Is intermittent fasting right for you?

Intermittent fasting is a popular diet trend, but what does the science say? And is it right for you?

Intermittent fasting is on an upward trend. Some believe it’s not good for you, and others believe it’s the best thing for weight loss, but what does the science say? 

In this blog, we answer: 

What are the main types of intermittent fasting?

From 12-hour fasting to The Warrior Diet, there are several types of intermittent fasting. Intermittent fasting can be broadly split into three categories: 

  1. Alternate-day fasting  
  2. The 5:2 diet  
  3. Time-restricted eating  

What is alternate-day fasting?

Alternate-day fasting is exactly what it says on the tin – you alternate the days that you fast. 

On fasting days, you can either have a zero-calorie day and only drink water, or you can limit yourself to 25% of your energy requirement (usually between 500 and 800 calories) [1]. On the other (non-fasting) days, you can eat whatever you like, with no restrictions.  

Alternate-day fasting is slightly different to the more well-known 5:2 diet.  

What is the 5:2 diet?

The 5:2 diet is similar to alternate-day fasting, except you eat for five days and fast for two. These can be whichever days you like. On your fasting days, you eat around 25% of your energy requirements, and on your non-fasting days, you eat your standard recommended calorie allowance.  

Recently, the 5:2 diet has gained more attention, especially after Michael Mosley’s documentary Eat fast and live longer aired. Dr Mosley discussed all the benefits the 5:2 diet had to offer, including reducing your risk of heart disease, stroke, and cancer [2]. 

What is time-restricted eating?

Time-restricted eating is when you limit the time that you eat – this could be that you limit yourself to only eating within a specific 10-hour window.  

It includes: 

  • 12-hour fasting — i.e. only eating within a 12-hour window 
  • 16-hour fasting — i.e. only eating within an eight-hour window 
  • A weekly 24-hour fast 
  • The Warrior Diet  

What is The Warrior Diet?  

The Warrior Diet is an intermittent fasting technique by Ori Hofmekler based on the eating patterns of ancient warriors. It involves fasting or undereating for 20 hours of the day but then eating all your recommended calories within a four-hour window – usually in the evening. It has different phases, and, in each phase, you’re limited to certain foods alongside the hours that you eat.  

The Warrior Diet is split into three phases [3]: 

  • Phase one aims to remove toxins from your body and neutralise substances that can trigger fat gain. In this phase, dieters are encouraged to avoid any kind of animal protein, and instead, eat raw fruit and vegetables alongside small servings of plain dairy products. You can drink tea and coffee (with no sugar) and fruit juices. It’s recommended that when it comes to your eating window, you should eat your first portion and then give yourself a 20-minute break. If you’re then still hungry, eat some more of the same type of food – this should help regulate your body’s signals of feeling full.  
  • Phase two aims to improve the use of fat for energy. In this phase, the Warrior Diet recommends you eat foods that are high in healthy fats such as nuts and seeds. The rest of your diet should be the same as in phase one.  
  • Phase three aims to improve the use of carbohydrates for energy. In this phase, you should alternate between days of high-carbohydrate and high-protein meals, such as one to two days of high carbs followed by one to two days of low carbs and high protein, and then continue this in an alternating pattern.  

After these three initial phases, you can rotate between whichever phases suit you. Hofmekler recommends that you take a nutritional supplement such as a multivitamin when initially starting this diet.  

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What are the benefits of intermittent fasting?

So, now you know the types – but what are the benefits of intermittent fasting?  

Each technique has different benefits, but many of the benefits cross over.  

The benefits of intermittent fasting include [1,2,4,5]: 

  • Aiding weight loss (with 5:2 and alternate-day fasting providing the best results for weight loss) 
  • Reducing blood pressure  
  • Improving brain function [6] 
  • Improving appetite regulation (at least in the short term) 
  • Reducing insulin resistance and HbA1c in people who are overweight or have pre-diabetes 
  • Improving sleep in people who have insomnia  
  • Potentially lowering bad cholesterol with alternate-day and 5:2 fasting (although results are highly variable) [7] 
  • Reducing insulin resistance and HbA1c in people who are overweight or have pre-diabetes 
  • Aiding autophagy (where your body recycles old and damaged cells to regenerate new healthier cells) [8] 

Some research suggests that intermittent fasting can also help in the management of thyroid conditions.  

Can you intermittent fast if you have a thyroid condition?

If you have a thyroid condition, you may be wondering whether intermittent fasting is safe for you and if it may be helpful for your condition. 

With the thyroid gland being the metabolic powerhouse of the body, it's unsurprising that intermittent fasting impacts thyroid function. In healthy adults, alternate-day fasting and very low-calorie diets have been shown to reduce T3 [9,10]. 

