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. During eating periods there are no restrictions on what can be eaten contrary to other popular diets. 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. There are also diets that allow food consumption during “fasting” days albeit at a much lower calorie intake than non-fasting days.
IF is different from prolonged fasting, which is loosely defined as the abstinence of food for over 48 hours and which comes with its own risks and possible benefits. There may be no pattern to prolonged fasting, unlike IF.
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, e.g. 12pm-8pm.
A day of eating followed by a day of fasting.
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.
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.
Non-fasting days can be valued a bit too much like a treat. 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. IF is not a free pass to eat whatever you want without gaining weight. Calories in vs calories out still applies, which you can find out more about here.
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.
What IF does well is simplicity. The rules are easy to follow, which can help some stick to this eating pattern.
There are many ideas around why IF might be beneficial. One of the most prominent 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. Circadian rhythms are affected by the environment, notably light and darkness. When there are environmental disturbances, such as working a night shift, this has a negative effect on the body’s internal clock and metabolism, consequently impacting health.
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.
From an evolutionary perspective, this is quite logical. Our ancestors did not have constant access to food and likely went through periods with no food, i.e. fasting. On top of this, food was either gathered or hunted during the day and eaten relatively soon after its procurement suggesting an eating pattern during daylight and fasting at night.
Fasting may be an opportunity for the body to “reset” and repair. Evolutionary proponents suggest this allowed our ancestors to prepare and be ready again to search for food.
Moreover, 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.
There are many benefits touted for IF; some have more evidence behind them than others. As IF is still a relatively new research area, many of the effects have only been seen in cells or in animals and are yet to be replicated in humans, which is important to remember.
Potential benefits with relatively strong evidence include a reduction in C-reactive protein (a blood marker of inflammation), total cholesterol and triglycerides[11, 12]. 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.
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.
Increased fat oxidation is often put forward as a strong benefit of IF. Contrary evidence has shown that participants who skipped breakfast had the same oxidation of carbohydrates and fats over 24 hours as those who didn’t.
A similar lack of difference has been shown in overall weight loss between IF and controls, which is usually a continuous calorie restriction diet. This has been replicated in several randomised control trials with different IF regimes up to 2 years in duration[16-18]. What’s more is adherence to an IF diet is comparable to other diets[19, 20].
An interesting benefit of IF could be the maintenance of free-fat mass (FFM), in other words muscle, during weight loss resulting in favourable changes in body composition[21, 22].
IF has been touted as a diet for increasing life expectancy, but should this be the case? This belief centres around the concept of autophagy. Autophagy is the breakdown of the components within cells during metabolic distress to provide a nutrient source to those cells. Examples, where autophagy can be increased, include exercise and a lack of available nutrients, e.g. during starvation.
In relation to life extension, the general theory is that fasting induces autophagy which helps to clear up damaged components in the body, thereby keeping cells healthy. IF, and calorie restriction in general, increases 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[25, 26].
Calorie restriction studies conducted in rhesus monkeys offer some insight. They illustrated a lower incidence of ageing-related deaths and a stalled onset of age-related diseases and so may suggest delays in the onset of ageing. Improvements in metabolic profile and a possible reduction in oxidative stress have also been highlighted. Nonetheless, there was no increase in life pointing towards the fact calorie restriction effects in long-lived animals are complex and likely dependent on a variety of environmental, nutritional and genetic factors.
Moving the conversation on to humans, the outcomes are unclear, and due to the restrictions required to study over whole lifetimes, we may never fully know.
Overall, it’s important to determine what works for the individual. IF has straightforward guidelines; however, this doesn’t necessarily make it any easier to follow than other diet regimes[19, 20]. IF is not a way to be healthy while eating a poor-quality diet, but it can be a method that leads to greater control over dietary choices.
There are many different IF regimes, allowing flexibility in which regime to follow. On the other hand, this complicates research comparisons and real-life application. IF research is still in its infancy so it’s advisable to remain open-minded in respect of health claims made regarding IF at this stage.
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