Since 2012, when a BBC documentary caused the 5:2 diet to explode in popularity, intermittent fasting diets have become all the rage. Many have become seduced by this alternative weight-loss/health-boosting strategy, shunning the conventional form of dieting in which you continuously watch what you eat.

Intermittent fasting diets involve cycles of ‘fasting’ (either no or very limited calories) and ‘non-fasting’ (basically, you can eat whatever you like). This may take various forms:

– 5 days of ‘normal eating’: 2 days of 500kcal/day for women and 600kcal/day for men.

– 6 days of ‘normal eating’: 1 day of fasting, i.e. only calorie-free fluids such as water.

– 8 hours of ‘normal eating’: 16 hours of fasting (some may call this going to sleep without dinner!).

– fasting for 24 hours, once or twice a week, or ‘Eat-Stop-Eat’.

– fasting or consuming 500-600kcal every other day.

– fasting during the day and eating a large meal at night (again, some may call this the ‘I-don’t-have-time-for-breakfast-and-am-so-darn-busy-at -work-so-have-no-time-to-eat’ diet), popularly known as the Warrior Diet.

There is a body of evidence to support intermittent fasting diets that, I have to admit, is rather convincing:

Firstly, it is assumed that going for long periods of time without food was a typical way of life during the prehistoric hunter-gatherer era, particularly during colder seasons when food was scarce. As such, our bodies adapted and clung on (for want of a better word) to the energy provided from food when it was lucky enough to get it. In the in-between times, fat synthesised and stored in the body would be the principal energy source. In the modern Western world, many people are fortunate enough not to have to worry about food availability and the longest period of fasting would normally just be overnight. Whilst it’s reassuring for us to have our fridge permanently stocked, our bodies may not necessarily thank us, as they are thought to be programmed to the aforementioned patterns of food availability and aren’t used to the types of (often high-carbohydrate) food consumed frequently by many people today.

Studies in humans have shown that restricting calories in the long-term, whilst maintaining adequate intake of nutrients, reduces the risk of developing Type II diabetes, hypertension, cardiovascular disease and cancer. It is also thought to counteract stiffening of the heart muscle, alterations in the nervous system and gene expression, which are all associated with ageing. This past month, a study was published showing that healthy participants who followed a ‘fasting-mimicking-diet’ – low in calories, sugars and protein but high in unsaturated fats – for 5 consecutive days per month for 3 months had reduced body weight, body fat (both overall and in the trunk area), lowered blood pressure and decreased insulin-like growth factor 1 (a hormone that mediates the effects of growth hormone [this does what it says on the tin] and is involved in many other pathways, including those linked with ageing) compared to those following an unrestricted diet. Whether these benefits are to do with calorie restriction/intermittent fasting per se, or rather/also the fact that the people in these studies ate nutrient-dense food (minimally processed and all that jazz), remains to be seen. Similar benefits have been shown in people following ketogenic – very low-carbohydrate, high-fat – diets, which aim to mimic the state of starvation on the body, perhaps suggesting that calorie restriction, or at least your body thinking it is getting less calories, is at least partly responsible for such phenomena.

I am slightly more sceptical about intermittent fasting diets used for weight loss. It makes perfect sense that periodically depriving yourself of food means that, overall, you consume less calories. Unless, of course, you really truly go overboard during your unrestricted hours/days…which is possible, but probably unlikely for the majority of people. A recent review found that overweight and obese adults following intermittent fasting diets lost on average 0.2-0.8kg/week. “What’s wrong with that?”, you say? Well, nothing at all, except that similar weight loss was found in those who restricted their calorie intake at all times (i.e. the conventional form of dieting). The authors also reported no difference in outcomes with regards body fat, muscle mass or waist circumference. This suggests that intermittent fasting is certainly an option for weight loss, but is not necessarily any more effective than conventional dieting. And let’s not forget that if you ‘go on a diet’ (this may or may not involve fasting) to lose weight and then go back to your previous eating habits after a certain period of time, it’s pretty likely that you’ll regain any lost weight, and then some.

Furthermore, it is also important to mention that intermittent fasting may carry some risks.

– According to PEN: Practice-based Evidence in Nutrition®, ‘fasting may actually be deleterious to health by increasing insulin resistance while decreasing important liver detoxifying enzyme functions, muscle mass and body nutrients, and one’s ability to work and exercise.’ These authors were not focusing on intermittent fasting, although some of the studies cited in the ‘evidence clip’ included fasting on alternate days.

– If you are restricting your food intake at any time, it is important to ensure that what you do eat (both on fasting and non-fasting days) is of high nutritional quality. Your best bet is to follow the principles of the EatWell Guide.

– If you are feeling ill, it is probably best not to fast as this will put additional stress on your body.

– Be aware of the potential consequences of fasting. According to the official 5:2 diet website, ‘some people report headaches or constipation’, although is likely to be due to dehydration. Food provides around 20% of our daily fluid intake, so you have to compensate for this on fasting days.

I certainly don’t advocate ‘diets’ and am the first to scrutinise the latest wonder regime. “All in moderation; a little bit of what you like“, I harp on to anybody who seeks my advice on the matter. However, if weight loss is needed and the person is simply not going to manage changing their culinary customs day-in-day-out, I do believe that intermittent fasting has its place. And the same goes for people at high risk of diseases such as Type II diabetes. I wouldn’t personally practise intermittent fasting unless I knew that my health depended on making some serious lifestyle changes, and I would recommend everybody who is considering starting such a regime to seek advice from a dietitian, but I am strongly swayed by the evidence behind the intermittent fasting concept. And, if you have enough willpower to cope with consuming only 500-odd calories throughout the day whilst still working/studying/doing-whatever-you-do, I take my hat off to you!

Natasha Schoeler

Natasha has just finished her postgraduate Dietetics degree at KCL. She previously completed her PhD on the genetics of response to the ketogenic diet and is keen to follow a career combining clinical paediatric dietetics and ketogenic diet research.

Foodtalk blog posts are written by a variety of health and care professionals in order to showcase different perspectives in the world of nutrition and health.