Here’s the skinny on fasting for weight loss – the 5:2 diet
By Surinder Baines, University of Newcastle
The "new" weight-loss strategy known as the 5:2 diet has been receiving much attention in the media since the book The Fast Diet: The Secret of Intermittent Fasting – Lose Weight, Stay Healthy, Live Longer was launched late last year.
The 5:2 diet allows you to eat as usual for five days and to fast for two days. On fasting days, the dieters need to restrict intake of food to approximately 2000 kilojoules (500 calories) a day for women or 2400 kilojoules (600 calories) for men.
The two days of fasting don't have to be consecutive and you can decide how you want to spread your food intake on those days as long as you adhere to energy restriction. The food consumed during the two fasting days should have little fat and carbohydrate content and alcohol consumption is not recommended.
During the two fasting days, you are typically allowed protein foods such as eggs, or low-fat yogurt or cheese for breakfast and protein foods such as chicken, fish, lean meat, along with salad or other non-starchy vegetables for lunch or dinner. You are permitted water, green tea, or black coffee. While you can have milk with your beverages, it must be counted toward your caloric intake.
Not a fad?
Intermittent fasting or restricting energy intake for weight loss, which is what the diet is based on, is not a new concept. And there are other kinds of fasting diets around, such as "alternate day fasting". But while energy restriction in the form of various weight-loss diets has been investigated in both humans and animals, there's little research regarding the utility of intermittent fasting in humans.
A 2011 study in the United Kingdom that investigated the effects of intermittent energy restriction (to approximately 2266kJ a day for two days) compared to continuous energy restriction (approximately 6276KkJ a day for seven days a week) over six months, in 107 young overweight or obese women. It reported that both diets were equally effective for weight loss, as well as other markers of good health.
But there seemed to be potential difficulties in adherence. At the completion of the study, only 58% of the women in the intermittent fasting group planned to continue with the diet, compared to 85% of those in the energy-restricted group.
This study was one of the largest undertaken in this area so far and the few previous studies in the field have had a much smaller number of participants. Although these smaller studies have been conducted for shorter time periods, the UK study is also considered to be relatively short term.
Weight loss within the first six months is common with a lot of different types of diets. But research studies have shown that the majority of people put much of the weight back on within three to five years.
Need for caution
Many people who have tried the 5:2 diet report that they have been successful in losing weight but this is the case for most weight-loss diets in the short term. The issue of long-term compliance with the two days of energy restriction remains unresolved, as does long-term weight maintenance because people usually are not able to keep to their new weight.
Difficulties in adherence resulting in weight regain may encourage some people to try another dieting attempt and this can lead to the cycle of weight loss and weight regain being repeated. This happens in most cases of dieting-related weight loss.
The risks or the potential to overeat or gorge on non-fasting days also needs to be investigated. Diet quality is of particular significance for those who fast intermittently to ensure that all nutritional requirements are met and that the intake of some nutrients that have low intakes anyway (such as calcium) is not further compromised.
What's more, we still need to investigate whether intermittent fasting is a safe weight-loss strategy, especially for people with diseases such as diabetes. Starvation-type diets have side-effects such as dehydration, anxiety, irritability, tiredness and lethargy and whether we should be looking out for these in the 5:2 diet remains to be determined.
Intermittent fasting is reported to be effective among those who have used it for weight loss and it seems to be as effective as an energy-restricted diet in the short term. It may be a viable weight-loss option for some people but we need to research its effects beyond those reported, especially since many of these effects are anecdotal at present.
It's best to follow healthy eating dietary guidelines and seek advice from your doctor before embarking on intermittent fasting as a weight-loss strategy.
Surinder Baines does not work for, consult to, own shares in or receive funding from any company or organisation that would benefit from this article, and has no relevant affiliations.
- Surinder Baines
The University of Newcastle acknowledges the traditional custodians of the lands within our footprint areas: Awabakal, Darkinjung, Biripai, Worimi, Wonnarua, and Eora Nations. We also pay respect to the wisdom of our Elders past and present.