Fasting, the intentional restriction of calorie intake over a period of time, has numerous health benefits including weight loss, protection against disease and improved longevity [1]. It comes in many forms, the most popular for weight loss being prolonged (lasting longer than two days) and intermittent (alternating between cycles of fasting and feasting). Both methods have different advantages and disadvantages depending on your lifestyle –but which is better for weight loss?

How does fasting work?

Caloric restriction promotes weight loss through the process of ketosis, during which the body metabolises fat for energy. Around 12-16 hours into a fast, blood glucose and insulin levels drop and the nutrient signalling pathways that are regulated by the mTOR kinase protein are deactivated [1].

This forces the body into the fasting state and it starts to switch from using glucose from food for energy to metabolising fat stores, producing ketones that are oxidised by the brain. As food is reintroduced, nutrient signalling pathways are reactivated and ketosis plateaus. This process can promote weight loss as well as improve metabolic function, reduce inflammation and enhance immune functioning. Weight loss has numerous benefits to health and longevity, reducing the risk of morbidity from heart disease, stroke, type 2 diabetes and some cancers [2].

Ketosis has the additional advantage of burning fat supplies without using protein, making it possible to continue weight training for maintaining and growing muscle during caloric restriction [3].

What is prolonged fasting?

The human body has adapted to withstand long periods of food deprivation, with the longest fast ever recorded lasting 382 days [4].

Prolonged fasting is an extreme form of caloric restriction that lasts for two days or more. Usually, they last anywhere between 2-5 days to a maximum of two weeks and are only broken by the consumption of water and plain tea or coffee. For those new to fasting, it is recommended that you practise intermittent fasts of increasing lengths before trying prolonged, as your body becomes adjusted to ketosis [5].

For example, start out with the 12:12 intermittent fast before moving on to a 24-hour fast and finally a two-day prolonged fast.

Benefits to health and longevity

Prolonged fasting can promote weight loss, reduce abdominal fat and improve blood pressure levels, even in those who are already a healthy weight [6].

It is not a quick weight-loss fix and should be repeated regularly, for example once a month, in order to see the physical results of long term weight loss. Prolonged has an advantage over short intermittent fasts in that it induces autophagy, the process of cellular rejuvenation. Autophagy is an ongoing cellular process in which cells reuse their damaged organelles to renew [7].

It is also used during nutrient stress to balance available energy sources for survival, and thus can be triggered by caloric restriction. Around 24 hours into a prolonged fast, nutrient-sensing pathways repress the TOR kinase protein in response to nutrient deprivation, inducing autophagy.

The cell renewal from autophagy is thought to protect against age-related disease like neurodegeneration, cardiomyopathy, diabetes and cancer, as well as increase longevity. Autophagy reaches its highest level around day two of a prolonged fast, so is more difficult to achieve using shorter intermittent fast methods.

Potential side effects

An obvious drawback to prolonged fasting is the constant hunger that accompanies sustained calorie restriction. Considering that most prolonged fasts only allow the consumption of water, tea and coffee, it is a method that requires serious commitment and motivation. However, our bodies can adapt to hunger and one study found that 93% of subjects reported an absence of hunger [6].

While prolonged fasting is generally safe, it is one of the riskier methods of caloric restriction that can be accompanied by mild side effects including dehydration, electrolyte imbalance and fatigue. Users also may experience irritability and low-energy as the body transitions into ketosis. This can be countered by taking sensible measures such as increasing your water intake, adding ½-1 teaspoon of sea salt to drinking water to prevent electrolyte depletion, and taking regular rest [5].

An alternative type of prolonged fast is the fasting mimicking diet, a five-day fast that provides nutrients with a specially-designed meal plan without triggering the body’s nutrient-sensing pathways and maintaining the fasting state. This is an easier option for those who want to try prolonged fasting while avoiding the dreaded hunger that comes with conventional fasts. Read Longevity.Technology’s product review of ProLon from L-Nutra, if you need any more convincing.

What is intermittent fasting?

This is a popular type of fast that requires less commitment than prolonged fasting. It involves alternating between periods of eating and fasting either within one day or over several days and therefore can be practised potentially indefinitely. Crucially, it is not a conventional diet as it does not restrict what you eat during feasting periods. There are many types of intermittent fast, including alternate day fasting, the 5:2 and the one meal a day fast (OMAD).

Additionally, time restricted eating (TRE) alternates between periods of fasting and eating within one day, for example the 12:12, the 16:8 and the more intensive 20:4 [8]. Everyone practises TRE to some extent during sleep, therefore extending this natural fast is an easy way to incorporate TRE into any lifestyle.

Benefits to health

Since ketosis begins around 12-16 hours into a fast, you can achieve the same fat burning benefits of prolonged fasting with even the shortest of intermittent fasts [1].

Intermittent fasting has an advantage over prolonged in that it avoids severe feelings of hunger and can be practised daily. This makes the fast easier to commit to and lessens its impact on other areas of life such as socialising. Indeed, practising it as a daily habit can help reset your relationship with food, with users reporting reduced feelings of hunger, a re-calibrated response to eating and better well-being [8].

However, since autophagy begins at least 16 hours into a fast depending on the individual, it becomes more difficult to achieve its cellular renewal benefits with short intermittent fasts compared with prolonged fasts.

Potential side effects

Intermittent fasting comes with minimal risk when compared with prolonged, with mild side effects including headaches and fatigue. However, some users may unconsciously reduce their physical activity and increase energy intake before, during and after a period of fasting to compensate for the caloric restriction, reducing the benefits of the fast [9].

A way to stop this is by avoiding overeating and partaking in light exercise – which is not recommended during prolonged fasts. Both intermittent and prolonged fasting are effective weight loss methods, and while prolonged has the added benefit of autophagy, its intensive calorie restriction makes intermittent fasting an easier option for long term weight loss.

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7877980/
[2] https://rb.gy/u5ktmx 
[3] https://pubmed.ncbi.nlm.nih.gov/27550719/
[4] https://rb.gy/ez4ikd
[5] https://fasting.com/fasting-methods/prolonged-fasting/
[6] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6314618/
[7] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2990190/
[8] https://fasting.com/fasting-methods/intermittent-fasting/
[9] https://rb.gy/dmbkos