Updated: Aug 30, 2020
"Intermittent fasting" (IF) was the most googled diet of 2019 in the United States (1). Its promoted as a method to boost weight loss, optimise health and to keep you looking young forever. But is this type of eating (or rather not eating) all its cracked up to be?
What is Intermittent Fasting?
Intermittent Fasting as the name suggests involves a fast. Its different to most diets out there today as the focus is on when you eat rather than what you eat.
In most intermittent fasting diet regimes, you have a small time window when you are allowed to eat (usually fewer than 8 hours), and you fast for the remainder of the day (16 hours). This is called time restricted feeding.
There is also alternate day fasting. This involves fasting on a few days per week e.g. the 5:2 diet where you consume less than 500-600 calories 2 days per week and consume your usual intake on the remaining 5 days of the week.
Will IF help me lose weight?
Because IF usually leads to an overall reduction in calorie intake, it can help some people to lose weight, therefore can be a viable strategy.
However in that respect a traditional diet (e.g. caloric restriction eat smaller meals, more fruit and veg etc.) approach offers similar benefits. Multiple studies have found that IF leads to the same amount of weight loss as a traditional diet (2,3).
What matters most is consistency. People who find eating less often easier than eating less could benefit from trying IF. As for unique metabolic health benefits of fasting the debate is still ongoing.
Mechanism & Claims
The concept is, when we intermittently fast our body slows down into repair mode. This allows for beneficial changes in our body at a cellular level e.g. an increase in insulin sensitivity and improvement in cellular repair. The idea is that this can help people lose weight, reduce the progression towards age-related disease and even improve longevity.
There is a lot of ongoing research into IF however much of the research is in its early stages. Many of the studies so far have been too short to show significance or carried out in animals.
Other Considerations & Risks
IF is not recommended if you’re pregnant/lactating, underweight, younger than 18, or have a history of eating disorders/disordered eating.
Eating is a health-promoting activity, denying yourself to eat within certain time frames despite feelings of hunger is ignoring your bodies internal cues.
Some of us may experience digestion problems when we eat large amounts of food in a short amount of time. Larger volumes of food translates to more time needed to digest. This can cause additional stress on your GI tract, leading to indigestion and bloating. This can have huge implications for those with IBS.
Restriction can lead to binging. Some people may take the eating periods as an opportunity to eat more calories than they really need. When you anticipate a period of fasting coming up, it can be very tempting to over-eat. This can lead to binge-eating followed by feelings of guilt which can manifest into a dangerous cycle.
Eating is also very much a social activity- fasting for certain periods can interfere with this.
Depending on the length of your fast, you may experience stress, headaches, constipation, or dehydration. Staying hydrated is particularly important; it’ll also help mitigate any headaches or constipation.
Limiting periods of eating may lead to nutrient deficiencies if poorly planned.
As always seek advice from a registered dietitian and your GP if you are planning on starting any form of a restrictive diet.
(1) https://trends.google.com/trends/yis/2019/US/ Accessed Aug 2020
(2) Headland, Michelle & Clifton, Peter & Carter, Sharayah & Keogh, Jennifer. (2016). Weight-Loss Outcomes: A Systematic Review and Meta-Analysis of Intermittent Energy Restriction Trials Lasting a Minimum of 6 Months. Nutrients. 8. 354. 10.3390/nu8060354.
(3) Davis et al. (2015) “Intermittent energy restriction and weight loss: a systematic review” (http://www.nature.com/ejcn/journal/v70/n3/full/ejcn2015195a.html)