4 Nutritionists Break Down The 5:2 Diet

Food trends are constantly evolving, in response to the seasons, popular ingredients, or what’s going viral on social media. A large part of this involves discussions around ways of eating which go into minute detail, with strict rules that can feel overwhelming or impossible to stick to. Advocates confidently share the quick-fixes or benefits of certain diets, but it’s important to get all the facts before changing your eating patterns. In this second instalment of a new series, 4 of our expert nutritionist contributors will break down the most popular diets, advising you from a science-backed, nutritionally-informed perspective about the pros and cons. This week, they offer their take on the 5:2 diet. Click here to read their take on the "keto" diet, and here for the Mediterranean diet lowdown.

Jessica Shand, Naturopathic Nutritionist (NTP)

This 5:2 diet is an intermittent fasting diet, also known as The Fast Diet.

 It’s called the 5:2 because five days of the week are regular eating days whereby you eat what you would usually eat, which is largely why people like it so much as this does not feel like a standard diet whereby calorie restriction is continuous, but two days of the week are drastically calorie controlled to 500 for women and 600 per day for when, which is approx. 25% of usual daily intake. Providing the two fasting days have one non-fasting day in-between the two days can be any days of the week.

 The theory behind this diet is that limiting calorie intake on the two days switches the body’s energy source from food to fat stores and helps with weight-loss, particularly in people who are overweight and obese (1).

Sinéad Berry, Registered Nutritionist (MSc, mBANT, rCNHC)

While there’s some evidence to suggest that the 5:2 diet may help with short-term weight loss (2), it’s likely because it offers a structured way to consume fewer calories overall. In fact some studies concluded that weight loss was no greater than on a regular low calorie diet (3) and what works for one person may not work for you. 

However, the 5:2 diet is relatively simple to follow as it doesn’t involve strict meal timing or food restrictions on non-fasting days and some people may find this flexibility easy to manage. More long-term studies are needed to determine whether this way of eating is linked to a lower risk of heart disease (4) and type 2 diabetes (5). But ultimately the focus is on counting calories, which potentially means that, if not carefully planned, there’s a risk of not eating enough nutritionally-dense foods and this may lead to deficiencies. If you’re restricting your food for two days, it’s possible to become fixated on food on ‘normal’ days, potentially leading to overeating or bingeing (6). 

Fuelling your body with nutritious, balanced meals will leave you feeling satisfied, full of energy and will have a positive impact on your health. Healthy eating is about long term sustainability, rather than depriving yourself through a restrictive diet. It’s also important to remember that food is only part of the picture when managing a healthy weight — good-quality sleep, regular physical activity and stress management are also essential.

Rosemary Martin, Dietitian (BSc MSc RD)

If adhered to over time, the simple reduction in energy through a lower calorie intake can result in weight loss. Supporters of the diet also claim benefits for longevity and risk of cancer and dementia due to a reduction in the chemical insulin-like growth factor 1 (IGF-1).

 Although the simplicity of the 5:2 diet makes it an attractive one, it does not appear to hold any superior weight loss power in the long-term when compared with other strategies (7). One study found a significantly higher energy intake on days after a fasting day, suggesting that the diet may result in over-compensation due to hunger (8). Evidence remains limited on its long-term health effects and so many additional claims remain unproven.

 The 5:2 diet is one strategy that may help some people to achieve weight loss. On fasting days however, you may also experience fatigue, headaches, bad breath, constipation, and low mood. This strategy is not recommended for adolescents, those with diabetes, pregnant or breastfeeding women, those with a history of eating disorders or anyone who feels unwell.

Rohini Bajekal, Nutritionist and Board-Certified Lifestyle Medicine Professional (MA Oxon MSc Dip IBLM)

Worryingly, the 5:2 diet is focused on calorie intake and timing and does not suggest what foods to eat. There have been no long-term studies on the 5:2 diet, but the lack of emphasis on nutritious, fibre-rich foods may lead to unhealthy habits. It may also encourage a negative mindset around restricting foods and calorie counting, leading to fear and guilt when the diet is inevitably challenging to sustain. Fasting is not suitable for some people such as those who have diabetes or with eating disorders.


1. https://pubmed.ncbi.nlm.nih.gov/27708846/

2. R. Schübel, J. Nattenmüller, D. Sookthai, et al. Effects of intermittent and continuous calorie restriction on body weight and metabolism over 50 wk: a randomized controlled trial. Am. J. Clin. Nutr. 2018;108:933-945.

3. Welton S, Minty R, O'Driscoll T, Willms H, Poirier D, Madden S, Kelly L. Intermittent fasting and weight loss: Systematic review. Can Fam Physician. 2020;66(2):117-125.

4. Varady KA, Bhutani S, Klempel MC, Kroeger CM, Trepanowski JF, Haus JM, Hoddy KK, Calvo Y. Alternate day fasting for weight loss in normal weight and overweight subjects: a randomized controlled trial. Nutr J. 2013;12(1):146.

5. Harvie MN, Pegington M, Mattson MP, Frystyk J, Dillon B, Evans G, Cuzick J, Jebb SA, Martin B, Cutler RG, Son TG, Maudsley S, Carlson OD, Egan JM, Flyvbjerg A, Howell A. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomized trial in young overweight women. Int J Obes (Lond). 2011;35(5):714-27.

6. Cook F, Langdon-Daly J, Serpell L. Compliance of participants undergoing a '5-2' intermittent fasting diet and impact on body weight. Clin Nutr ESPEN. 2022;Dec;52:257-261.

7. Hajek P, Przulj D, Pesola F, et al. A randomised controlled trial of the 5:2 diet. PLoS One. 2021;16(11):e0258853. Published 2021 Nov 17. doi:10.1371/journal.pone.0258853.

8. Cook F, Langdon-Daly J, Serpell L. Compliance of participants undergoing a '5-2' intermittent fasting diet and impact on body weight. Clin Nutr ESPEN. 2022;52:257-261. doi:10.1016/j.clnesp.2022.08.012

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