Why Your Gut Matters for Weight Loss: Part 4

Fibre: When More Is Not Always Better

If I asked you what fibre does, I suspect you would say something along the lines of, “It keeps you regular.” You would not be wrong, but it is a bit like saying your smartphone makes telephone calls. Technically true, but we are missing rather a lot of what else it can do.

Fibre has become one of those nutritional words we all know is supposed to be good for us.   We are constantly bombarded with the advice.  Eat more wholegrains. Eat your vegetables. Add some seeds. Get your 30 g a day. Yet in clinic I regularly see people who have followed that advice faithfully and ended up feeling considerably worse. They add bran to breakfast, lentils to lunch and an enormous pile of vegetables to dinner, then wonder why by 8pm their stomach resembles a beach ball.

So is fibre good for us or not?

Absolutely. But, as is so often the case with nutrition, the answer is a little more interesting than simply “eat more”.

Fibre is essentially the part of plant foods that escapes digestion in the small intestine and travels further along the digestive tract. Some fibres help hold water and add bulk to stools, some form gels that can slow the absorption of nutrients, while others are readily fermented by our gut bacteria. Most foods contain a mixture, which is why I am far more interested in variety than becoming obsessed with whether something is technically soluble or insoluble.

And this is where fibre gets really interesting, because some of it isn't really feeding you at all. It is feeding your microbes.

Think back to our flower bed analogy from earlier in this series. We talked about beneficial bacteria acting like gardeners, helping keep the weeds under control and creating an environment where the whole garden can thrive. Fibre is part of what we feed those gardeners. Give them a varied supply of the plant foods they like, and they can get to work. Give them very little to eat, and the garden becomes much harder to maintain.

But those bacteria do something rather clever with their dinner. When certain fibres reach the large intestine, bacteria ferment them and produce compounds called short-chain fatty acids. The three you will hear about most often are acetate, propionate and butyrate. You certainly don't need to remember their names, but it is worth remembering what they do.

Butyrate, for example, provides an important fuel source for the cells lining the colon. Short-chain fatty acids also help maintain the gut barrier, immune signalling, and metabolic health. Suddenly fibre starts looking rather more interesting than something we sprinkle on breakfast to help us go to the loo.

And for those who want to cut through the food noise, there is also a fascinating link to appetite.  

The gut contains specialised cells that respond to nutrients and microbial metabolites and release hormones involved in hunger and fullness. Two of these are GLP 1 and PYY. You are probably already familiar with GLP-1 because it is the hormone pathway targeted by many of the weight-loss medications. Your body, however, was producing GLP 1 long before anyone put it into an injection pen.

When bacteria ferment certain fibres, the short-chain fatty acids they produce can interact with these appetite signalling pathways. Research suggests this may contribute to the release of hormones including GLP 1 and PYY, helping communicate with the brain that food has arrived and influencing feelings of fullness. Fibre can also affect how quickly food moves through the digestive system and how quickly glucose reaches the bloodstream, which can help with blood sugar control and appetite.

Now, before we all get overexcited, this does not mean adding a spoonful of fibre to breakfast is the natural equivalent of taking a GLP-1 weight-loss medication. It isn't. The effect is much smaller, and human studies show that different fibres do not all produce the same response. But what it does show us is something I find far more interesting: the foods we eat are having conversations with our appetite hormones through the gut and the microbiome every single day.

That matters when we are trying to lose weight. A meal containing protein, healthy fats and fibre is generally far more satisfying than one based predominantly on rapidly digested refined carbohydrates. Fibre adds bulk, certain fibres slow digestion and absorption, and fermentation products may contribute to satiety signalling. Put those things together, and it becomes much easier to understand why improving diet quality can change hunger, rather than weight loss simply being an exercise in trying to ignore it.  Remember the saying, you can’t outrun a bad diet; well, this can help explain a little more about why.  

Fibre may also help with another very important piece of the weight loss puzzle we have talked about throughout this series: blood sugar. Certain viscous fibres can slow the rate at which carbohydrate is digested and glucose enters the bloodstream. Rather than a rapid rise followed by the familiar dip that has you eyeing up the biscuit tin two hours later, the response can be steadier. That doesn't make fibre a magic shield against sugar, but it is another example of how a meal's structure changes how our body responds.

So far, fibre sounds rather wonderful. And it is.  But now we need to talk about the person sitting in my clinic saying, “Every time I try to eat more fibre, I blow up like a balloon.”  This is where generic nutrition advice can start to fall apart.

