Intermittent fasting has become popular in recent years, with fans claiming it as a life-hack to help you lose weight, and reduce your risk of developing diseases such as heart disease and cancer.
These are impressive claims, but as we know, we can’t always trust health claims we see in the media. We decided to have a look at the evidence.
What is intermittent fasting?
Fasting is not a new concept, and has been incorporated into religious celebrations like Ramadan or Easter for thousands of years, however intermittent fasting generally allows some food intake, whereas religious fasting generally involves complete abstinence from food intake. The most popular type of intermittent fasting is known as alternate day fasting, as seen in the much-publicized 5:2 diet. It generally involves fasting for two days per week, and eating regularly for the remaining 5 days. The fast days normally allow 500 calories per day for women, and 600 calories per day for men. Others promote time-restricted feeding, where food is limited to a certain window of time- for example, eating from 12pm to 8pm, and fasting for the other 16 hours of the day.
Animal studies suggest that fasting may help to increase length of life while simultaneously reducing the risk of diseases like diabetes, cardiovascular disease and cancer. Studies in mice have also indicated fasting may improve cognitive function and improve the immune system. While these results are exciting, results in animal studies often fail to translate when the same research is carried out in humans.
Studies in humans are more limited, and have had mixed results. One human study published earlier this year found that fasting for five days per month reduced blood pressure, inflammation markers, waistlines and total body fat. This improvement in biomarkers may have been due to weight loss rather than fasting specifically, as losing excess weight generally improves blood biomarkers in the short term.
Another study published this week found alternate-day fasting did not produce better weight loss, weight maintenance, or improvement in blood biomarkers compared with daily calorie restriction. This echoed a large systematic review published in 2016, which found that fasting produced the same weight loss as daily calorie restriction.
This new study also found that people in the fasting group had a harder time sticking to the diet, and more people in that group dropped out of the study.
Fasting during cancer treatment
There is also emerging research in recent years that fasting before or after chemotherapy many help to reduce side effects. The theory behind the potential benefits of fasting is that it may sensitize cancer cells to treatment while simultaneously protecting healthy cells from treatment-related damage.
There have been some small studies where patients have self-reported that reduced treatment side effects from fasting, however larger studies would be required before any actual benefit could be proven.
Fasting is not recommended for anyone with a history of disordered eating, as it may trigger a binge/purge mentality.
It is also not be suitable for anyone going through cancer treatment who is losing weight, as fasting would likely contribute to this, and malnutrition is known to worsen treatment outcomes. Those who are at a healthy weight and who wish to trial fasting should speak to their oncologist prior to making a change. Some oral chemotherapy drugs must be taken with food, so to not do so may cause harm.
People with diabetes or any other chronic medical condition should also speak to their doctor prior to fasting to avoid unnecessary risks.
Intermittent fasting appears to be a safe way for healthy individuals to lose weight, and may be a good option to try for those who struggle with limiting their intake on a daily basis. It may help improve health biomarkers, although further studies are needed, as to date there are very few published studies in humans linking fasting to clinical outcomes like cancer, heart disease or diabetes.
Fasting may be beneficial to reduce chemotherapy side effects in those not suffering from weight loss, but may contribute to malnutrition. Anyone who wishes to try intermittent fasting should speak with their physician prior to undertaking any change.