We are living in an era where the Western World is excessively struggling with overweight and obesity. There is an abundance of diets around these days, from Keto to Paleo, from FODMAP to Intermittent Fasting. Most of these diets will allow quick weight loss in the beginning. However, more than often, these health trends lead to the so-called “yo-yo effect” where the kilos come flying back as soon as the diet is stopped [1, 2]. Multiple “yo-yo” diet cycles have proven to significantly increase fat deposition, even more than a consistent high-fat diet [3].

Today, we will dive into the world of intermittent fasting. What are the health benefits and, most importantly, what are the side effects?

There are various approaches to intermittent fasting, all of which aim to have periodic breaks from eating. The most popular method of intermittent fasting allows an eating window of 8h a day followed by a fasting period of 16h, which is known as time-restricted feeding (TRF) [1]. However, some fasting strategies also consist of having only a couple “fast days” a week, with little to no energy consumption, accompanied by normal “non-restricted” food intake for the rest of the week, such as alternate-day fasting (ADF), periodic prolonged fasting (PF), or intermittent calorie restriction (ICR) [2].

The theory behind fasting is that during the “fasting state”, the body burns down fats as an energy source, thereby shrinking fat mass. It has been studied scientifically that temporary nutritional deprivation is associated with health benefits such as improved glucose metabolism, reduced lipid accumulation, and a risk reduction for cardiovascular diseases [ 4,5].

However, the successes of these scientific studies have been slightly exaggerated by the press. Most of the aforementioned research results come from animal studies, and the evidence on human health is only preliminary [6]. There have been reports of decreases in triglycerides (fats) and LDL (bad cholesterol) levels among participants of an intermittent fasting study, while other studies show no effect [2, 7]. A suitable explanation for this is that study participants that start the diet while being overweight are also the ones that lose the most amount of weight, and subsequently triglycerides and LDL. However, for participants that were not extremely overweight, the results are not so striking. Another issue is that most human research is performed on small, selected groups which do not represent the entire population properly. So far, it is unclear if healthy people will also achieve the same positive effects [6]. 

Typical side effects encountered at the start of an intermittent fasting diet include fatigue, mood swings, and increased feelings of hunger [8]. Furthermore, research has revealed that prolonged fasting dysregulates the circadian rhythm. This, in turn, affects the biological clock that influences the release of hormones such as insulin and the stress hormone cortisol [9]. The circadian rhythm controls many aspects of the sleep-wake cycle, such as blood pressure, heart rate, body temperature, and hormonal secretion. For example, insulin levels are partly regulated by the circadian rhythm to increase hormone levels early in the morning, preparing the body for the intake of breakfast. Mice have an opposite sleep-wake cycle, making research on rodents challenging to translate to humans [9]. Nevertheless, intermittent fasting in mice resulted in a decreased level of insulin throughout the day. This proved to be true even when the fasted mice ate twice as much during the eating period compared to ad libitum fed mice, resulting in a similar caloric intake [4, 10]. 

Lastly, while weight loss is known to positively reduce cortisol levels, extensive fasting has been shown to increase the level and frequency of cortisol secretion, which induces an acute stress reaction in the body [11]. Whereas insulin’s function is to remove glucose from blood for storage, cortisol releases glucose into blood for immediate energy usage. Short-term cortisol is healthy for the body as it has anti-inflammatory properties. However, chronically high cortisol levels lead to increased levels of glucose, which increases the risk of developing diabetes and obesity, but also weakens the immune response and increases the risk of cardiovascular diseases [10, 12].

To conclude, even though enough research implies that intermittent fasting is safe for most people (a physician should always be consulted before starting), there can still be negative side effects. Unfortunately, the adverse effects have not been studied thoroughly yet, mostly because the majority of research on intermittent fasting has been done on rodents.