If you have a healthy digestive system and currently eat very little fibre, gradually increasing it is usually a very sensible thing to do. But if you are already significantly bloated, constipated or dealing with excessive fermentation or certain gut conditions, suddenly adding lots of highly fermentable fibre can make you feel considerably worse. It does not necessarily mean you have suddenly become intolerant to all those healthy foods. Sometimes it is a sign that we need to look at what is happening within the gut and why those foods are causing so much discomfort.

I used the broken leg analogy in the last article, and it is worth repeating because it explains this beautifully. Exercise is brilliant for us. Nobody would argue otherwise. But if you have broken your leg, I am not going to tell you to go for a run because running is healthy. We deal with the problem first, let things recover and then gradually build exercise back in. Fibre can sometimes be similar. Address what is going wrong in the gut first, then gradually reintroduce all that lovely fibre goodness as it becomes better tolerated.

If our digestive conveyor belt is already crawling along, food is hanging around for too long and certain microbes are having rather too much of a party, throwing a large quantity of fermentable fibre into the mix can provide even more substrate for fermentation. The result can be more gas, more distension, more discomfort and a patient staring at me saying, “But I thought this was supposed to be good for me.”
It is good for you. It may simply not be the right type, the right amount or the right time.
That distinction is really important.

This is also why I am wary of fibre challenges where people suddenly decide they must reach 30 g tomorrow. The UK recommendation for adults is around 30 g per day, and most people eat considerably less. It is a useful population target, but your gut does not know that Tuesday is the day you have decided to become healthy. If you have been eating 12 or 15 g and suddenly double it overnight, your microbiome has quite a lot of extra work to do.

Increase fibre gradually. Drink enough fluid. Pay attention to what your gut tells you. And, most importantly, aim for variety rather than simply chasing a number.

Different plants provide different fibres and other compounds, which in turn support different members of the microbial community. Vegetables, berries and other fruits, nuts, seeds, beans and lentils if tolerated, oats and other wholegrains all bring something slightly different to the table. Resistant starch is another interesting example. When starchy foods such as potatoes or rice are cooked and then cooled, some of the starch changes structure and becomes more resistant to digestion in the small intestine, meaning more reaches the large bowel where microbes can use it.

This is why I would rather see someone gradually eating a broader range of plant foods they tolerate than forcing down huge quantities of bran because an app says they are 6 g short of their fibre target.

And if fibre consistently makes you feel worse, don't simply assume you are doing it wrong. Persistent bloating, pain, constipation or excessive wind deserve attention. Sometimes we need to work out why the gut is struggling before we start feeding it more. Poor digestion, altered motility, changes within the microbiome or conditions such as SIBO can all affect how someone responds to certain foods and fibres. In SIBO, for example, excessive numbers of bacteria in the small intestine can start fermenting carbohydrates before they reach the large bowel, contributing to gas, bloating and discomfort. Address what is driving the problem and, as the digestive environment improves, gradually reintroduce foods that were previously problematic in amounts that suit the individual.

This becomes particularly important during weight loss. When people start eating less, whether through deliberate calorie restriction or because they are using GLP 1 based medication and simply feel less hungry, the volume and variety of food can shrink dramatically. If vegetables, pulses, nuts, seeds and other fibre rich foods disappear at the same time, the microbiome is receiving less nourishment just when we want to protect gut health.

The answer is not to force huge salads into someone who already feels uncomfortably full. It is to make the food they can eat count. Protein remains important for preserving muscle, but we also want appropriate fibre, healthy fats and as much plant variety as the gut comfortably tolerates. Weight loss is not simply about making the plate smaller. It is about making every mouthful work harder nutritionally.

Remember, your gut is not a passive tube that simply processes whatever you put into it. It is a living ecosystem, an immune organ, a hormone signalling centre and a vital part of the way your body regulates appetite, blood sugar, hormones, inflammation and metabolism.

Looking after it does not require perfection. It means noticing when something doesn't feel right, feeding the microbes that help you thrive, keeping the digestive conveyor belt moving and understanding that sometimes the answer isn't more restriction, but better nourishment. In general, we want more real, minimally processed food, less ultra processed food, less added sugar and plenty of fibre from a varied diet. But, as we have seen throughout this series, context matters. The healthiest food in the world isn't particularly helpful if your gut is currently making you miserable every time you eat it.

Your gut has been talking to you throughout this whole series. Perhaps it is time we started listening. 

Next time, we are going to talk about something I hear about all the time in clinic: food noise and cravings. Why can you eat a perfectly good meal and still find yourself thinking about chocolate an hour later? Why are some days so much harder than others, and why does willpower often have very little to do with it? We will look at the role of blood sugar, appetite hormones, stress, sleep and even your gut, and most importantly, what you can actually do to quieten the noise and feel more in control around food.