The FastDay Forum

The results of a questionnaire sent out to members of the 5:2fastdiet forum were analysed statistically to try to discover what are the main factors influencing weight loss with this diet and how great an influence each factor has. As is the case with all diets, gender, starting BMI and duration of time on the diet were, unsurprisingly, key factors influencing the speed of weight loss. Overall, a quarter of the variation in speed of weight loss is accounted for by these three factors. As few men answered the questionnaire and some factors appear to have a different effect for men and women, our full analysis had to be restricted to only the women participants. While physical activity might be expected to enhance weight loss, we were unable to show this in our analysis. This is likely due to insufficient data about change in activity levels before and after starting 5:2. An important factor influencing weight loss is the number of hours with no calorie intake on fast days. Fasting fully (no calories) for more than 20 hours on fast days increased weight loss by around 120 grams per week compared with a 12 to 16 hour fast. Only a few people fast for less than 12 hours or longer than 24 hours but the difference between these extremes in terms of weight loss appears to be even larger at 240 grams per week. Adding an extra fast per week (i.e., 4:3 rather than 5:2) seems to result in faster weight loss, but the participants using 4:3 were too few for this effect to show in the full analysis. Although the main calorie restriction is conferred by the 2 days’ fasting, participants’ eating behaviour on the non-fast days had an important effect on the speed of weight loss. Firstly, bingeing on feed days can, unsurprisingly, slow down the weight loss. Compared with women who never binge, those who binge sometimes or often experience slower weight loss: in the order of 120 grams per week. Bearing in mind that the average weight lost per week for the women in our study was 490 grams per week, an effect size of 120 grams is important. Secondly, a change in dietary habits to a ‘healthier’ diet (defined as eating fewer snacks, less sugary food, fewer ready meals and eating more vegetables) appeared to increase weight loss by around 250 grams per week. Roughly one half of this effect is estimated to be the result of a change in food content, the other half is estimated to be due to eating less food (change in food quantity). Our final analysis, which takes into consideration the five main factors (BMI, weeks on diet, binge behaviour, healthy diet and length of fast) was able to explain almost half (40%) of the differences in rate of weight loss seen in the women who answered our questionnaire.