A few years ago, intermittent fasting, such as the 5:2 diet, became popular. And for good reason: they are an effective way to lose weight — if done properly. I’m a big fan of intermittent fasting, having practiced it for over 6 years now and it having been perhaps the biggest factor in my own weight-loss success story.
What is intermittent fasting?
Intermittent fasting is simply defined as taking a break from food for at least 12 hours. A popular type of fasting is called 16:8 fasting. This involves fasting for 16 hours and eating within an 8-hour window. For example you would finish your last meal of the day by 8 p.m. then leave 16 hours before your first meal of the next day at 12 noon (which really just means skipping breakfast). Another popular type of intermittent fasting involve having a very low calorie intake on two days a week (called 5:2 fasting).
There are two elements to a healthy way of eating: what to eat and when to eat. Both will influence many aspects of our health.
Intermittent fasting has been shown to benefit health in many ways:
Improvements in diabetes and factors that lead to diabetes [1–3]
Improvements in cardiovascular disease and factors that lead to cardiovascular disease [1–3]
Improvements in neurological conditions [3,4]
Improvements in inflammation 
Improvements in gut health 
Improvements in body composition .
Rules of fasting
During your fast, ideally you should avoid all calories but, because it is important to stay hydrated, it’s okay to have a splash of milk in your tea or coffee or to have a salty drink like miso soup or vegetable bouillon during your fasting period. Sugar or artificial sweeteners are not allowed in your drinks.
Stay well hydrated: drink plenty of water. And, of course, any kind of tea, coffee or herbal infusion without milk or sweetening is absolutely fine.
You can include the time you are asleep as part of your fasting period, so if you’re aiming to do a 16-hour fast, you can simply skip breakfast and fast through to lunchtime (or skip dinner and fast through to breakfast). During a 24-hour fast you skip two meals. During a 36-hour fast you would skip all three meals and break your fast the next morning.
Break your fast wisely: have a small meal to start with. You will likely be surprised that you will feel full sooner than you were expecting. Over-eating at this stage can cause digestive upsets. The longer your fast, the more likely this is to happen. After your light meal, wait around 20 minutes to see if you are still hungry and if so, you can have something else.
Avoid any refined carbs in your “break-fast” meal: your insulin levels will be very low and your body may be slow to respond to the rise in blood sugar after eating carbs. Big peaks in blood sugar are harmful to health, so ensure your breakfast is a healthy one.
Allow your body time to adapt and avoid intense or long exercise during your fast at first.
How often to fast
Too much fasting can stress the body  and so the longer you choose to fast the less often you should do it:
16- to 19-hour fasts are can be done daily (this is what I do), some people do have a weekend off if doing 19-hour fasts.
24- to 36-hour fasts are usually done twice a week (a variant is the 5:2 diet where you fast for 36-hours but you are allowed a small amount of food — around 500 calories-worth — during this time).
48-hour fasts should be limited to once a week or less often
Longer fasts than 48 hours should only be done with medical supervision.
It’s important to work out what suits you best. Play around with different lengths of fast. Although 24-hours or more may sound daunting, once you have tried it you’ll never feel a fear of hunger again!
You don’t have to fast on the same days each week or the same number of times: fasting is very flexible. Fit your fasting round your lifestyle not the other way around.
Side-effects of fasting
If you are well-established in a healthy way of eating, you should find that fasting does not cause any problems at all, although it is wise to avoid all but gentle exercise during your first fasts (after a short period of adaptation you can resume your usual exercise regime).
If you do suffer any ill-effects, these are very short-lived and, as your body adapts to your fasting regime, will soon disappear.
The most common side-effect is a headache, which is often due to not drinking enough. There is a surprising amount of water in our food and so when we stop eating we can easily become dehydrated. Also, when you fast you tend to lose sodium from your body which can also result in a headache. The answer to this is a salty drink as mentioned above — miso soup or vegetable bouillon will do nicely.
You may also feel cold: because eating results in some heat being produced in the body you won’t be getting this effect. A brisk walk a hot drink or simply putting on some more clothes is the answer.
As mentioned, fasting can be mildly stressful on the body so if after some time of practising intermittent fasting you show signs of increased stress: e.g., you feel excessively hungry, constantly tired or are sleeping badly, it is time to take a break.
1. Moro, T. et al. Effects of eight weeks of time-restricted feeding (16/8) on basal metabolism, maximal strength, body composition, inflammation, and cardiovascular risk factors in resistance-trained males. J. Transl. Med. 14, 290 (2016).
2. Patterson, R. E. & Sears, D. D. Metabolic Effects of Intermittent Fasting. Annu. Rev. Nutr. 37, 371–393 (2017).
3. Mattson, M. P., Longo, V. D. & Harvie, M. Impact of intermittent fasting on health and disease processes. Ageing Res. Rev. 39, 46–58 (2017).
4. Mattson, M. P., Moehl, K., Ghena, N., Schmaedick, M. & Cheng, A. Intermittent metabolic switching, neuroplasticity and brain health. Nature Reviews Neuroscience (2018). doi:10.1038/nrn.2017.156
5. Gasmi, M. et al. Time-restricted feeding influences immune responses without compromising muscle performance in older men. Nutrition (2018). doi:10.1016/j.nut.2017.12.014
6. Zarrinpar, A., Chaix, A., Yooseph, S. & Panda, S. Diet and feeding pattern affect the diurnal dynamics of the gut microbiome. Cell Metab. (2014). doi:10.1016/j.cmet.2014.11.008
7. Rothschild, J., Hoddy, K. K., Jambazian, P. & Varady, K. A. Time-restricted feeding and risk of metabolic disease: A review of human and animal studies. Nutr. Rev. (2014). doi:10.1111/nure.12104