Non-communicable diseases (i.e., cardiovascular diseases including heart attacks and strokes, cancers, chronic respiratory diseases and diabetes) are still a major cause of mortality, especially in countries with high incomes. It is also worth noting that a diet high in sodium and low in whole grains, as well as alcohol use, are among the top 10 risk factors that explain the number of deaths worldwide.
While the ideal gut microbiome is far from being known, scientists have detected some features in fecal samples from patients with an altered gut microbiome composition or functions associated with non-communicable chronic diseases. Briefly, those features consist of low gut microbial richness or diversity, a depletion of short chain fatty acid-producing bacteria and an instability in the gut microbiota’s composition over time.
Dietary variety is related to gut microbiome diversity and a greater abundance of some potentially beneficial bacteria. In particular, the quantity and diversity of fibers in an individual’s diet is key to promoting a diverse gut microbiome packed with a wide range of functions that benefit the host. However, limited studies have explored how particular food staples can promote this gut microbiome diversity.
A new small study in adult subjects with some cardiovascular risk factors showed that consuming 150 g per day of multi-fiber bread over two months modified gut microbiome composition and functions and reduced cholesterol and insulin sensitivity parameters, compared to consuming standard bread.
The authors explored the impact of both quantity and quality of dietary fiber on cardiovascular health through the gut microbiome in adults with high triglycerides and/or low high-density lipoprotein cholesterol (“good cholesterol”) and/or high low-density lipoprotein cholesterol (“bad cholesterol”) and/or high total cholesterol. In an initial step, participants were randomly allocated to the multi-fiber bread or standard bread group for 8 weeks and, afterwards, participants followed the intervention they had not received initially for another 8 weeks.
The daily consumption of 150 g (equivalent of three slices) of multi-fiber bread with a mixture of 7 types of soluble and insoluble fibers (16 g of fiber in total) changed both the gut microbiome’s composition and its functions. Specifically, the multi-fiber bread impacted the abundance of eight different gut microbe enzymes specialized in degrading complex carbohydrates.
In contrast, the daily consumption of 150 g of standard sourdough bread (5.6 g of fiber) did not alter gut microbiome composition and functions.
Compared to standard bread, multi-fiber bread led to a decrease in total cholesterol, low-density lipoprotein cholesterol, insulin and insulin resistance, assessed by the Homeostatic Model Assessment of Insulin Resistance (HOMA-IR), which tells us how much insulin the pancreas needs to synthesize to keep blood sugar levels under control. Although the decrease in LDL cholesterol blood levels was small (-0.36 mM), the authors acknowledged that such a decrease in “bad cholesterol” is relevant in terms of reducing cardiometabolic risks, as 1 mM of LDL cholesterol reduction has been shown to be associated with a 19% fall in cardiovascular mortality risk.
Interestingly, the increase in some microbial gene families involved in digesting dietary fiber was inversely related to peaks of glucose and insulin after meals. That means that the improvement of cholesterol levels and insulin sensitivity parameters observed may be partly mediated by the activity of gut microbes.
Although the study is small, it suggests the gut microbiome is involved in mediating the cardiovascular health benefits of increasing both quality and quantity of dietary fibers. While we have known for a while about the many benefits of dietary fiber, it was not until recently that scientists elucidated why it is so beneficial. On the whole, it seems that prioritizing multi-fiber breads over single-fiber breads as food staples in daily diets offers potential metabolic benefits in people showing high cardiovascular risk.
Roser M, Ritchie H. Burden of disease. Our World in Data, September 2021. Available: https://ourworldindata.org/burden-of-disease (Accessed: 4 July 2022).
Our World in Data. Number of deaths by risk factor. 2019. Available: https://ourworldindata.org/grapher/number-of-deaths-by-risk-factor (Accessed: 4 July 2022).
Huang X, Gao Y, Chen W, et al. Dietary variety relates to gut microbiota diversity and abundance in humans. Eur J Nutr. 2022. doi: 10.1007/s00394-022-02929-5.
Ranaivo H, Thirion F, Béra-Maillet C, et al. Increasing the diversity of dietary fibers in a daily-consumed bread modifies gut microbiota and metabolic profile in subjects at cardiometabolic risk. Gut Microbes. 2022; 14(1):2044722. doi: 10.1080/19490976.2022.2044722.
Zimmer C. Fiber is good for you. Now scientists may know why. The New York Times, January 1, 2018. Available: https://www.nytimes.com/2018/01/01/science/food-fiber-microbiome-inflammation.html (Accessed: 4 July 2022).