Intermittent Fasting: What the Evidence Actually Says

Intermittent fasting (IF) is an eating pattern of cycling between periods of eating and voluntary abstinence from food. It’s much more about when you eat rather than what you eat.

The short version

• There are many benefits touted for IF; some have more evidence behind them than others.
• There are many different regimes for IF - leading to flexibility for which regime to follow.
• IF research is still developing, so it’s advisable to remain open-minded in respect of health claims made regarding IF at this stage. That being said, potential benefits with relatively strong evidence include a reduction in fasting insulin.
• It's not for everyone. You should always check with a medical professional before starting IF or any weight management plans.

What intermittent fasting actually is

Intermittent fasting is an eating pattern that alternates between periods of eating and periods of fasting. The focus isn't on what you eat — it's on when.

The main approaches:

• During eating periods, there are no restrictions on what can be eaten.
• Depending on who you talk to, what is allowed during fasting periods varies. Generally, water and non-caloric beverages such as coffee and sugar-free soft drinks are allowed.
• IF is different from prolonged fasting, which is loosely defined as the abstinence of food for over 48 hours.
• During periods of eating, a balanced, nutritious diet should still be consumed and will make the fasting days a little easier. A diet high in fibre, fruits and vegetables, and protein will help with satiety.

Types of Intermittent fasting:

• Time restricted feeding: eating takes place during a restricted window of time. For instance, the 16:8 diet involves fasting for 16 hours per day and eating within an eight-hour window
• Alternate day fasting: a day of eating followed by a day of fasting.
• Modified fasting regimes: on “fasting” days some food can be eaten, typically 20-25% of daily calorie needs. The most popular of these regimes is the 5:2 diet, which requires, for two days a week, a calorie intake of 500kcal for women and 600kcal for men.
• Religious fasting: the most well-known example is Ramadan, which requires Muslims to not eat or drink anything during daylight hours for 29/30 days. This is often very different from fasting undertaken through dietary choice where eating during daylight hours, in line with circadian rhythms, is more common.

How intermittent fasting works

One of the main theories behind IF focuses on circadian biology. The human body has an internal clock that operates on an approximate 24-hour cycle (circadian rhythm) and influences a number of processes, such as sleep. Read more on circadian rhythms here.

Feeding and fasting can affect the body’s circadian clock. The thought is IF is more aligned with this clock than modern eating patterns and so leads to metabolic benefits. Eating the majority of calories earlier in the day rather than late at night may also exert health benefits in much the same way.

Fasting periods can result in the body using up all its glycogen stores. This leads to the body switching from glucose as an energy source to ketones. Ketones are provided by the oxidation of fat, which could help to preserve muscle mass, amongst other roles, hence the theory behind IF providing benefits beyond a reduction in calorie intake.

What the research suggests

Weight loss and body composition

One of the main reasons people undertake IF is for the purpose of weight loss. A smaller eating period can naturally result in fewer calories consumed over the week. IF could reduce the body’s compensatory mechanisms that cause a decrease in energy expenditure with weight loss through the inclusion of non-fasting days, i.e. days where calories are not restricted. However, this is currently still a theory without sufficient practical data to support it.

Health markers

A reduction in C-reactive protein (a blood marker of inflammation), total cholesterol and triglycerides have been reported across several studies. The issue is that these positive results are hard to separate from the effects of weight loss, which causes similar changes and occurs in a large proportion of the studies on IF.

Ageing

Autophagy is linked to life extension and ageing in a similar manner through clearing up damaged components in the body, thereby keeping cells healthy. IF, and calorie restriction in general, can increase the rate of autophagy and reduces oxidative stress.

Animal models provide evidence that calorie restriction and IF could delay the ageing process. The mechanisms for calorie restriction and IF in ageing appear to share several similarities.

Moving the conversation back on to humans, the outcomes are unclear, and due to the restrictions required to study over whole lifetimes, we may never fully know.

Who should (and who should not) try intermittent fasting?

IF is not suitable for type 1 and 2 diabetics due to an impaired insulin response and hypoglycaemia. IF can also be a risk factor for relapse in those with a history of eating disorders. Additionally, pregnant and breastfeeding women and people with a diagnosed chronic disease should not attempt IF.

During fasting days, particularly when starting out, some symptoms may arise such as headaches, dizziness and a lack of energy and concentration. These symptoms can be reduced by staying properly hydrated.

Edited by The Digest team