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Does Intermittent Fasting Really Work?

Updated: Feb 15, 2021

A scientific comparison between 3 types of intermittent fasting:

the 5:2 diet, the 16:8 diet, and Alternate Day Fasting.

“Does intermittent fasting really work?” is a common question I get asked by my clients. In recent years intermittent fasting has increasingly become a topic of interest when it comes to weight loss, however there is a lot of confusion as to what it actually is. The aim of intermittent fasting is to lead to calorie restriction, which has been found to increase lifespan, and reduce lifestyle diseases such as cardiovascular disease, diabetes and cancer. The 3 most popular intermittent fasting diets are the 5:2 diet, the 16:8 diet and alternate day fasting. But what exactly is involved in each of these diets and is there strong evidence to show that they help with weight loss? I have reviewed the most recent scientific studies to find out.

5:2 diet

The 5:2 diet involves eating normally for 5 days of the week, and restricting calories for 2 days of the week to 500 calories a day for women and 600 calories a day for men. This diet was popularised in 2012 by Dr. Michael Mosley’s TV documentary Eat Fast, Live Longer. The diets effectiveness on the show was promising, however whether the effects of the diet were long-lasting is an unanswered question.

A human trial was published in 2013 by investigators at the Genesis Breast Cancer Prevention Center at the University Hospital of South Manchester, Manchester UK (Harvie et al., 2013). It was conducted on 100 overweight women who consumed a Mediterranean diet for 5 days of the week and restricted their calories by 70% on 2 days of the week. The women did this for 3 months, and were compared to a group who restricted calories by 25% every day.

The results showed that the group who followed the 5:2 diet lost significantly more body fat, and had better insulin sensitivity than those following daily calorie restriction. This diet shows promising results, however the participants following the 5:2 diet in this study had a lower starting BMI than the daily energy restriction group, which could cause potential unreliability, therefore stronger human trials are needed to test the effectiveness of this diet and it’s long term effects.

16:8 diet

The 16:8 diet is also known as time restricted feeding (TRF), and involves fasting for 16 hours, and eating normally within a eight hour window, (such as between 12-8 pm).

The 16:8 diet has had success in weight loss in animals, but not consistently in humans. A review (Rothschild et al., 2014) found that there was only low level evidence on humans and 10-12 hour TRF was more effective at lowering LDL cholesterol (bad cholesterol), which is similar to how most of us eat every day anyway.

The TREAT Randomised Clinical Trial (Lowe et al., 2020) was the first large scale randomised human trial, aiming to test the effects of the diet on 116 adults for 12 weeks. They were compared to a control group eating 3 meals a day at controlled times. The trial found that the 16:8 diet resulted in a small average weight loss (-0.1kg), however there was no significant difference in weight loss between the 16:8 and consistent meal timing group. This trial, which has the strongest clinical evidence to date on the 16:8 diet concluded that the 16:8 diet is not more effective in achieving weight loss than eating throughout the day.

Alternate Day Fasting

Alternate day fasting is fasting every other day. A moderated version of this is eating 400-600 calories on the fast days. On normal days you can eat whatever you want. Research has found that although in theory you can eat twice as much on normal days, most people only eat around 10% more than they normally would, therefore overall you eat less per week.

A 4-week randomised controlled trial on 60 participants (Stekovic et al., 2019) found that alternate day fasting produced a 37% calorie reduction on average, with a significant reduction in visceral fat (fat around internal organs), while muscle did not reduce. Additionally, the fat to lean ratio reduced, so subjects became more lean.

This strong level of evidence study concluded that alternate day fasting does not alter bone mass, significantly reduces the risk of cardiovascular disease in healthy adults, forms a leaner body type, and increases anti-ageing markers.


Overall the research shows that the 5:2 diet and alternate day fasting have better effects on our body than restricting our calories every day, which has shown to compromise our immunity (Meydani et al., 2016).

These diets show promising evidence for effective weight loss, however the long effects of these diets still haven’t been studied, therefore caution should be taken if you decide to follow them, and you should always consult your doctor before starting a diet that involves fasting to ensure you are fit and healthy to do so.

Some quick tips:

· On feed days try to follow a Mediterranean Diet full of wholegrains, fruits, vegetables and oily fish

· On fast days, sip a total of 2-3 litres of water throughout the day, try coconut water to keep things interesting!

· Take a multivitamin on fast days to keep your micronutrient levels topped up.

Ishika Sharma (ANutr)

Ishika Sharma is a registered associate nutritionist from London, specialising in weight loss, gut health and healthy ageing. She graduated from King's College London in Nutrition BSc, and has had clinical dietetic training in the NHS in weight loss, malnutrition, paediatrics and gut issues such as irritable bowel syndrome. She keeps up to date with nutrition research and critically appraises scientific literature to ensure all nutrition advice is current, and evidence-based.

Ishika sees patients who wish to lose weight and want a personalised approach, with regular guidance and check ins. She also gives nutrition advice for gut issues, skin problems, optimal ageing through nutrition and fatigue issues.


Harvie, M., Wright, C., Pegington, M., McMullan, D., Mitchell, E., Martin, B., Cutler, R.G., Evans, G., Whiteside, S., Maudsley, S., Camandola, S., Wang, R., Carlson, O.D., Egan, J.M., Mattson, M.P., Howell, A., 2013. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr 110, 1534–1547.

Lowe, D.A., Wu, N., Rohdin-Bibby, L., Moore, A.H., Kelly, N., Liu, Y.E., Philip, E., Vittinghoff, E., Heymsfield, S.B., Olgin, J.E., Shepherd, J.A., Weiss, E.J., 2020. Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial. JAMA Internal Medicine 180, 1491–1499.

Meydani, S.N., Das, S.K., Pieper, C.F., Lewis, M.R., Klein, S., Dixit, V.D., Gupta, A.K., Villareal, D.T., Bhapkar, M., Huang, M., Fuss, P.J., Roberts, S.B., Holloszy, J.O., Fontana, L., 2016. Long-term moderate calorie restriction inhibits inflammation without impairing cell-mediated immunity: a randomized controlled trial in non-obese humans. Aging (Albany NY) 8, 1416–1426.

Rothschild, J., Hoddy, K.K., Jambazian, P., Varady, K.A., 2014. Time-restricted feeding and risk of metabolic disease: a review of human and animal studies. Nutrition Reviews 72, 308–318.

Stekovic, S., Hofer, S.J., Tripolt, N., Aon, M.A., Royer, P., Pein, L., Stadler, J.T., Pendl, T., Prietl, B., Url, J., Schroeder, S., Tadic, J., Eisenberg, T., Magnes, C., Stumpe, M., Zuegner, E., Bordag, N., Riedl, R., Schmidt, A., Kolesnik, E., Verheyen, N., Springer, A., Madl, T., Sinner, F., de Cabo, R., Kroemer, G., Obermayer-Pietsch, B., Dengjel, J., Sourij, H., Pieber, T.R., Madeo, F., 2019. Alternate Day Fasting Improves Physiological and Molecular Markers of Aging in Healthy, Non-obese Humans. Cell Metabolism 30, 462-476.e6.


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