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Get Started Social media is filled with influencers claiming to know all about fitness and nutrition. Some posts go viral and soon become a trend. Few nutrition trends though, have managed to make beans and oats look exciting on social media. Fibremaxxing though, has done it. If you’ve ever talked about or searched online for anything fibre related, your social media feed may now be full of it. Scroll for a few minutes and you will find people stacking their breakfast bowls with seeds and counting grams of fibre the way gym-goers count grams of protein.
Unlike a lot of online food fads, this one does tend to point in a sensible direction. Most people in the UK eat far less fibre than they should, and the health research behind eating more of it is some of the most consistent in nutrition science.
But the “maxxing” part is where it can go wrong. Jumping from a low intake to a very high one overnight, or getting the timing wrong around training, can leave you feeling bloated and convinced that fibre does not agree with you. So what should we be doing?
Fibremaxxing grew out of the same corner of social media that gave us protein-maxxing and every other “maxxing” trend. The idea is simple. Push one part of the diet as high as it will go and post the results.
In this case, the push is towards fibre. Influencers share high-fibre breakfasts, swap white pasta for lentil pasta, add psyllium husk to smoothies and track daily totals of 50 grams or more. The claims that come with it range from a flatter stomach to easier weight loss.
Some of those claims do have good evidence behind them. Some are a bit stretched. And a few of the methods are probably going to cause more discomfort than better health.
Clients are seeing this content every day, so a personal trainer who can sort the useful parts from the hype is a trusted one. The starting point is how much fibre people eat in the first place.
In 2015, the Scientific Advisory Committee on Nutrition (SACN) reviewed the evidence on carbohydrates and health and set a new target for fibre. Its Carbohydrates and Health report recommends that adults eat 30g of fibre a day, with lower targets for children (15g at age 2 to 5, 20g at 5 to 11 and 25g at 11 to 16).
The UK population is a long way off that figure. The latest National Diet and Nutrition Survey, covering 2019 to 2023 and published in June 2025, found that adults aged 19 to 64 averaged 16.4g a day. Adults aged 65 to 74 averaged 16.9g. Only 4 per cent of adults met the 30g recommendation.
Put simply, the average adult in the UK eats a little over half the recommended amount. Fewer than one in twenty reach the target.
That gap is the strongest argument in favour of the trend. Very few of your clients are at risk of eating too much fibre. Most of them would benefit from eating more.
Fibre is the part of plant foods that our own digestive enzymes cannot break down. It travels through the small intestine largely intact and reaches the large bowel, where much of it is either fermented by gut bacteria or adds bulk to the stool.
It is often split into two groups.
Soluble fibre dissolves in water and forms a gel. Oats and barley are good sources, as are beans and lentils. This gel slows digestion a little, which is one reason soluble fibre helps with fullness and with the rise in blood sugar after a meal. Some types, such as the beta-glucan in oats and barley, also help to lower blood cholesterol.
Insoluble fibre does not dissolve. Wholegrains and wheat bran are rich in it, and so are nuts and seeds. It holds on to water and adds bulk, which helps food move through the gut and keeps stools softer and more regular.
There is a third way of looking at fibre that explains a lot of the side effects people report. Fermentable fibre is the portion that gut bacteria break down for fuel. They produce short-chain fatty acids, which are useful compounds for the lining of the bowel and beyond, and they also produce gas. Beans and lentils are highly fermentable, and so are many fibre supplements.
We covered how that bacterial community responds to exercise in our article Your Gut Microbiome Is Listening to Your Training, so we will not repeat it here. For this article, the main point is that more fermentable fibre means more fermentation, and more fermentation means more gas, at least while the gut adjusts.
The strongest single body of evidence on fibre comes from a series of systematic reviews and meta-analyses commissioned by the World Health Organization to inform its guidelines on carbohydrates (Reynolds et al., 2019). The team pulled together 185 prospective studies, with just under 135 million person-years of data, and 58 clinical trials involving 4,635 adults.
When they compared the people eating the most fibre with those eating the least, the observational data showed a 15 to 30 per cent lower risk of death from any cause and from cardiovascular disease. There was a similar reduction in the risk of coronary heart disease, stroke, type 2 diabetes and colorectal cancer.
