Monday, 11 February 2013

A Fast Way to Lose Weight?


First things first, real fasting involves abstaining from all food and drink.

Various plans over the years, including detox diets, have advocated fasting, the more sensible of which allows for fluids.

These newly arisen fasting diets don’t advocate anything as extreme as avoiding all food and drink, but instead involve a significantly reduced calorie intake on ‘fasting’ days – usually a maximum of 600 kcal for men, and 500 kcal for women.

So this diet essentially advocates days of significant calorie restriction rather than actual fasting. But they use the term fasting, so I’ll stick with it for the sake of clarity.

The frequency of the fasting varies depending on what you read, but usually takes the form of either alternate day fasting (fasting every other day) or fasting on a 5:2 ratio - the latter involves fasting for two days per week. Both forms of the diet can be considered as intermittent fasting.

So what does it involve?

The pattern of the diet is variable – the frequency of the fast, as already said, varies depending on what plan you follow. Whichever it is it, the fast days involve restricting your calorie intake quite dramatically.

The general advice around calorie intake per day is 2000 kcal for a woman, and 2500 kcal for a man. Now, these are far too general to be of real use to the individual (but that’s another story), but do give us some indication of the rough energy intake that’s considered the norm/common. And it does give you some indication as to just how much of a calorie restriction you’re expected to follow on an intermittent fasting diet.

Let’s take a 500 kcal fast as an example. What do you get for your calories?

The answer, unsurprisingly, is not a lot. But it very much depends on how careful you are with your calories as to how much you have to eat.

For 500 kcal you could have a couple of slices of toast and peanut butter, and a Mars bar. And that’s it. Or you could have a couple of scrambled eggs for breakfast, a bowl of tomato soup and a salad at lunch, and a bowl of vegetable and lentil stew for dinner. This diet requires thought and preparation, and some knowledge and skill. And a liking for vegetables will certainly help.

How does it work?

Essentially, the diet involves days of calorie restriction. And when you restrict calories to a level below that which you use, you create a deficit which leads to weight loss. It’s fairly basic maths.

However – and there’s usually a however with diets – there are drawbacks.

There are various potential problems with fasting:
  •  Hunger. As a society we’ve got used to being surrounded by food. For most of us there’s never any need or desire to allow ourselves to feel actual hunger. It’s not a pleasant sensation, but it is a natural one. It lets us know when we need to eat – not a drawback as such, but something to be aware of!
  • Fasting can affect your mood – don’t be surprised if you find yourself feeling just a little bit grumpy on your fasting days.
  • Low energy consumption can lead to low energy levels – on fast days, you may find you feel tired and struggle with normal activities.
  • Following on from the above, if you exercise or are training for a sport you may well find your efforts feel that much more difficult than usual. By fasting you are essentially depriving your body of fuel, and the extra demands of being active may be one step too far for your dwindling energy levels.
  • Fasting affects hunger hormones, causing a down-regulation of appetite. So after a few days of fasting you may find that you’re not bothered as much by feelings of hunger. However, on non-fasting days you may find that there is a rebound increase in the hunger hormones, stimulating a greater appetite than you would normally experience. This can lead to bingeing and overeating – and the excess calories eaten on non-fasting days can sabotage weight-loss.
  • Fasting has an effect on your metabolic rate – low energy consumption causes a down-regulation in how much energy our bodies use, so whilst you may be eating less, your body is also using less. This down-regulation can linger even when you return to your normal diet, so it’s quite possible that you will gain weight on returning to a normal eating pattern.

However – yes, there’s another however – these diets are not well studied.

The known effects of fasting/very low calorie diets are usually applied to periods of extended fasting/calorie restriction. How these transfer to intermittent calorie restriction is not fully understood – and whilst that’s not necessarily something to deter you from following the diet, it is something to be wary of. The long-term consequences of this diet are simply not known.

Non-fasting days

There are different ideas about what and how you should eat on non-fasting days.

Should you eat ‘normally’, that is, stick to whatever dietary pattern you usually follow? Or can you splurge – reward your virtuosity for sticking to your calorie restriction, and devour anything and everything you desire?

Research into these diets advocates that you should maintain a healthy balanced diet on your non-fasting days. You need to get in the nutrients you’ve missed out on during the fast, and an excessively high calorie intake on your ‘feed’ days can sabotage any efforts on your ‘fast’ days. So whilst you may want to eat nothing but cream cakes and pies, try and stick to balanced meals and snacks. And remember there’s nothing wrong with cake and chocolate, in moderation.


In addition to the potential for weight loss, the fasting diets also seem to confer other benefits to general health.

In the few studies that have been published on these forms of dieting, improvements in various health markers were noted, including cholesterol and blood glucose.

Calorie restriction has been studied for years, with the focus being on its effect on longevity – and the findings suggest that there are benefits to be had. In addition to a reduction in levels of IGF-1 (insulin-like growth factor, a compound responsible for the ageing process), cholesterol and blood glucose are reduced, and with these so are your risks of cardiovascular disease and type 2 diabetes.

Anything else?

There’s always a last word to be had – is this eating pattern to be recommended? It’s hard to say. As a dietitian I like to work from evidence – high quality, rigorous evidence. And in the case of Alternate Day Fasting, 5:2 fasting, or any other configuration of calorie restrictive diets, the evidence just isn’t there yet.

The studies that have been published are promising, but there are some questions yet to be answered – what pattern of restriction works most effectively? How should you eat on non-fasting days? What level of calorie restriction is most effective?

Perhaps most importantly you need to consider how sustainable this eating pattern is. Do you just adhere to the plan for a finite period of time for the purposes of weight loss? What happens when you reach your goal? Do you revert back to your previous diet, which was probably responsible for your weight/cholesterol/etc. in the first place? Sustainability in diet and lifestyle is key to life-long health, and is intermittent fasting/severe calorie restriction sustainable?

It’s something to think about.

A word of warning – if you are diabetic, pregnant or breast-feeding, DO NOT follow this diet plan. It’s also strongly contraindicated if you have a history of eating disorders.
If you have any other health conditions, be safe and check with an expert before embarking on this (or indeed any other) diet. 

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