Updated: Apr 16
You’ve probably heard about intermittent fasting; either on Instagram or on the cover of a magazine. But what is it, and is it worth trying?
What is it?
Intermittent fasting is a broad term that covers various types of fasting. The foundation of intermittent fasting is creating a smaller time frame in which you can eat. Whether that be alternative day fast (think about the 5:2 diet where you fast for 2 days of the week) or restricting the hours in a day you can eat (12-21 hours fasted). Typically, the hours or days that you do not fast, individuals eat a restricted number of calories or intuitively.
The idea behind this diet is that there is less time in the day to eat leading to a reduction in total calorie intake. Some researchers have also proposed that fasting reduces the levels of a hormone called insulin, which is responsible for letting nutrients (including sugar) into the cell. This is thought to aid weight loss as it leads to fat stores being used to provide energy. Hence most people attempt this diet in the pursuit of weight loss.
Moreover, there are also claims that this diet can incur a range of different health benefits such improving cholesterol, cognition and inflammation, or preventing Alzheimer’s and cardiovascular disease.
This type of diet is considered unsafe for those with advanced diabetes or who are on medications for diabetes, those with a history of eating disorders, women who are pregnant or breastfeeding women.
Will it help you lose weight?
When looking at body weight, research often looks at the components that make up our total body weight – fat free mass and fat mass. Fat mass is as it sounds – it is essential to have some of fat mass in order to maintain life, and support reproductive functions. Whereas, fat-free mass (sometimes called lean) refers to muscles, bones, ligaments, tendons and internal organs. The key to a healthy weight loss is to reduce body fat stores (fat mass) and preserve fat-free mass. Reductions in fat-free mass are typically bad news; particularly as lean muscle plays a key role in calorie burning (or energy expenditure).
A fairly recent systematic review found that intermittent fasting led to reductions in total body weight and fat mass. However, the studies included in paper had a wide range of variability. The number of people taking part and the study length ranged widely. The type of intermittant fasting used also differed; some studies used continuous calorie restriction, whilst others used approaches similar to that of the 5:2 diet. This makes it harder to determine whether these results occurred due to intermittant fasting or as a result of other factors in the study.
When comparing styles of fasting diets, one trial compared intermittent fasting (5:2) to continuous daily calorie restriction over 50 weeks and found that both diets produced comparable weight loss results. These findings could suggest that 5:2 style fasting works simply by creating a calorie deficit, not by some kind of voodoo magic. But this study is fairly short in length and needs replication to ensure that the results did not occur by chance. More studies looking at the long-term effect of fasting on human body weight are needed to determine if intermittent fasting is safe and effective.
There is evidence that weight loss can decrease the number of calories you use whilst at rest (resting energy expenditure); meaning that you burn less calories as a result of the fast which may lead to plateau. Moreover, fasting may lead to reductions in fat-free mass; which would further decrease your calorie burning potential.
Though some research suggests that sufficient protein intake and resistance training during intermittent fasting regimes increase fat-free mass preservation whilst enabling losses from fat mass in short term studies.
As I said previously, there is a wide range of intermittent fasting styles available; which makes it harder to evaluate exactly what type of style is effective. A lot of studies have been conducted on animals, not humans; so the findings are not translatable to humans. A lot of the human studies are observational designs (cannot prove causation) carried out on individuals during Ramadan or have modest sample sizes; so they do not reflect the diverse, wider population.
What the evidence does suggest, is that intermittent fasting and conventional daily calorie restriction provide similar weight loss results in the short term. This may be simply because having less hours to eat in the day, means you have less time to consume as many calories.
Eating in accordance with day light and night time (circadian rhythms) is a sensible way to plan your meals; as night time eating has been associated with a higher risk of becoming obese and developing type 2 diabetes. Moreover, eating rich or heavy meals late at night may disrupt your sleep.
It is important to note that there is not a “one-size-fits-all” when it comes to the way you eat. Intermittent fasting is still considered a diet, which is likely to set you up for disappointment as over 90% of diets fail. Choosing a way of eating that you can stick to, whilst encompassing general healthy eating advice, is most important when it comes to achieving healthy weight loss. Moreover, physical activity, sleep and stress levels can all impact your food choices and inevitably weight changes. So considering your lifestyle habits as a whole is a smarter move.
If you are considering changing your diet, consult with a registered dietitian or nutritionist, or your GP to ensure you are safe to do so.
Harris, L. et al.(2018) ‘Intermittent fasting interventions for treatment of overweight and obesity in adults’, JBI Database of Systematic Reviews and Implementation Reports. doi: 10.11124/jbisrir-2016-003248.
Heilbronn, L. K. et al.(2005) ‘Alternate-day fasting in nonobese subjects: Effects on body weight, body composition, and energy metabolism’, American Journal of Clinical Nutrition.
Peterson, C. M. (2019) ‘Intermittent Fasting Induces Weight Loss, but the Effects on Cardiometabolic Health are Modulated by Energy Balance’, Obesity. doi: 10.1002/oby.22384.
Schübel, R. et al.(2018) ‘Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: A randomized controlled trial’, American Journal of Clinical Nutrition. doi: 10.1093/ajcn/nqy196.
Trepanowski, J. F. et al.(2017) ‘Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: A randomized clinical trial’, JAMA Internal Medicine. doi: 10.1001/jamainternmed.2017.0936.