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Will the Fast Diet Work for You?

Joan Ransley is a nutrionist, a food writer and a friend of ours. She holds a PhD in Human Nutrition and for several years worked in research at the University of Leeds where she conducted several weight loss studies. We thought she was just the person to write a  guest blog on the Fast Diet.

From working with patients on various weight loss studies I know how difficult it is for people to stick to a diet, lose weight and keep the weight off.

Interestingly there have been several clinical trials which compare diets like the ‘Fast Diet’ known as Intermittent Energy Restriction [IER] to more conventional energy restriction known as Continuous Energy Restriction [CER]. The results look encouraging although not spectacular over a 6 month period.

The Fast Diet may help the ‘dieter’ to think about calorie intake over the period of a week and in theory it should lead to weight loss of about 0.5kg [l lb] a week if the diet is adhered to. This is based on an energy deficit of the two ‘fasting days’ per week which amounts to 3,200 kcals less than you would normally eat in that time. (The recommended energy intake of an adult woman is 2,100kcals and on each of the fasting days this is reduced to 500kcal for a woman). Other improvements in health such as increased insulin sensitivity, reduction in some cancer risks and reduced cardiovascular risk should also be expected.

In one study I looked at ‘The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women’ reported in the International Journal of Obesity (2011), researchers compared the effectiveness of the ‘Fast type diet’ to a ‘Usual reducing diet’ in approximately 100 women.

The study showed both types of diet were equally effective for weight loss at 6 months. The percentage of women in the IER (Fast type diet) and CER (Usual reducing diet’) groups losing 5–10% body weight were 30 and 33%, respectively, and losing 10% or more body weight were 34 and 22%, respectively, indicating the ‘Fast type diet has a slight edge for women who lost more than 10% of their initial body weight.  Both groups experienced comparable reductions in body fat, fat-free mass, hip, bust and thigh circumference.

The researchers found the ‘Fast type diet’ did not lead to overeating on non-fasting days which they checked by looking at the complete food diaries of participants in the study – a strength of the diet.

One important question to ask about any diet is – How easy is it to stick to? The results of the above study showed 60% of the people who had started the diet were still following it after 3 months and 54% – just over half at 6 months which is good – showing the dieters can stick with the diet. Interestingly the participants did not find the ‘Fast type diet’ any easier to stick to than a ‘Usual reducing diet’, particularly in the longer term. Predictably, ease of following the diets varied between individuals.

In another study I read the researchers hypothesised Intermittent Energy Restriction may be easier to follow than a daily or continuous energy restriction because it may help people overcome adaption to the weight reduced state (which occurs when your resting metabolic rate slows due to energy restriction – making losing weight harder).

From what I have read there does not seem to be enough evidence to show whether eating the 500/600 kcals in one go vs with a 12 hour gap in between 2 small meals is better. I suspect if studies were conducted to compare the two methods there would be a high relapse rate in the group who fasted for 12 hours between the day time meals. Most people find it very difficult not to eat for such long periods of time. They tend to fall off the wagon if they get too hungry. Hunger is a very powerful drive.

What I do know from the exercise diet studies is that if people exercise aerobically first thing in the morning before breakfast they are less likely to put on weight. There may be something in delaying the time when you eat breakfast. Remember if you sleep for 8 hours and then eat breakfast at 11.30am you will have had at least a twelve hour fast between meals anyway.

Studies have consistently shown that breakfast skippers tend to gain weight because they miss the opportunity to have a low calorie meal and by the time they get hungry overeat on higher fat/ calorie food. I don’t think there is any harm in delaying breakfast if you find it works for you.

So to conclude; the Fast Diet will lead to weight loss of about 0.5kg per week over the long term (6 months) if you stick to it. It should not lead to over eating on the non fasting days. It is also important to continue to exercise because of the synergistic effects of energy restriction and exercise on both weight loss and other markers of health. Good Luck.

Dr Joan Ransley
Honorary  Lecturer in Human Nutrition
University of Leeds

Posted on 26 Jan 2013 by Jill

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