Fasting for weight loss

Fasting has been practiced for millions of years, and recent research has shown adaptive cellular responses that reduce oxidative damage and inflammation, and boost cellular protection – in short, fasting can delay the ageing process. It reduces hypertension, asthma, and rheumatoid arthritis. Studies of intermittent fasting show reduced blood pressure, lower cholesterol levels and improved insulin sensitivity, the latter facilitating a reversal of type 2 diabetes... and of course, it can be used for weight loss.


If fasting continues past a few days, resting metabolic rate can reduce by as much as 20% as energy expenditure adapts to cope with the lower caloric intake (adaptive thermogenesis). Historically, it has been thought that a calorie intake below one's basal metabolic rate resulting in a slowing down of metabolism would then encourage weight gain. However, it appears that as long as there is sufficient protein in the diet and muscle mass is maintained, metabolic rate does not reduce; in fact some experts suggest that as fasting puts the body in a state of mild stress, the metabolic rate is actually elevated.

As it is thought that any drop in metabolic rate is predominantly as a result of lean muscle loss, ways to limit the loss of lean tissue include:

  • consuming sufficient amounts of protein
  • restricting other foods but consuming a high fat diet to enter into ketosis, which conserves muscle
  • exercising to promote lean tissue maintenance

Whether an ample amount of protein or fat can be consumed will depend on the type of fast, and exercise will be dependant upon energy levels. This is where intermittent fasting has some benefits, as food is eaten in most types of intermittent fasting, such as the 'Fast diet' or '5:2 diet', so protein and/or fat can be consumed, and also energy levels are unlikely to be affected, so exercise can continue.

Research illustrates different results when comparing the effectiveness of fasting to other types of calorie restrictive diets. A 2018 study found no difference in the time it took to achieve a 5% weight loss between participants who were on an intermittent fasting regime, compared with those on continued energy restriction. However, a 2014 review found that intermittent fasting regimens demonstrated body weight reductions of 3–8% after 3–24 weeks in comparison to energy restriction diets, which demonstrated 4–14% reductions in weight after 6–24 weeks. Both weight loss strategies yielded comparable reductions in visceral fat, fasting insulin, and insulin resistance.

The metabolic effects of fasting

If a meal is missed, blood glucose levels drop. The resulting secretion of the hormone glucagon stimulates the breakdown of stored carbohydrate, which releases glucose back into the bloodstream, supplying the body with glucose for between 12-24 hours with sufficient stores. Even if a fast is continued and carbohydrate stores have been largely used up, the human body manufactures glucose from non-carbohydrate sources such as amino acids, lactic acid and glycerol, so we really don't need to 'top up' our energy levels with regular snacks - if the goal is to lose weight, we should allow the body to use up some of its stored energy.

Our stored fats (triglycerides) are broken down into glycerol (which is turned into glucose for energy) and fatty acids. The fatty acids form something called ketones, and the level of ketones rises as our carbohydrate or caloric intake reduces, providing fuel and reducing the break down of protein and/or muscle tissue. In the early phases of fasting we are using some ketones for energy, but will not necessarily be in a full state of ketosis.

Energy during a short fast will be taken mostly from the breakdown of stored carbohydrate, and a higher amount of stored fat will be broken down for energy. There will then be a rising level of something called gluconeogenesis (making new glucose from proteins, glycerol and lactic acid), and then, as carbohydrate restriction continues and stores reduce further, the breakdown of muscle tissue may occur, which is not conducive to good weight loss as this will reduce overall metabolic rate. However, consuming sufficient protein and/or fat as mentioned above can help to maintain muscle mass and offset this natural slow down in metabolism.

So, whereas pure fasting alludes to a complete fast (consuming nothing but water), fasting nowadays can mean a reduction in calorie intake for part of a day (time restricted eating), or on chosen days (Fast diet, 5:2 diet), simply reducing calorie intake and stimulating metabolic pathways to use up existing energy stores and make increasing amounts of alternative energy such as ketones. Ketogenic diets, therefore, provide many of the metabolic and weight loss benefits that fasting does, but without the actual fasting!

Fasting in its many forms is a viable option for weight loss, and many people find fasting for part of a day, or for a couple of days a week, an easier option than changing their usual diet into a calorie-restricted 'weight loss diet'. However, one thing is certain - whichever form of calorie restriction/reduction you choose, it's still important to make healthy food choices for long term benefits. It's also important to seek advice before you start any new eating regime to ensure you are appraoching it safely. Speak to your GP to check you have no health concerns that may prevent you from fasting or get expert advice from a professional nutritionist or dietician


Antoni R, Johnston KL, Collins AL, Robertson MD. (2018) Intermittent v. continuous energy restriction: differential effects on postprandial glucose and lipid metabolism following matched weight loss in overweight/obese participants. British Journal of Nutrition, Volume 119(5):507-516. doi: 10.1017/S0007114517003890.

Barnosky AR, Hoddy KK, Unterman TG, Varady KA. (2014) Intermittent fasting vs daily calorie restriction for type 2 diabetes prevention: a review of human findings. Translational Research – the journal of laboratory and clinical medicine, Volume 164:302–311.

Hall KD and Guo J. (2017) Obesity Energetics: Body Weight Regulation and the Effects of Diet Composition. Gastroenterology, Volume 152(7):1718-1727.e3. doi: 10.1053/j.gastro.2017.01.052. 

Harvie MN and Howell T. (2016) Could Intermittent Energy Restriction and Intermittent Fasting Reduce Rates of Cancer in Obese, Overweight, and Normal-Weight Subjects? A Summary of Evidence. Advances in Nutrition, Volume 15;7(4):690-705. doi: 10.3945/an.115.011767. 

Longo VD, Mattson MP. (2014) Fasting: molecular mechanisms and clinical applications. Cell Metabolism, Volume 4;19(2):181-92. doi: 10.1016/j.cmet.2013.12.008. 

Westerterp KR. (2004) Diet induced thermogenesis. Nutrition and Metabolism, Volume 1: 5.

The views expressed in this article are those of the author. All articles published on Nutritionist Resource are reviewed by our editorial team.

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Wadebridge PL27 & Newquay TR7
Written by Sara Kirkham, BSc.(Hons) Nutritional Medicine, MBANT, CNHC
Wadebridge PL27 & Newquay TR7

Sara Kirkham has a 1st class honours degree in Nutritional Medicine, published research and 13 published books. She has helped people lose weight for 30 years in the health and fitness industry and now in nutritional therapy practice, making weight loss a speciality, and her book 'Weight Loss-The Essential Guide' is a great DIY guide to weight loss

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