Fasting has been practised throughout history for both religious and practical reasons. If we look back at our ancestors, hunter-gatherers only had access to food when they killed or collected it. Even during times of war, food was scarce and fasting was a part of everyday life. Our ability to store body fat for long periods of time has helped the survival of our species over the years. Interestingly, our bodies have evolved with mechanisms that make it difficult for us to lose weight, as a protective measure for surviving periods of famine. Unexpected food shortages over decades have evolved humans to take advantage of food when it is present – which is why it is common to feel hungry every time we see food, even though our body may not need it.
What is “intermittent fasting”?
Intermittent fasting is an eating pattern where you cycle between periods of eating and fasting. Relative to other diets, it is not restrictive on what you eat but rather when you eat. Some people use the 16:8 method, where you can eat for a duration of 8 hours in a day and fast for the remaining 16 hours. Others prefer the 5:2 method, where you can eat for 5 days and fast the remaining 2 days of the week. During the fasting period, it is okay to take limited-caloric beverages like water, tea, coffee (with no added sugars of course!) or even vitamins/supplements.
What’s the science behind it?
Short-term fasting activates a process called autophagy, in which your cells use old and dysfunctional proteins for energy. Autophagy has shown to have protective effects against Alzheimers, cancer and ageing. Moreover, dietary fasting makes our body go into ketosis, by increasing the production of ketone bodies. Ketone bodies are made from fatty acids by the liver and can be used as a source of energy instead of glucose, when our body’s glucose levels are low. Interestingly, ketogenic diets are used as method to prevent seizures in adults and children with intractable epilepsy. Whilst the neuroprotective effects of ketone bodies cannot be refuted, the exact science behind it is not yet fully understood.
What is the scientific evidence?
There is an immense number of animals studies dating from 1935 to present, which have shown similar results replicated in independent labs across the world. In rodents, intermittent fasting has repeatedly been shown to have neuroprotective effects, reduce insulin levels (as a consequence of overall lower blood glucose levels), and reduce inflammatory markers like TNFalpha (TNFa). TNFa is considered the “leader of the pack” in inflammation, as it triggers the cascade of other inflammatory molecules to be produced. Inflammatory markers are involved in most age-related diseases and in insulin resistance. Therefore, reducing levels of TNFa has several beneficial effects. Despite the vast amount of evidence in rodents, the evidence from humans is mostly anecdotal. Surprisingly, the human studies we found were very limited which may, in part, be due to lack of funding. Food and drug companies would not reap any profits from confirming the health benefits of intermittent fasting. After all, it is a practice that does not promote consumerism, it does quite the opposite. Nonetheless, we found one clinical study (60 human participants) looking at the effects of alternate-day fasting in healthy people, which demonstrated a number of health benefits, similar to those observed in rodents. The participants alternated between 36 hours of zero-calorie intake with 12 hours of unlimited eating throughout the week.
Risks and threats associated with intermittent fasting
Fasting diets have been heavily criticised for encouraging eating disorders, such as anorexia and bulimia. Another potential pitfall is the risk of people overindulging in unhealthy dietary habits after a period of fasting, which may counteract the potential positive effects of intermittent fasting. We found several internet articles suggesting that fasting can lead to difficulty sleeping, hair loss, and increased levels of stress. However, according to our research, there are no scientific studies backing these claims.
A final note
At no point in history has food been so easy to obtain as it is now. In many ways, this is a glorious phenomenon. However, obesity in first-world countries has become just as big of a problem, affecting 107.7 million children and 603.7 million adults, as hunger in under-developed countries, affecting 820 million people globally (WHO, 2018). Our biological mechanisms have not been able to adapt to our new environment in which food is constantly available. There is a large body of evidence from animal studies highlighting the beneficial effects of intermittent fasting. Human studies are limited, but show positive results. Larger scale and long-term human studies are required to confidently confirm that intermittent fasting is not just another diet fad and that there are no significant adverse effects associated with it in the long run.