Balancing dietary fibre and protein for a healthy gut
By Dr. Silvia W. Gratz, Rowett Institute of Nutrition and Health, University of Aberdeen
Many people recognise that maintaining an appropriate body weight is very important for health. However, losing weight is not easy and so diets which are high in protein and restricted in carbohydrate are popular as they help to promote a feeling of being full at a lower calorie intake than normal.
However, low intakes of carbohydrates and especially dietary fibres from cereal and vegetable sources are problematic in terms of gut health. For example, a low intake of dietary fibre is linked to an increased risk of developing colorectal cancer. Low intakes of carbohydrates and dietary fibres change the populations of bacteria that live in the human colon. This decreases the production of beneficial bacterial fermentation products such as the short chain fatty acid butyrate, a small molecule that maintains healthy intestinal cells.
A diet high in protein also leads to more protein being transported into the colon and colonic bacteria use these proteins and amino acids as an energy source. When amino acids are fermented in such a way, several potentially harmful by-products are released into the gut lumen. Among these are N-nitroso compounds, which are a group of cancer causing chemicals, formed in the gut. These compounds are increased in the stool of individuals who consume diets high in red and processed meat.
Therefore, a healthy balance between dietary protein and red meat which helps us feel full, and dietary fibres which are important to keep our gut healthy is essential to promote overall health.
- Gut bacteria in the colon break down carbohydrates to produce beneficial short chain fatty acids. If diets are high in protein, gut bacteria will also break down protein and amino acids and produce an array of different products.
- Some bacterial breakdown products from protein and red meat will damage DNA in gut cells. This damage is believed to be an early event in cancer development.
- The World Cancer Research Fund and the UK Scientific Advisory Committee on Nutrition (SACN) therefore recommend that red meat consumption should be limited to no more than 500 g a week, with as little processed meat as possible to minimise the risk of developing colorectal cancer.
- New evidence suggests that the potential adverse effect of high protein and high meat diets in the gut could be counteracted by incorporating increased dietary fibre into the diet. However, this needs to be proven in humans.
Our research focuses on exploring the relationship between dietary protein, red meat, carbohydrate and dietary fibre on the formation of potentially harmful breakdown products in the human gut. We conduct carefully controlled dietary intervention studies in human volunteers, which enable us to precisely monitor the food consumed by each volunteer over several weeks. Stool samples are collected from each volunteer and an array of bacterial fermentation products are measured. We can then study the effect of diet on the bacterial fermentation products found in the stool samples.
We found a decrease in beneficial short chain fatty acids when volunteers consumed low carbohydrate diets. At the same time we found increased levels of N-nitroso compounds with high red meat intake (Russell et al. 2011). Besides red meat, we confirmed that high dietary nitrate from lettuce, spinach and some root vegetables also increases the formation of N-nitroso compounds in the human gut. On the other hand, foods rich in vitamin C and dietary fibre from cereal products protect against the formation of such toxic compounds in the gut (Holtrop et al. 2012).
This observation suggests that dietary fibre can ameliorate the changes in gut fermentation seen with high protein diets. In ongoing studies we are incorporating different kinds of dietary fibres into diets that are relatively high in protein and red meat. With these modifications we aim to prevent or limit the production of harmful fermentation products inside the human gut.
Our research highlights the importance of balancing potentially problematic foods in our diet such as red meat and high-nitrate foods, with protective foods that are rich in vitamin C and dietary fibre. These findings will help us to develop diets that will benefit from the satiating (feeling full) effect of dietary protein while counteracting any potential negative effects on intestinal health. Our results may also assist the food industry with the development of new products, or reformulation of existing products, which can aid weight loss as part of a healthy balanced diet.
Russell W. R., Gratz S. W., Duncan S. H., Holtrop G. et al. (2011): High protein, reduced carbohydrate weight loss diets promote metabolite profiles likely to be detrimental to colonic health. Am J Clin Nutr 93(5):1062-1072.
Holtrop G., Johnstone A. M., Fyfe C. & Gratz S. W. (2012): Diet composition is associated with endogenous formation of N-nitroso compounds in obese men. J Nutr 142(9):1652-58.
Source: The Nutrition Society