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Many scientists agree that intermittent fasting may be effective for weight loss.1 But, how does it affect your body outside of this? In particular, how might it affect your gut? Our aim here is to help you understand the relationship between intermittent fasting and the gut, and what you need to consider when thinking about adopting the eating plan.
Nicknamed 'the science of going without', intermittent fasting (IF) describes a group of diets which involve alternating between periods of fasting or low calories, and periods of eating normally.
IF hit the headlines in 2012, after it appeared on the BBC documentary 'Eat, Fast and Live Longer'. Since then, it's become a popular way to lose body fat, with research suggesting that it's is as effective for weight loss as cutting calories on a daily basis.2
By eating fewer calories than your body needs to fuel daily activities, it's forced to tap into stored energy, which comes from either body fat or lean tissue, in order to make up the shortfall - which ultimately results in weight loss. An initial benefit of IF is that it's been shown to protect lean tissue more effectively than cutting calories daily, which is good news for body composition, especially if you're a regular at the gym.3
One of the most popular fasting plans is the 5:2 diet, which involves eating normally for 5 days a week, and eating low calories (around 500 a day) the remaining 2 days. This is a type of modified fasting, in that you don't do a complete fast, and you still get in a limited amount of calories. This version is one that many people find easier to follow than a complete fast, so it may be advisable for those new to this style of eating.
Other types of IF include alternate day fasting (ADF), which involves a full 24-hour fast followed by 24-hours of eating normally, and time-restricted feeding (TDF).4 TDF involves fasting for 16-20 hours (from 7pm-11am for example), with all calories eaten in the remaining 4-8 hour period. There's not enough evidence to say which is the most effective approach, although the majority of research has focused on ADF.4
Importantly, fasting doesn't mean ditching healthy eating advice. Our food choices play a fundamental role in many aspects of our lives: from moods to energy levels, and general health and wellbeing. So, making sure you're getting a healthy balance on fasting and non-fasting days matters. On fasting days, opting for foods rich in protein and fibre will put the brakes on hunger, making appetite control that bit easier.
As IF has become more popular, scientists have started to look at how it can affect different parts of the body, including the gut. Stretching from mouth to bottom, the gut (also called the digestive tract) is a long muscular tube, which carries out some of the most important jobs in the body. Responsible for the breakdown, digestion, and absorption of everything we eat and drink, it also defends against illness, and removes harmful waste products.
As food enters our stomach, it begins an epic journey. Stomach acid fends off harmful bacteria and activates enzymes that start to break down nutrients. From the stomach, food enters the small intestine, which stretches about 5-6 metres in length. The walls of the small intestine are covered with tiny little bumps called villi, which whisk nutrients away into the bloodstream. From the end of the small intestine, the large intestine takes over. As well as removing waste, the large intestine is home to trillions of helpful bacteria. Together with the genes they instruct, this community is known as the microbiome.
A healthy microbiome has a rich variety of bacterial species (or families). If the microbiome loses bacterial diversity or becomes dominated by one type of bacteria, our health can suffer. Thankfully we now know that we can shift the balance of bacteria in our microbiome by changing the way we eat. Prebiotic foods, like onions, leeks, asparagus, garlic, and under-ripe bananas, in addition to fibre, can help to stimulate the growth of helpful bacteria, while probiotic foods and supplements 'top-up' existing levels.
So what happens to this delicate ecosystem during fasting? Research is limited when it comes to humans, but we do have some clues. In a 2014 study supported by the American Gut (a microbiome research project), researchers looked at how various 3-day fasts affected the microbiomes of 19 adults from the U.S and U.K.5 Volunteers in the study were asked to follow one of 3 fasts: water only, fruit and vegetable smoothies plus the probiotic #VSL3, or water plus lemon, maple syrup, and cayenne pepper, before returning to their normal diets. Stool samples were taken before, during, and for 2 weeks after the fast.
Results showed that fasting did change the microbiome, but not in the same way for everybody. Some of the volunteer's microbiomes showed huge changes, while others had so much variety in their species to begin with that it was hard to detect a difference. Of most interest were the fruit and vegetable fast, which caused a spike in a species of bacteria known as Akkermansia - which is linked with protection against diabetes and obesity.
But, within 10 days of volunteers returning to a normal (but unhealthy) diet, numbers had dropped back to their previous levels. More research is needed in this area, but we can see that the microbiome can be affected (varying in how much from person to person) during fasting. Which, lead us nicely onto...