The trials backed this up with direct measurements. Higher fibre intakes lowered body weight, systolic blood pressure and total cholesterol compared with lower intakes.
The biggest reductions in risk were seen when people ate between 25g and 29g of fibre a day, which sits very close to the UK target. And the dose-response curves suggested that intakes above that range might bring further protection against several of these diseases. The certainty of the evidence for fibre was graded as moderate, which is a good rating for nutrition research.
A newer umbrella review, which gathers and grades existing meta-analyses, reached a similar conclusion (Veronese et al., 2025). It included 33 meta-analyses covering 38 health outcomes and more than 17 million people. Higher fibre intake was linked with lower risk for 29 of those outcomes.
The authors rated the evidence as convincing, their top grade, for a lower risk of dying from cardiovascular disease, pancreatic cancer and diverticular disease. They rated it as highly suggestive for a lower risk of dying from any cause, and of cardiovascular disease more broadly.
There are limits here that we should keep in mind. Almost all of this evidence is observational. People who eat a lot of fibre tend to eat more fruit and vegetables, smoke less and move more, and studies can never fully separate the effect of fibre from those other habits. Only six of the 33 meta-analyses in the umbrella review were rated as high quality.
Even so, the consistent findings and the supporting trial data make fibre one of the better-supported recommendations in nutrition. The case for eating more is definitely there. The case for eating as much as humanly possible is not.
Many clients come to fibremaxxing through weight loss content, so it helps to know what the evidence does and does not support in this area.
High-fibre foods tend to be bulky and take longer to chew and digest. A bowl of porridge or a lentil soup fills the stomach for relatively few calories. In the Reynolds trials, higher fibre intakes led to lower body weight, although the differences were modest rather than sizeable.
Blood sugar is the other common claim. A 2025 meta-analysis looked at 51 randomised controlled trials in 3,420 adults with overweight or obesity who did not have type 2 diabetes (Colak et al., 2025). The trials lasted between one and twelve months and compared added fibre with a placebo.
Added fibre lowered fasting blood glucose, fasting insulin and a measure of insulin resistance. The changes were small. Fasting glucose fell by an average of 0.07 mmol/L, which is helpful at a population level but not something a client would notice on a home glucose meter.
So the fair summary for a client is that fibre helps with fullness and blood sugar, and it works best as part of an overall eating pattern. It is not a substitute for a calorie deficit and regular training when it comes to weight loss.
The problems with fibremaxxing come from the speed and the scale, not from fibre itself.
The most common mistake is going from a typical UK intake of around 16g a day to 40g or more within a week. The gut bacteria that ferment fibre need time to adjust to a bigger supply. Until they do, a sudden jump in fermentable fibre can bring wind, bloating and changes in bowel habit. Many people give up at this point and decide fibre does not agree with them.
The fix is simple. Build up gradually, adding a few grams every few days rather than overhauling every meal at once. A client who adds one higher-fibre swap a week can reach the mid-20s within a couple of months with far less discomfort.
Fluid is the second issue. Insoluble fibre works by holding water in the stool. If a client sharply increases fibre without drinking more, the stool can become harder rather than softer, and constipation can get worse. NHS advice on increasing fibre pairs it with drinking plenty of fluids, and that is the line to follow.
The third issue is the number itself. The research supports getting to around 30g and suggests there may be further benefit beyond that. It does not support the idea that 50 or 60 grams is automatically better, and very high intakes can be uncomfortable for most people. For most clients, reaching the UK target consistently is a big step forward.
A lot of fibremaxxing content leans on psyllium husk and inulin powders. These products can help some people, and the Colak meta-analysis found that isolated fibres added to the diet did improve markers of blood sugar control (Colak et al., 2025).
It is worth looking at this in more detail though. The same analysis found that no form of fibre supplement could be considered superior, and the effects of all forms were small. Supplements also deliver fibre and very little else.
The foods behind the large observational benefits in the Reynolds review were wholegrains, pulses, vegetables, fruit, nuts and seeds. These bring a wide mix of fibre types along with plenty of vitamins and minerals. A scoop of powder in a smoothie supplies none of that.
Many concentrated fibre products are also highly fermentable. Inulin and chicory root fibre, common in “gut health” bars and drinks, are well known for causing wind and bloating in larger amounts. A client who adds several of these products on top of a sudden change in diet is likely to feel it.
