Intermittent fasting: the ultimate guide to weight loss

Fasting is a method of restrictive eating used to effectively burn fat and lose weight, as well as benefitting health and lifespan. Prolonged fasting can be a challenging commitment as it requires low caloric intake over long periods of time.

Alternatively, intermittent fasting (IF) follows regular cycles of fasting and feasting and does not restrict your diet when you can eat, making it easier to incorporate into your lifestyle long term.

There are various types of intermittent fasting that all work to metabolise fat reserves through the process of ketosis – but which is right for you?

What is intermittent fasting?

Intermittent fasting involves alternating between periods of fasting and feasting, ignoring the three meals a day tradition common to western diets.

Notably, it is not a diet as it does not restrict which types of food you eat or your calorie intake during feasting hours, making it easy to adapt into everyday life and to continue long term.

This is in contrast to prolonged fasts of 48 hours and over which, although providing cellular health and longevity benefits less achievable with shorter fasts, require major commitment and involve greater risk of dehydration, electrolyte imbalance and fatigue than other methods [1].

There are several types of fasts within intermittent fasting including the 5:2, which involves restricting calorie intake for two days out of the week (typically consuming less than 500kcals per day) and eating unrestrictedly for the rest.

This is a variation of alternate day fasting, which, as the name suggests, consists of alternating days of fasting and feasting. The eat-stop-eat method involves restricting calories for 24 hours at least once a week, starting the fast after dinner on day one and resuming eating with dinner on day two, achieving a 24 hour fast.

Another variation is time-restricted eating (TRE) that uses a daily cycle of feasting and fasting. TRE is therefore easy to incorporate into a busy lifestyle as it does not require caloric restriction for more than 24 hours.

TRE methods vary in length and difficulty; the 12:12 for beginners balances 12 hours of fasting with 12 hours of feasting that most people can achieve by simply skipping breakfast. Increasing in difficult is the 16:8 that has a shorter 8-hour feeding window and is the most popular method for weight loss as it is effective in burning fat, increasing muscle mass and improving performance.

The most intense of the TREs is the 20:4, which is not recommended to first-time fasters as it requires eating all your food for the day within a short 4-hour period, normally in one large meal [2]. Considering that most people fast overnight without realising it, most TRE methods are easy to achieve by simply extending this natural fast by skipping breakfast in the morning and dessert or snacks at night.

How does it work?

Intermittent fasting effects your body through several key pathways. Firstly, eating only during certain hours limits your overall calorie intake without restricting what foods you can eat during feast hours, helping weight loss.

Secondly, caloric restriction metabolises fat and promotes weight loss through ketosis, a metabolic process during which the body switches from using glucose for energy to fat and ketones – known as the G-to-K switch [3].

This begins 12-16 hours into a fast, meaning that you can achieve the fat burning benefits of fasting with even the shortest of intermittent fasting methods.

Ketosis onset also depends on pre-fast liver glycogen content, last meal composition and physical activity. Glucose and insulin levels in the bloodstream drop, while nutrient signalling pathways that are regulated by the mTOR kinase protein are deactivated.

This forces the body into the fasting state as glucose supplies are no longer available to convert into energy and the body instead metabolises its fat reserves, releasing free fatty acids (FFAs) and ketones.

FFAs are oxidised for energy by most tissues while ketones supply energy to the brain during ketosis, which subsides as food is reintroduced into the diet during feasting. Using ketosis to generate energy has the advantage of burning fat while retaining muscle mass [3].

Read more about what happens to your body during caloric restriction here.

Weight loss and health benefits

Intermittent fasting can lead to weight loss, improved metabolic function, reduced inflammation and oxidative damage to tissue, as well as enhanced immune system functioning, of value to both those seeking to lose weight as well as those already at a healthy weight [3].

The health benefits of weight loss are well known: it can reduce the risk of high blood pressure and high blood cholesterol as well as diseases like heart disease, stroke, type 2 diabetes and some cancers [4].

Since intermittent fasting is practised as a daily eating habit it can also help to retrain your relationship with food, and may have added benefits such as reduced feelings of hunger, a re-calibrated conditioned response to eating and improved well-being [2].

Caloric restriction for longer periods has additional benefits from autophagy, the cellular process by which cells renew themselves reusing their damaged organelles. Autophagy is usually a protective mechanism that occurs in tissues all the time, however it can be stimulated by stress in response to caloric restriction.

Cellular rejuvenation is thought to protect against metabolic diseases, neurodegenerative diseases, infectious diseases and cancers, as well as having anti-aging effects – however these benefits are best obtained by longer intermittent fasts of more than 24-hours [5].

Side effects of intermittent fasting

While intermittent fasting is a safe and sustainable weight loss method, new users may experience minor side effects such as headaches from electrolyte imbalances and hunger pangs. To minimise this, you can take a multi-mineral tablet or add a little salt to drinking water [2].

Hunger is unfortunately an unavoidable fact of fasting at least initially, however continued adherence to intermittent fasting is associated with less overall hunger [6].

Another potential pitfall is compensatory behaviour, in which physical activity is reduced and energy intake is increased before, during and after a period of fasting to compensate, possibly impairing the weight loss benefit of the fast [7].

To reduce this, avoid overeating during feast hours and continue to do light aerobic exercise, especially before or after a large meal during feasting hours to avoid a spike in postprandial blood glucose.

Overall however, intermittent fasting is an effective way to lose weight and experience the additional benefits of fasting to health and lifespan long term, without enduring an arduous prolonged fast.