Before we dive in, if you're looking for a quick and easy way to track your fast using an app, consider January. It's completely free to download and use, and allows you to choose between numerous types of fasting. It also provides insight into the food you do eat, so you can make the best choices possible for your health goals.

Download the free January app here.

What happens metabolically when you fast?

Simply put, when food is consumed regularly during the waking hours, it is metabolically converted to glucose — the body's primary source of energy. However, when fasting for eight to 12 hours, glucose levels start to dip and the body begins to tap into stored carbohydrates (called glycogen) and eventually fat as sources of energy. Therefore, intermittent fasting has been linked, mostly in animal research, to revving up the body’s fat-burning mechanisms. 

What are the types of intermittent fasting?

There are many different types of fasting that fall under the intermittent fasting umbrella. The most common ones discussed in the literature are: 

• Alternate-day fasting: Fasting days with little or no calorie consumption are alternated with non-fasting days of unrestricted intake.‍
• Modified intermittent fasting: Restricted intake days followed by a longer series of non-fasting days, this is commonly called the 5:2 approach, in which a low-calorie intake (~20% of needs or 500 calories) is consumed on two consecutive days and the other five days are normal (~2,000 calories).‍
• Religious fasting: Among the many variations of religious fasting, the practice of eating a large meal after sunset and a small meal before sunrise is most common and creates a fasting period of about 12 hours. ‍
• Time-restricted eating: This practice limits food intake to a certain number of hours per day. Compared to other types of IF, the literature suggests that most individuals find time-restricted eating a realistic regimen because it is a similar pattern to the way many people already eat (with much of the fasting taking place overnight). 

Regardless of what type of intermittent fasting you may be considering, it is important to first consult with your healthcare provider — especially if you are pregnant or breastfeeding, have type 1 diabetes, are under age 18, or have a history of disordered eating. 

How long should you fast? 

Research suggests that extended periods of fasting, independent of dietary composition and caloric intake, could have potential as an effective intervention for the treatment of chronic and age-related disorders. 

While the ideal fasting period has not been proven in clinical (human) studies, most of the research points to a span of 12 to 18 hours as most beneficial. This fasting range appears to have various positive effects on the cardiovascular system, reduces markers of inflammation, and stimulates the metabolic switch between glucose and fat oxidation. In the January app, we enable you to select the fasting schedule that works best for you.

Some advocates tout the benefits of longer periods without food — such as  24, 36, 48 and even 72 hours of fasting. However, many healthcare professionals are concerned about the safety of such practices and recommend that these much longer fasting periods be adopted only under medical supervision. 

What is the 16:8 method?

Simply put, this intermittent fasting approach involves fasting daily for 16 hours and restricting food intake to an eight-hour window. For example, you could eat your first meal of the day at 11 a.m. and be finished with dinner by 7 p.m. In a small study, obese subjects following a 16:8 diet pattern for 12 weeks were compared to those who ate “normally.” In one study, an 8-hour time-restricted feeding resulted in ~350 less calories per day, an approximate weight loss of 3%, together with a reduction in systolic blood pressure. This is the method we typically prefer in the January app.

What is the 14:10 method?

Like the 16:8 intermittent fasting method, the 14:10 method is reported to be easy to adhere to as it involves only a 14-hour fasting period paired with a 10-hour eating window. A study investigating the effectiveness of a 14:10 eating pattern compared to a 12:12 pattern in obese individuals found that those participants adhering to a 14:10 schedule lost more weight and had significantly improved blood glucose levels after eight weeks. 

Although further human studies are needed, fasting has emerged as a method for controlling the cluster of metabolic abnormalities known as metabolic syndrome (MetS). In a study of patients with MetS who followed the 14:10 method for 12 weeks, improvements in waist circumference, percent body fat, and percent visceral fat were demonstrated, leading the authors to suggest that a 10-hour eating period promotes weight loss in those with MetS. 

Key takeaways

Further evidence is needed to determine the ideal fasting duration period that would most benefit metabolic and other long-term health outcome measures. The early research, however, does suggest that — particularly for individuals prone to late-night snacking — a few simple eating behavior changes could result in the achievement of a 12-hour fasting window, which in turn could improve blood glucose regulation, fat-burning, and overall cardiometabolic health. To track your fasting and get individualized recommendations, check out the January app.

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