IF may have some benefits for the microbiome, but these are likely to vary according to the current state of your microbiome in the first place - and what you eat on non-fasting days. Diet can reliably change the microbiome in as little as a day, which means filling up on high fat, high sugar foods (like biscuits, crisps, sweets, and refined cereals) on non-fasting days is a no-no.6
Until more research has been conducted, the best way to care for your microbiome may well be to adopt healthy eating habits on both fasting and non-fasting days. This involves eating a diet with plenty of fibre, in addition to pre and probiotic foods. So, whether you fast or not, the following tips should help you to protect your microbiome:
Whether you're a meat-eater or vegetarian, getting plenty of fibre is vital. In fact, studies from around the globe show how fibre rich diets boost the levels of helpful bacteria in the gut.7 In the UK, average daily fibre intakes hover around 14 grams, but latest advice from SACN, the government's advisory body, suggests you should be aiming for 30 grams a day. Make this your goal on non-fasting days, and you'll be off to a good start. On fasting days, plan smaller meals around fruits, veggies, and pulses, as these are richer in fibre and more filling than refined foods.
Fermented foods, like kombucha, sauerkraut, sourdough, miso, tempeh, and kefir, are a rich source of lactic acid bacteria - a large family of probiotic bacteria that have positive effects in the gut. More research into their benefits is needed, but there doesn't seem to be any harm in adding them to your diet.
Treat your bacteria with meals containing artichokes, asparagus, onions, garlic, leeks, and greenish bananas, as these prebiotic foods can encourage the growth of helpful bacteria in your gut. Asparagus soup, for instance, is simple, healthy, and great for fast days. For non-fasting days, try garlic toast with sourdough bread, topped with tomato and onion salsa.
Finally, you might want to include a probiotic drink or supplement to give your gut bacteria a helping hand. Studies suggest probiotics are most helpful for balancing levels of helpful bacteria in digestive conditions like irritable bowel syndrome, or after a period of illness.8
From the research we have, fasting (at least in the short term) does seem to have some positive effects on gut health. However, the benefits (especially on the levels of helpful gut bacteria) seem short-lived if you return to an unhealthy diet between fasts. The best way to maintain the benefits, and protect your microbiome in the longer term, is to fertilise your gut bacteria with a fibre-rich diet, rich in fruits, veggies, and whole grains. Adding fermented or probiotic foods may add some extra benefits - as well as adding more diversity to your diet, which for your microbiome, can only be a good thing. If you struggle to get probiotic or prebiotic bacteria in the form of food, there are a number of supplements that can help you aid your microbiome, without inconveniencing you.
If you'd like to read more about the benefits of keeping a healthy gut, as well as find more information on how you can promote good gut health, select Digestion from the Your health menu above.
Nothing beats a healthy, balanced diet to provide all the nutrients we need. But when this isn’t possible, supplements can help. This article isn’t intended to replace medical advice. Please consult your healthcare professional before trying supplements or herbal medicines.
1 M. N., Pegington, M., Mattson, M. P., et al. (2010). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomised trial in young overweight women. International Journal of Obesity, 35(05).
2 Trepanowski, J. F., Kroeger, C. M., Barnosky, A., Klempel, M. C., Bhutani, S., Hoddy, K. K., et al. (2017). Effect of alternate-day fasting on weight loss, weight maintenance, and cardioprotection among metabolically healthy obese adults: a randomized clinical trial. JAMA Internal Medicine, 177(07).
3 Aragon, A. A., Schoenfeld, B. J., Wildman, R., et al. (2017). International society of sports nutrition position stand: diets and body composition. Journal of the International Society of Sports Nutrition, 14(16).
4 Varady, K. A., and Hellerstein, M. K. (2007). Alternate-day fasting and chronic disease prevention: a review of human and animal trials. Journal of the International Society of Sports Nutrition, 86(01).
5 Thompson, L. (2014). What does a three-day dietary cleanse do to your gut microbiome? American Gut.
6 David, L. A., Maurice, C. F., Carmody, R. N., et al. (2014). Diet rapidly and reproducibly alters the human gut microbiome. Nature, 505(7484).
7 Tap, J., Furet, J. P., Bensaada, M., et al. (2015). Gut microbiota richness promotes its stability upon increased dietary fibre intake in healthy adults. Environmental Microbiology, 17(12).
8 Huey Shi, L., Balakrishnan, K., Thiagarajah, K., et al. (2016). Beneficial Properties of Probiotics. Tropical Life Sciences Research, 27(02).