Low-carb diets and gut health
In my first post on gut health, I talked about some dietary changes you could make to support your beneficial gut bacteria. For part two, I'm looking a little closer at the long-term effects of "low carb" diets on both gut health and overall health.
If regular consumption of wholegrain and other high-fibre carbohydrates supports gut health by providing a food source for beneficial bacteria, what happens when we follow a low-carb diet?
Studies have shown a reduction in the population of members of the Bifidobacterium genus of bacteria, as dietary carbohydrates are reduced. These probiotic bacteria are known to have a positive effect on both gut health and overall health. Other populations of beneficial bacteria were also affected. In addition, low carbohydrate and high protein diets have lead to a decrease in the production of butyrate, a short-chain fatty acid with anti-inflammatory properties, known to protect against colon cancer. Butyrate also helps to maintain the health of the lining of the small intestine.
Along with the negative effects on beneficial gut bacteria, a low-carbohydrate, high-protein diet can be deficient in a number of important nutrients. Dietary fibre is an obvious one, but such diets are also typically low in vitamins A and E, vitamins B1, B6 and B9, calcium, magnesium, iron and potassium.
Reduced fibre consumption could increase the risk of cardiovascular disease and Type 2 diabetes. Vitamins A and E are integral to skin health and are powerful antioxidants. The B vitamins listed above are involved in energy metabolism, red blood cell formation and DNA synthesis. Calcium is essential for bone health, as is magnesium, while potassium helps maintain fluid balance in the body, and iron is essential for oxygen transport, immunity and cognitive function.
Due to the higher intake of protein, especially if the protein is from animal sources, such diets are often higher in saturated fat and cholesterol. An increase in saturated fat and cholesterol in the diet is associated with an increased risk of cardiovascular disease.
Another major consideration with low-carb diets is the reduction in fruit and vegetable consumption required to maintain a reduced carbohydrate intake. While many people view reducing their intake of starchy foods (such as breads, cereals, baked goods and potatoes) as following a low-carb diet, fruit and vegetables also contain carbohydrates. Consequently, if an individual is to follow a truly low-carb diet, their intake of fruit and vegetables will be significantly lower than current recommendations. As this food group is a major source of compounds that protect against cancer, such a reduction in their consumption can lead to an increased risk of many forms of cancer.
Finally, in a large study of over 15,000 adults in the United States, both low-carbohydrate (below 40% energy from carbs) and high-carbohydrate (over 70% energy from carbs) diets were associated with an increase in mortality (death), with the carbohydrate intake associated with the lowest mortality at around 50-55% of total energy. This moderate carbohydrate "sweet spot" lines up with current dietary guidelines, and allows for a diet that includes a variety of foods from all four food groups. For more information on current dietary guidelines for New Zealanders, read this.
All these factors paint an interesting picture of the potential effects of low-carb, high-protein diets in the long term. If you are seriously considering this way of eating in the short-term, seek the advice of a qualified nutrition professional. They will take into account any pre-existing risk factors that may be exacerbated by this diet, and help ensure you meet your nutrient needs.
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Brinkworth, G.D., Noakes, M., Clifton, P.M., & Bird, A.R. (2009). Comparative effects of very low-carbohydrate, high-fat and high-carbohydrate, low-fat weight-loss diets on bowel habit and faecal short-chain fatty acids and bacterial populations. The British Journal of Nutrition, 101(10), 1493-1502.
Conlon, M.A., & Bird, A.R. (2014). The impact of diet and lifestyle on gut microbiota and human health. Nutrients, 7(1), 17-44.
Seidelmann, S.B., Claggett, B., Cheng, S., Henglin, M., Shah, A., Steffen, L.M., Folsom, A.R., Rimm, E.B., Willett, W.C., & Solomon, S.D. (2018). Dietary carbohydrate intake and mortality: A prospective cohort study and meta-analysis. The Lancet, 3(9), 419-428.
Willson, K., & Situ, C. (2017). Systematic review on effects of diet on gut microbiota in relation to metabolic syndromes. Journal of Clinical Nutrition and Metabolism, 1(2).