How can lipids in the diet impact our gut microbiota?
Our dietary habits have changed considerably over the last few decades, with the excessive consumption of energy-dense foods, which is partly responsible for the development of numerous chronic diseases with a nutritional component, such as obesity, diabetes and cardiovascular disease. It is now recognised that certain nutrients in our diet can play a role in the prevention and progression of these diseases, provided that they are consumed in sufficient quantities.
Consumption of omega 3 below recommendations
In the specific case of lipids, the most recent data on the consumption and eating habits of the French population have shown that fatty acid intakes are qualitatively unbalanced in relation to nutritional recommendations (INCA31 study). While intakes of omega-6 polyunsaturated fatty acids (PUFAs) appear to be adequate, intakes of omega-3 PUFAs and monounsaturated fatty acids (MUFAs) are insufficient. Consequently, improving the bioavailability of inadequately consumed fatty acids without increasing total fat intake is a proven nutritional objective with positive public health implications.
To exert their health effects, fatty acids in the diet must be made bioavailable through the digestion/absorption process. Various factors can affect their initial bioaccessibility in our digestive tract (i.e. the fraction ingested that can be absorbed) and then their bioavailability in the body. Among them, the intestinal microbiota has been considered for about ten years as a major parameter capable of modulating the fate of fatty acids within the cells of our intestine.
The intestinal microbiota: an ally of choice for our health
The gut microbiota is the most complex ecosystem in our body, consisting of nearly 100,000 billion commensal bacteria, fungi and viruses distributed throughout our gastrointestinal tract. It performs many functions that are essential for our health. It is involved in maintaining the integrity of our intestinal cells, the maturation of our immune system, energy recovery, the production of essential vitamins for the body, as well as the absorption of dietary fat and the storage of fat. The balance of the intestinal microbiota, which is specific to each individual, is important to ensure its functions and its composition can be modified by different compounds and nutrients, including fatty acids. In certain metabolic pathologies such as diabetes or obesity, an imbalance of the intestinal flora (called dysbiosis) is observed, which can be corrected by compounds with prebiotic properties: the lipids in our diet can play this role.
Fatty acids in our diet modulate the composition and function of the gut microbiota
Omega 3 PUFAs, whether of plant origin (alpha-linolenic acid, ALA, present in rapeseed, walnut or flaxseed oils) or animal origin (EPA and DHA present in marine sources such as fish), have been the subject of several studies on this subject, with more data for EPA and DHA than for their metabolic precursor, ALA. Concerning the latter, it has been shown that the consumption of walnuts, a nut rich in ALA, could favour an enrichment of the intestinal microbiota in beneficial bacteria of the probiotic type, associated with a reduction of rather deleterious bacteria. The combined intake of ALA(via flaxseed oil consumption), EPA and DHA(via fish oil consumption) showed similar beneficial effects.
Concerning omega-6 PUFAs, fatty acids that are better known for their pro-inflammatory effects, few studies have investigated their impact on the modulation of the composition of the intestinal microbiota. Consumption of a high-fat diet containing corn oil, a source of linoleic acid (LA, a precursor of omega 6), induces an enrichment of the microbiota in bacterial species known to be involved in various inflammatory processes. The association with fish oil, a source of EPA and DHA, makes it possible to correct the dysbiosis thus induced by favouring probiotic-type bacteria. LA can also be converted by our intestinal microbiota into a derivative with beneficial effects on health, a basic production of this derivative by a specific bacterium of the human intestinal tract having been described. This synthesis would appear to be a detoxification mechanism for the bacteria in the gut, which is very useful for our gut health.
Other data have shown that the omega-9 MUFAs present in olive oil (oleic acid being the major representative) could have a beneficial impact on the composition of the intestinal microbiota. It has been shown that the consumption of extra virgin olive oil, a source of oleic acid, can reduce the levels of bacteria in the faeces that are known to produce compounds that are potentially harmful to our colon.
All these data related to the study of the impact of dietary fatty acids on the intestinal microbiota have been the subject of a research programme conducted by ITERG and its academic partners since 2017 as part of the UMT ACTIA BALI2 "Biodisponibilité Alimentation Lipides Intestin". For the past 5 years, the work of this programme has focused on a line of research that was not invested in at the beginning, aiming to explore the link between the bioavailability of lipid nutrients (such as dietary fatty acids) and the intestinal microbiota.
In summary
The quantity, type or combination of fatty acids in our diet are all parameters that can modulate our intestinal microbiota in a more or less beneficial way. The data acquired to date tends to associate omega-3 PUFAs and omega-9 MUFAs with beneficial changes in its composition. Because of the nutritional status of the French population for these fatty acids and the need to increase their daily intake without increasing the consumption of total lipids, modulation of the intestinal microbiota to increase the bioavailability of these nutrients of interest is part of current research. This is the focus of the UMT ACTIA BALI's work on this topic3.
An article published in AFI, La revue des industries agro-alimentaires (May-June 2021, n°73) and a review of our work to be published in Critical reviews in food science and nutrition (Vande Weghe et al, 2021, under revision) give more details on these modifications of the intestinal microbiota associated with dietary fatty acids.