So the message for clients is food first. Supplements have a place, particularly when a GP or dietitian recommends one for a specific reason, but they should support a higher-fibre diet rather than replace it.
Telling a client to take as much fibre as you do for example, can make some feel worse. Irritable bowel syndrome (IBS), one of the most common gut conditions seen by GPs, is the best example.
The British Society of Gastroenterology guidelines on IBS, written by a large group of UK gastroenterologists, draw a clear line between fibre types (Vasant et al., 2021). Soluble fibre, such as ispaghula (the medical name for psyllium), is recommended as an effective first-line treatment for global symptoms and abdominal pain. Insoluble fibre such as wheat bran is not recommended, because it may make symptoms worse.
The guidelines also recommend that a low FODMAP diet, which restricts a group of highly fermentable carbohydrates, is used as a second-line approach and supervised by a trained dietitian. That is a medical diet, and it sits well outside a personal trainer’s scope of practice.
The same caution applies to clients with inflammatory bowel disease such as Crohn’s disease or ulcerative colitis, those who have had bowel surgery and anyone who has been told to follow a low-fibre or low-residue diet. A low-residue diet limits fibre and other foods that leave material in the bowel, and it is usually prescribed for a medical reason.
If a client reports a lasting change in bowel habit, blood in their stool, unexplained weight loss or persistent abdominal pain, the advice is as we’ve all been taught. They should see their GP before changing anything.
This is the part the fibremaxxing influencers leave out, and it is where we can add the most value for our clients.
Fibre slows the emptying of the stomach and adds bulk to what is in the gut. That is fine at a desk, but far less welcome halfway through a hard interval session or a half marathon, when blood flow has shifted away from the gut towards the working muscles. A large, high-fibre meal eaten shortly before hard or long exercise is a common cause of stomach cramps, urgent toilet stops and nausea.
Endurance athletes already know this. A study of 137 endurance athletes with a history of exercise-related gut symptoms asked them how they managed those symptoms (Scrivin et al., 2022). Reducing fibre was the most commonly reported dietary strategy, and it was rated among the most successful. The athletes also rated dietitians as their most useful source of advice. This was an early, self-reported survey, so it shows what athletes do and how they rate it rather than proving cause and effect.
A controlled study from Liverpool John Moores University adds some physiology (Foo et al., 2022). Nineteen healthy men ate their usual diet, which contained around 30g of fibre a day, for a week. They then switched to a diet with less than 10g of fibre a day for four days, matched for energy and macronutrients.
On the low-fibre diet, body mass fell by an average of 0.58kg by the morning of the fifth day, most likely because there was less food residue in the gut. Bowel movements dropped from two a day to one, and hunger rose slightly. Eighteen of the 19 men found the diet tolerable.
Put together, this supports a simple approach. Clients should eat a high-fibre diet most of the time, but keep the meal in the two to four hours before a hard training session or competition lower in fibre. For a marathon or other long event, some athletes reduce fibre for a day or two beforehand. That is a short-term tactic for specific situations, with a return to the usual high-fibre diet afterwards. Anything new should be tried in training first, well before race day.
Getting an extra 13 to 14 grams sounds like a lot. In practice, it takes four or five simple changes spread across the day.
The figures below are rounded and approximate, based on typical UK portions. Exact values vary by brand and by how foods are prepared.
| Food swap | Roughly how much fibre it adds | Practical tip |
|---|---|---|
| White bread to wholemeal (2 slices) | About 3g | Try a 50/50 loaf first if the client dislikes the texture |
| Sugary cereal to porridge oats (40g) | About 2 to 3g | Top with berries or a sliced apple for a further 1 to 2g |
| Add 80g of beans or lentils to a meal | About 4 to 5g | Stir a tin into a chilli or a pasta sauce |
| White pasta or rice to wholewheat pasta or brown rice | About 2 to 4g per portion | Mix half and half while the family gets used to it |
| Peeled potato to a baked potato with skin | About 2 to 3g | Leave skins on for roasties and wedges too |
| Add a portion of vegetables or frozen peas (80g) | About 2 to 4g | Frozen veg is cheap and counts the same |
| Crisps or biscuits to a small handful of nuts (30g) | About 2 to 3g | Keep a pre-portioned bag in a gym bag or desk drawer |
| Fruit juice to a piece of whole fruit | About 2 to 3g | A pear or an apple with the skin on is a good choice |
| Add a tablespoon of chia or ground flaxseed | About 3g | Sprinkle on yoghurt or porridge |
Two or three of these changes in a day can take a client from around 16g to the mid-20s. Four or five can take them close to 30g. Introduce them one at a time over a few weeks, and remind clients to drink more water as they go.
