What happens to your body during fasting for weight loss?

Fasting boasts numerous health benefits including weight loss, improved mental focus and cellular rejuvenation. But how does fasting actually work and what does it do to your body? Fasting promotes health and longevity through several key metabolic pathways.

Firstly, fasting causes the body to switch from using glucose for energy to metabolising the body’s fat reserves through ketosis (fat burning).

It also induces autophagy, during which cells reuse old and damaged organelles to renew themselves. Fasting can be supported by programmes such as ProLon, a fast mimicking diet build around is a five-day programme that allows you to fast with food.

What is fasting?

Fasting is defined as the voluntary abstinence or strong limitation of calorie ingestion over a limited period of time [1].

It is an ancient practice that has been leveraged across history and cultures for weight loss, to protect against disease and improve mental clarity.

Fasting promotes weight loss by forcing the body to metabolise its own fat reserves, rather than rely on readily available blood glucose from food consumed throughout the day for energy.

Types of modern-day fasting that have become popularised for weight loss include intermittent (an alternating cycle of fasting and feasting, usually lasting 12-48 hours), prolonged (fasting for longer than two days) and even the fasting mimicking diet (a five-day diet of small meals that avoid triggering cellular nutrient sensors).

Whatever the method, fasting has been shown to aid weight loss, protect against disease and may even boost longevity [2].

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How long should fasts last?

Depending on the type, fasts can last anywhere between 12 hours to weeks. However, beginners should avoid extreme prolonged fasting as it can cause negative side effects including headaches, dizziness and fatigue in the uninitiated.

A safer, alternative type of prolonged fast is the fasting mimicking diet, a five-day fast that provides the body with nutrients whilst maintaining its fasting state. Intermittent fasting is an even easier option which follows the body’s natural circadian rhythm that controls metabolism.

For example, restricting food consumption in the morning and evening can achieve an easy overnight fast of 12-13 hours, during which the body uses up the nutrients consumed throughout the day.

In fact, some people follow this type of fast without even realising, hence the etymology of ‘breakfast’ — to break one’s fast with the first meal of the day. The 16:8 method, fasting for 16 hours and feasting for 8 hours, is more intensive but more effective as no new nutrients are consumed so the body uses up its own glucose and fat reserves overnight.

What happens to your body during fasting?

The human body has adapted to survive prolonged periods of food deprivation and is therefore highly suited to fasting for weight loss.

During the hunter-gatherer days, humans adapted to natural cycles of food scarcity over millions of years. Now, the availability of high-fat, low-nutrient foods combined with increasingly sedentary lifestyles has caused a global obesity epidemic.

Fasting is a safe, sustainable way to combat obesity as it reduces calorie intake as well as triggering ketosis, the process of fat burning.

During fasting, the restriction of calorie intake prevents the body’s nutrient-sensing pathways from being activated, forcing the body into a fasting state that triggers its dietary stress response and disrupts key metabolic pathways.

Fasting for fat burning

The main mechanism by which the body loses weight during fasting is ketosis. Ketosis promotes weight loss by metabolising the body’s fat reserves without using its protein supply from muscle.

During fasting, the body undergoes a ‘metabolic switch’ from using readily available glucose for energy to ketones produced through ketogenesis (known as the G-to-K switch). This is controlled by the inhibition of the mTOR signalling pathway [1].

12 to 16 hours into a fast, the body’s glucose and insulin levels reduce. Since glucose is the body’s primary source of energy, the body must either rapidly produce more glucose through gluconeogenesis or rely on its fat stores.

In the early stages of fasting, the body may start to convert amino acids and protein from muscle into glucose as it prepares for ketosis.

However, since gluconeogenesis is a costly process that uses up precious calories that the fasting body does not have, the body will instead begin ketosis and produce ketones that are oxidised by the brain as an energy source.

Once ketones become the primary source of energy, the body reaches the metabolic state of ketosis [3].

The onset of this metabolic switch depends on the body’s base glycogen levels at the start of the fast as well as the composition of the last meal, energy expenditure and physical activity. Ketosis continues with fasting, reaching a plateau after 4 days of prolonged fasting and decreasing when food is reintroduced to the diet.

Fasting for autophagy

After around 24 hours of fasting, the process of autophagy begins, triggered by signals that sense nutrient deprivation. Autophagy is a catabolic process by which cells reuse their damaged organelles to rejuvenate in response to nutrient stress and is associated with protection against diseases including diabetes, cardiovascular disease and cancer.

Autophagy is always active in most types of cells where it plays a housekeeping role in maintaining intracellular organelles and proteins. However, autophagy can also be induced by starvation as it is a key process in cells’ adaptive response to nutrient deprivation.

Autophagy is controlled by TOR kinase that detects when nutrients are available and promotes growth. TOR kinase is inhibited by signals that sense nutrient deprivation, therefore increasing autophagy. As food is reintroduced to the diet, autophagy and ketosis wind down as nutrient-sensing pathways are reactivated and the metabolic switch is reversed [4].

What does fasting feel like?

Physically, you are likely to experience food cravings during the first 24-hours of fasting as hunger hormones like ghrelin are released. The stress hormone cortisol also starts to increase due to nutrient stress from reduced calorie intake.

As cortisol levels increase, grehlin levels decrease, reducing the sensation of hunger as the fast goes on. You may experience improved alertness and mental clarity from this hormonal shift.

In fact, some fasters experience euphoria or extreme mental clarity during prolonged fasts, which may be related to the release of brain-derived neurotrophic factor (BDNF).

The presence of ketones increases BDNF levels, which improves memory and mood. Coldness, especially in the extremities, is another, negative side effect of fasting, as it reduces heart rate and body temperature by activating the parasympathetic nervous system.

Dizziness or light-headedness may be signs of mild dehydration and lack of salt, as the body stops absorbing water from food during fasting [3].

Most of the health and weight loss benefits of fasting are experienced during the first 1-3 days. Cell growth occurs post-fast, which offers an opportunity for the body to undergo cellular and tissue regeneration [1].