By Dr Sheena Fraser
13th Feb, 2023
As a practicing doctor for over 20 years, I see patients with constipation daily at work. It’s normal practice to advise a more fibre rich diet and more fluids but inevitably we end up prescribing laxatives long term for many people. We rarely consider the long-term risks of having a sluggish gut despite us knowing that it often goes hand in hand with other disease. I never considered the significance of this until my recent research into the gut microbiome. Time and time again the research papers I was reading discussed the importance of a healthy gut transit time (time it takes for food to travel from mouth to anus). This year a paper in “Gut” by Nicola Prochazkova et al1 discussed the role of gut transit time in determining the health of the gut microbiome and how this links with many diseases. I want to share with you the conclusions of the paper and why I believe that we should all start taking more interest in the gut transit time of our patients and ourselves.
So, what is a normal gut transit time and what influences it? The average gut transit time in the UK is 30-40 hours but it is still considered normal to have a gut transit time up to 72 hours. The definition of constipation is less than 3 bowel movements a week. Women suffer from more constipation than men and our incidence of constipation increases as we age from 15% of children and adults under 60, to 33.5% of adults over 60. People living in nursing homes are 50% likely to suffer from constipation and a whopping 70% of patients in long stay hospital wards are constipated (NiCE guidelines). However, if you are consuming a diet with adequate fibre (>30g per day) your gut transit time will be much lower, the average in my family is 24 hours.
We can measure gut transit time easily just by consuming foods that are easy to spot in our stool. Beetroot, sesame seeds and sweetcorn kernels are readily used for this reason. Researchers sometimes use blue dye in food to make it even more obvious. I have used the beetroot method and found it very easy to determine my own gut transit time and have recommended it to patients, most people find this acceptable, even my kids managed it with the help of some vinegar.
So why should we be bothered by a longer gut transit time? I guess apart from the obvious discomfort, bloating and loss of appetite that constipation brings, what really interests me is what is happening at a more microscopic level. The causes of constipation are numerous, from a genetic predisposition to a lack of fibre in the diet, diseases affecting gut motility, ultra-processed foods, drugs affecting gut motility, stress, low fluid intake and sedentary behaviour. By far the commonest reason for constipation that I see is a dietary lack of fibre and over-consumption of processed foods.
According to the British Dietetic Association (BDA), the average fibre intake in the UK is only 60% of the recommended 30g per day we should be ingesting. We also know that the energy intake from ultra-processed foods in the UK is 56.8% of our total energy intake from food according to a 2019 study 2. In other words, the average person is consuming a diet high in processed foods and low in fibrous whole foods.
So how do these foods influence our gut transit time? Fibre comes in different forms and I have discussed its role on the microbiome in a previous blog – The Trouble with Fibre – essentially it softens and bulks the stools, directly influences the gut wall and neurones within it speeding up motility. In addition, it is fermented in the colon by beneficial bacteria who then produce short chain fatty acids (SCFA’s) which aid serotonin synthesis. Serotonin speeds up gut motility too. The beneficial bacteria grow in number and support the health of the gut wall and prevent harmful bacteria from over growing.
Ultra-processed foods on the other hand are generally absorbed through the small bowel and are high in fat and sugar. They encourage the growth of more harmful bacteria and a build-up of toxic secondary bile acids which can directly affect the gut wall, influence motility, and lower the amount of SCFA’s being produced by outcompeting the beneficial bacteria. High levels of more harmful bacteria in the small bowel (small intestinal bacterial overgrowth) are associated with irritable bowel disease and constipation. In addition, if a diet is low in fibre the bacteria in the colon run out of fibre to ferment so start breaking down proteins from the diet or from the mucous lining the wall. This results in damage to the gut lining and can potentially trigger a leaky gut – look it up, it is real – or check out Microbiome Medics next podcast! When these proteins are metabolised, there are toxic metabolic by-products containing sulphur, ammonia and methane which are damaging to the gut wall creating inflammation and damage and this worsens constipation.
A prolonged gut transit time is associated with a loss of microbial diversity, altered composition, and reduced beneficial metabolic function of our gut microbiome. Unsurprisingly, many diseases are associated with prolonged gut transit time. These include irritable bowel syndrome and colitis, neurological disorders such as Parkinson’s disease, Alzheimer’s and Multiple Sclerosis as well as metabolic disorders like diabetes, ischaemic heart disease and some cancers Eg. bowel and breast 1.
I would urge all clinicians to take constipation seriously, if we can speed up gut transit time, we will alter the composition of the gut microbiota and in doing so reduce the risks of further disease. The best way of doing this is a change of diet, feeding the colonic bacteria with whole foods rich in fibre and polyphenols and restricting ultra-processed foods, maintaining an active lifestyle free from stress, and getting enough sleep to allow our beneficial bacteria to thrive. Laxatives can help initially but don’t address the harmful lifestyle choices that lead to constipation and will not prevent the diseases coming further down the line.
For more information on the gut microbiome check out “Gut Microbiome for Clinicians” an online course available via BSLM and the newly launched “microbiome Medics” podcast available on Apple and Spotify.
- Advancing human gut microbiota research by considering gut transit time. Nicola Prochazkova et al; Gut 2023; 72; 180-191
- Ultra processed foods and excessive free sugar intake in the UK: a nationally representative cross-sectional study. Fernandinho Rauber et al; BMJ open 2019; 9: c027546