Most research around intermittent fasting looks at healthy individuals. Most of this evidence is positive, but it's difficult to say how this applies to people with conditions like Hashimoto's and Graves' disease. It's possible it may reduce levels of inflammation and help manage symptoms like weight gain, but unfortunately, there's not enough evidence in this area. 

If you're trialling intermittent fasting with a thyroid condition, you can monitor your thyroid hormones before and during the process with our Thyroid Blood Tests.  

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You can find out more about our Thyroid Blood Tests in our Thyroid Blood Test Buying Guide.  

What are the drawbacks of intermittent fasting?

Intermittent fasting seems to have many positives – and has received high praise in the weight loss world. But as with everything, there can be drawbacks.  

Some research suggests that not all fasting techniques work as well as others and some may even be associated with side effects, including (in some cases) reduced lifespan [1]. 

One of the biggest risks with intermittent fasting may be a nutrient deficiency, as there is the risk that when not fasting, you overindulge on ultra-processed foods [11].  

To reduce this risk, ensure that when you’re not fasting, you eat nutrient-rich foods. 

Intermittent fasting isn’t recommended for certain groups, including people who:  

  • Are pregnant or breastfeeding 
  • Are elderly or frail 
  • Have or have recovered from an eating disorder 

Though there is some evidence that intermittent fasting may provide benefits for conditions like type 2 diabetes, it can also be dangerous if you’re taking medications to lower your blood sugar levels (like insulin) [12]. 

Read more about fasting and diabetes.  

Is intermittent fasting for you?

As with everything, intermittent fasting will either work for you, or it won’t. For many, intermittent fasting can aid weight loss and reduce the risks of certain health conditions. For others, it may result in a nutrient deficiency or possible ill health. Skipping meals or significantly limiting calories can be dangerous for people with underlying health conditions [13]. In these cases, it’s best to discuss the potential benefits and risks of intermittent fasting with your doctor first. 

So, is it for you? That’s up to you to decide.  


  1. K;, V.K.A.C.S.E.M.G. (no date) Cardiometabolic benefits of intermittent fastingAnnual review of nutrition. U.S. National Library of Medicine. Available at: (Accessed: January 23, 2023). 
  2. Mattson, M.P., Longo, V.D. and Harvie, M. (2017) “Impact of intermittent fasting on health and disease processes,” Ageing Research Reviews, 39, pp. 46–58. Available at:
  3. The warrior diet: Pros, cons, how to follow it, and example meal plan (no date) Medical News Today. MediLexicon International. Available at: (Accessed: January 23, 2023). 
  4. Regmi, P. and Heilbronn, L.K. (2020) “Time-restricted eating: Benefits, mechanisms, and challenges in translation,” iScience, 23(6), p. 101161. Available at:
  5. Mindikoglu, A.L. et al. (2020) “Intermittent fasting from dawn to sunset for four consecutive weeks induces anticancer serum proteome response and improves metabolic syndrome,” Scientific Reports, 10(1). Available at:
  6. Mattson, M.P. (2015) “Lifelong brain health is a lifelong challenge: From evolutionary principles to empirical evidence,” Ageing Research Reviews, 20, pp. 37–45. Available at:
  7. Tinsley, G.M. and La Bounty, P.M. (2015) “Effects of intermittent fasting on body composition and clinical health markers in humans,” Nutrition Reviews, 73(10), pp. 661–674. Available at:
  8. Bagherniya, M. et al. (2018) “The effect of fasting or calorie restriction on autophagy induction: A review of the literature,” Ageing Research Reviews, 47, pp. 183–197. Available at:
  9. Stekovic, S. et al. (2019) “Alternate day fasting improves physiological and molecular markers of aging in healthy, non-obese humans,” Cell Metabolism, 30(3). Available at:
  10. Wadden, T. A., Mason, G., Foster, G. D., Stunkard, A. J., & Prange, A. J. (1990). Effects of a very low calorie diet on weight, thyroid hormones and mood. International journal of obesity14(3), 249–258. Available at:  
  11. Vo, J. (2020) Nutrition & Health Info Sheets for health professionals - intermittent fasting, UC Davis Nutrition Department. Available at:,foods%20during%20the%20eating%20period (Accessed: January 23, 2023). 
  12. Vo, J. (2020) Nutrition & Health Info Sheets for health professionals - intermittent fastingUC Davis Nutrition Department. Available at:,foods%20during%20the%20eating%20period (Accessed: January 23, 2023). 
  13. Not so fast: Pros and cons of the newest diet trend (2019) Harvard Health. Available at: (Accessed: January 23, 2023). 

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