As personal trainers, we are in a good position to support healthy eating, provided the advice stays within general public health guidance.
Encouraging a client to eat more plant foods in line with the Eatwell Guide and the 30g target is general healthy eating advice. Showing them how to build up gradually and time meals around training fits comfortably within our scope of practice.
Diagnosing gut problems, putting a client on a low FODMAP diet or recommending specific supplements to treat symptoms does not. Those discussions belong with a GP or a registered dietitian. A PT who is willing to refer a client on earns trust from both clients and healthcare professionals they work alongside.
You have no doubt had a client who all of a sudden starts talking about a new food rule they picked up online. If they’ve talked about fibremaxxing, you’ll agree that it’s probably one of the better social media fads they could follow. The trend is right that most people need more fibre. It is however, wrong to suggest that more is always better or that it can be done overnight.
We know there is a target of around 30g a day, built mainly from wholegrains, pulses, vegetables and fruit. Clients can achieve this by adding fibre gradually and choosing food before powders, with plenty of water. Those training hard should keep the meal before a session lower in fibre and practise their race-day eating in training.
Anyone with ongoing gut symptoms or a bowel condition should get proper advice from their GP or a registered dietitian before making big changes.
Whether you coach marathon runners, busy parents or people taking their first steps into a gym, a few simple swaps and a little patience will do more for their health than any trend pushing extremes.
Colak, H., Larik, G.N.F., van Baak, M.A. and Canfora, E.E. (2025). Effects of isolated single fibers, fiber mixtures, and fiber-rich whole foods on glucose homeostasis in individuals with overweight and obesity: a systematic review and meta-analysis. Clinical Nutrition, 52, pp.236-251. Click here to review the full research article.
Foo, W.L., Harrison, J.D., Mhizha, F.T., Langan-Evans, C., Morton, J.P., Pugh, J.N. and Areta, J.L. (2022). A short-term low-fiber diet reduces body mass in healthy young men: implications for weight-sensitive sports. International Journal of Sport Nutrition and Exercise Metabolism, 32(4), pp.256-264. Click here to review the full research article.
Reynolds, A., Mann, J., Cummings, J., Winter, N., Mete, E. and Te Morenga, L. (2019). Carbohydrate quality and human health: a series of systematic reviews and meta-analyses. The Lancet, 393(10170), pp.434-445. Click here to review the full research article.
Scrivin, R., Costa, R.J.S., Pelly, F., Lis, D. and Slater, G. (2022). An exploratory study of the management strategies reported by endurance athletes with exercise-associated gastrointestinal symptoms. Frontiers in Nutrition, 9, 1003445. Click here to review the full research article.
Vasant, D.H., Paine, P.A., Black, C.J., Houghton, L.A., Everitt, H.A., Corsetti, M., Agrawal, A., Aziz, I., Farmer, A.D., Eugenicos, M.P. et al. (2021). British Society of Gastroenterology guidelines on the management of irritable bowel syndrome. Gut, 70(7), pp.1214-1240. Click here to review the full research article.
Veronese, N., Gianfredi, V., Solmi, M., Barbagallo, M., Dominguez, L.J., Mandalà, C., Di Palermo, C., Carruba, L., Solimando, L., Stubbs, B. et al. (2025). The impact of dietary fiber consumption on human health: an umbrella review of evidence from 17,155,277 individuals. Clinical Nutrition, 51, pp.325-333. Click here to review the full research article.
Fibremaxxing is one of dozens of nutrition trends your clients will ask you about. Knowing the research, where general advice ends and how to time food around training is what turns a quick chat on the gym floor into advice a client trusts and can act on. It also makes you the person they come back to when the next trend hits their feed.
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