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Behind the headlines: Fasting diets

The Fast Diet, also known as the 5:2 diet or intermittent fasting, is all the rage at the moment. Are these diets the answer to the obesity problem, or just another fad? HFG investigates.

What are the basics of the fast diet, 5:2 diet or intermittent fasting diet?

You may have heard of the Fast Diet, the 5:2 Diet, Intermittent Fasting or the Alternate Day Diet. The different diets vary, but the basic idea is to eat normally for five days of the week, and on two days to restrict food to a very low level (500 calories/2100kJ a day for women, 600 calories/2520kJ for men, or not eat at all).

Will this diet help me lose weight?

When you fast, your body is forced to dip into energy stores to get the fuel it needs to keep going. Restricting your food intake drastically, even on only two days in the week, is likely to result in weight loss. That’s provided, of course, you don’t overcompensate on the ‘regular’ eating days and pig out on whatever you can get your hands on. Proponents of fasting diets say this doesn’t happen in practice, and that people are more likely to focus on healthy eating during their regular days than they are to binge.

The bigger issue may be keeping the weight off. HFG Senior nutritionist, Rose Carr, says, “As with all weight-loss diets, going back to your old way of eating — ending the diet, essentially — without making any permanent changes to what and how you eat, is likely to mean that any weight you lose will quickly go back on.”

What are the downsides and risks? Could it be bad for me?

There has been hardly any research specifically on the 5:2 style of diets so any long-term negative effects are not known. Possible short-term side effects are likely to be the same as those on other restrictive diets: low energy, bad breath, anxiety, sleeping problems and dehydration. Cutting down your food intake of course cuts down your opportunities to get in all the nutrients, vitamins and minerals you need to be healthy, so fasters will need to pay even more attention to this to avoid deficiencies.

There is potentially a more serious risk to some people. If you are prone to disordered eating — for example, bingeing, restricting, emotional eating — intermittent fasting diets could encourage the disordered eating pattern.

Nutritionist Claire Turnbull says, “If you have struggled with an eating disorder, [a fasting diet] would not be a suitable or safe diet for you.” Nor would it be safe if you have diabetes or you are pregnant.

These diets also don’t teach us anything about healthy eating.

“If you already have a really healthy diet most of the time, you could be fine, but if you are an unhealthy eater to start with, this diet is probably not going to make you any healthier,” says Rose Carr.

Fasting could also be difficult to stick to long term. Meaning that as with all diets, unless you are prepared to stick to it for the rest of your life, and make a permanent change to the way you are eating, any weight lost is likely to go back on once you stop the diet.

Are there potential benefits? What does the science say?

There’s a lot that’s unknown at this stage. There’s no evidence on how intermittent fasting compares to conventional kilojoule-controlled diets in terms of weight-loss and health, particularly in the long term.

Obesity expert Professor Jim Mann points out, “There is absolutely no evidence these diets help with long-term weight-loss.”

There has been some research over the years into fasting’s effects on lifespan, cognitive decline and disease prevention, particularly heart disease and dementia (most of this research has been on animals). The results show some evidence that intermittent fasting may have benefits in these areas. Studies of fasting in rodents appear to indicate a connection between kilojoule restriction and longevity. Scientists agree, though, that more research is needed before we can say the theories are proven, or that fasting is better than any other diet for weight-loss and health.

What does a 2100 kilojoule day look like?

If you were to stick to 500 calories, ie. 2100kJ in a day, it would basically be one regular meal, or two small meals, such as an egg and a piece of fruit for breakfast, nothing during the day and a plate of vegetables or salad with a small amount of protein for dinner.

The bottom line

If you’re thinking of trying one of these diets, think about whether it is something you could stick to long term, for life. And make sure your diet is as healthy as it can be on the days you’re not fasting because if it isn’t, you’re unlikely to see lasting benefits and could suffer poor health from dietary deficiencies.

Professor Mann says, “While there is some evidence supporting fasting, that in no way translates into a recommendation that this diet is a good way to go. Really the only thing we know works for long-term weight-loss is re-educating yourself about appropriate eating, and creating long-term healthy habits.”

Nutritionist Claire Turnbull points out that the discipline required for fasting could be put to a better use.

“If you’re disciplined enough to fast two days a week, is it possible you’ve got the discipline to eat well all the time and cut down the portions of everything you eat, every day? That might be a better way to go long term,” says Claire.

We will watch the developing research on intermittent fasting with interest. Watch this space.


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