Fasting for Weight Loss

Low Carb versus Balanced Carbs: Same effect

Debates on effective and safe diets for managing obesity and type 2 diabetes in adults are ongoing. Low-carbs diets are widely promoted as being more effective for weight loss than other diets. A Cochrane Systemic Review of 61 RCTs (n= 6925) concluded that there is little to no difference in weight reduction or changes in cardiovascular risk factors between a low-carb and a healthy balanced carb diet (Naude et al. 2022). The most common diets compared were low-carbohydrate, balanced-fat (20 to 35% of total energy intake) and high-protein (> 20% of total energy intake) treatment diets versus control diets balanced for the three macronutrients. Restricting caloric intake and having regular contact with a researcher or healthcare practitioner can result in significant weight loss regardless of the specific diet intervention applied.

No value of adding protein supplement during very-low-calorie diets

This study randomised 108 adults with overweight or obesity (body mass index 28-40 kg/m2) to very-low-calorie diets (VLCD) with either 52 or 77 g/day protein for 8 weeks (total energy intake of 600 or 700 kcal/day, respectively). The attrition rate was 22% in both groups. Both VLCDs decreased body weight, fat mass, lean mass, and resting metabolic rate (all P < 0.05). Reductions in lean mass and resting metabolic rate did not differ between groups and the contribution of lean mass to total weight loss was identical. In other words, supplementation of VLCD with protein does not prevent lean body mass loss. Perhaps an exercise programme would have been more effective. While very-low-calorie diets can be effective, they are hard to stick to for patients and can be associated with loss of lean body mass. Intermittent fasting may be a better option as research has shown that the fasting mimicking diet is not associated with a loss of lean mass (Magkos et al. 2021).

Intermittent fasting with the 5:2 diet: 7 kg over six months

Weight-loss diets not only reduce body weight they can also significantly improve blood pressure control in hypertensive patients. This study compared the effects of intermittent energy restriction (5:2 diet) with those of continuous energy restriction on blood pressure control and weight loss in 205 overweight and obese patients with hypertension during a 6-month period. The continuous energy restriction was based on consuming 1,000 kcal/day for women and 1,200 kcal/day for men – which may have been about half their normal intake. The diets were equally effective for weight loss and blood pressure control. The weight loss was around 7 kg or a little over 1 kg per month of the six-month trial. The diet reduced the systolic blood pressure by 7 mm Hg and 5-6 mm Hg. There were also favourable improvements in body composition, HbA1c, and blood lipid levels, with no differences between groups (He et al. 2021). This weight loss does not seem great considering the effort required to stick to such a low calorie diet for such a long time.

In another year-long study the 5:2 diet with or without support group sessions was compared with a one-off standard weight-management advice in 300 adults with breast cancer.  The 5:2 diet did not seem particularly effective for weight loss in this group. This may have been related to adherence which declined to just 22% at the end of the study. Adherence to the 5:2 diet was initially high (74% at 6 weeks), but it declined over time (31% at 6 months and 22% at one year). The weight loss was only around two kilograms (Harvie et al. 2021).

A mega review of 130 randomised trials finds significant differences in various types of intermittent fasting diets

An umbrella review of 11 meta-analyses of 130 randomized clinical trials found six statistically significant associations of intermittent fasting supported by moderate to high quality of evidence. Outcomes associated with modified alternate-day fasting included a moderate reduction of body weight, body mass index, and cardiometabolic risk factors in adults with overweight or obesity. Only one association was supported by high-quality evidence: modified alternate-day fasting for 1 to 2 months, which was associated with moderate reduction in body mass index in healthy adults and adults with overweight, obesity, or non-alcoholic fatty liver disease compared with regular diet.

It is important to note that the modified alternate day fasting (MAFD) and the 5:2 diet were the only intermittent fasting types that were associated with statistically significant weight loss of more than 5% in adults with overweight or obesity. In contrast, zero-calorie alternate day fasting, and time restricted eating did not. The MADF is a very different diet to the 5:2 diet and incorporated repeated days of significant calorie restriction. It alternated between days of ad libitum eating and days of fasting with total caloric intake ranging from 0% to 40% or 0 to 600 kcal per day for 3 to 5 days per week. It is also worth noting that the IF diets were more successful in the initial phase (ie, 1-6 months), after which participants would frequently experience a plateau as additional weight loss was not further achieved because of the metabolic adaptation of the human body or decreased adherence to the assigned weight loss strategy. Lastly, many trials had a high degree of bias with partly because of the lack of masking participants and personnel (Patikorn et al. 2021).

Weigh loss during the Fasting Mimicking Diet (FMD)

Participants in the first clinical trial on the FMD, who were generally healthy and not very overweight, lost 2.5 kg after three cycles of fasting mimicking diet (Wei et al. 2017). Importantly, the weight loss is loss of body fat, especially around the waist, with a reduction in waist circumference of 4 cm. The 5-day FMD has been shown to preserve muscle mass during the programme. Much greater weight loss has been observed in clinical practice in overweight patients with repeated cycles of the FMD.


Harvie, M., M. Pegington, S. J. Howell, N. Bundred, P. Foden, J. Adams, L. Graves, A. Greystoke, M. P. Mattson, R. G. Cutler, J. Williamson, K. Livingstone, D. McMullen, K. Sellers, C. Lombardelli, G. Cooper, S. McDiarmid, and A. Howell. 2021. 'Randomised controlled trial of intermittent vs continuous energy restriction during chemotherapy for early breast cancer', Br J Cancer.

He, C. J., Y. P. Fei, C. Y. Zhu, M. Yao, G. Qian, H. L. Hu, and C. L. Zhai. 2021. 'Effects of Intermittent Compared With Continuous Energy Restriction on Blood Pressure Control in Overweight and Obese Patients With Hypertension', Front Cardiovasc Med, 8: 750714.

Magkos, F., M. F. Hjorth, S. Asping, M. I. Rosenkrans, S. I. Rasmussen, C. Ritz, A. Sjodin, and N. R. W. Geiker. 2021. 'A protein-supplemented very-low-calorie diet does not mitigate reductions in lean mass and resting metabolic rate in subjects with overweight or obesity: A randomized controlled trial', Clin Nutr, 40: 5726-33.

Naude, C. E., A. Brand, A. Schoonees, K. A. Nguyen, M. Chaplin, and J. Volmink. 2022. 'Low-carbohydrate versus balanced-carbohydrate diets for reducing weight and cardiovascular risk', Cochrane Database Syst Rev, 1: CD013334.

Patikorn, Chanthawat, Kiera Roubal, Sajesh K. Veettil, Viji Chandran, Tuan Pham, Yeong Yeh Lee, Edward L. Giovannucci, Krista A. Varady, and Nathorn Chaiyakunapruk. 2021. 'Intermittent Fasting and Obesity-Related Health Outcomes: An Umbrella Review of Meta-analyses of Randomized Clinical Trials', JAMA Network Open, 4: e2139558-e58.

Wei, M., S. Brandhorst, M. Shelehchi, H. Mirzaei, C. W. Cheng, J. Budniak, S. Groshen, W. J. Mack, E. Guen, S. Di Biase, P. Cohen, T. E. Morgan, T. Dorff, K. Hong, A. Michalsen, A. Laviano, and V. D. Longo. 2017. 'Fasting-mimicking diet and markers/risk factors for aging, diabetes, cancer, and cardiovascular disease', Sci Transl Med, 9.