Sustainable diets and considerations for the gut microbiome

Sustainability is a word that is fast becoming a household term in the food and nutrition sector. The concept of a sustainable diet is one that promotes optimal human health whilst having a low environmental impact, considers social and cultural differences and is economically accessible to all (World Health Organization (WHO)., 2019). Because of the multi-factorial nature of sustainability, significant questions remain around how food systems can collectively incorporate these dimensions and achieve sustainability targets in a time-sensitive manner.

Plant-based diets

One such suggestion for achieving sustainability and address environmental concerns is shifting towards a plant-based dietary pattern, which has a lower environmental impact per calorie or gram (British Nutrition Foundation., 2021; Food and Agriculture Organization of the United Nations (FAO) & WHO., 2019). Whilst plant-based dietary patterns are nutrient-dense and have been associated with favourable health outcomes, including lower incidences of non-communicable diseases and rates of all-cause mortality, animal-based foods offer notable quantities of many micronutrients that are highly bioavailable, such as vitamin B12, iron and calcium, that might be challenging to obtain in sufficient quantities in a diet solely made up of plants (Rosenfeld et al., 2023).

Plant proteins, the gut microbiome and proteolytic fermentation

Micronutrient bioaccessibility and bioavailability are not the only concerns of a plant-based diet as plant-based proteins also raise questions. Plant-based proteins are often described as incomplete due to lower quantities of essential amino acids and comprise of structures that can impact digestibility (Boven et al., 2024). Some of these structures, referred to as anti-nutrients, include phytates and oxalates that can interfere with the breakdown and absorption of nutrients by blocking the activity of digestive enzymes needed for these functions (Lonnie and Johnstone., 2020; Nath et al., 2022). Together, these factors can reduce protein bioavailability and make them harder to digest (Sá et al., 2019).

When amino acids are not taken up via the small intestine, they reach the colon where they are fermented by the gut microbiome. Gut microbes have a preference for carbohydrates and fibre, which serve as their primary energy source, leading to the production of metabolites such as short chain fatty acids (SCFAs), which offer a multitude of benefits to the host (Gilbert et al., 2018). This beneficial breakdown of dietary fibres by gut microbes is referred to as saccharolytic fermentation, however, a higher degree of undigested protein in the colon leads to a shift from saccharolytic to proteolytic fermentation (Boven et al., 2024). Where saccharolytic fermentation is associated with beneficial metabolites, proteolytic fermentation increases the production of ammonia, amines, phenols and branched short chain fatty acids which are associated with impaired gut barrier function and inflammation (Boven et al., 2024; Gilbert et al., 2018). Along with the production of undesirable metabolites, the more protein that is fermented in the gut also reduces the quantity of protein available for uptake and utilised by the body (Boven et al., 2024).

Solutions

It is important to note that plant and animal-sourced proteins have different overall nutrient profiles that mean they can be both be beneficial and included as part of a healthy, balanced diet (Day et al., 2022). However, to facilitate a population-wide shift towards a dietary pattern richer in plants, the concerns around the bioavailability of certain nutrients, including proteins, need to be addressed.

One such solution is the inclusion of complex carbohydrates, such as prebiotic fibres, which could promote saccharolytic fermentation. Prebiotics act as an energy substrate and can stimulate the growth of beneficial bacteria (Korpela., 2018). Whilst research in relation to plant-based proteins is in its infancy, theoretically, the growth of beneficial bacteria by prebiotics could support saccharolytic fermentation and reduce the production of harmful metabolites which may result from the fermentation of protein, whilst also increasing SCFA concentration and supporting a favourable microbiome composition (Boven et al., 2024; Korpela., 2018).

Conclusion

Despite the challenges of plant-based diets, when appropriately planned they can be sufficient to meet dietary requirements and beneficial to health. However, as research begins to uncover more about the complexities of human health and the importance of the gut microbiome, shifting more towards plant-based diets require further attention to address the nutritional question marks that exist.

By Róisín Pichon, ANutr.

References

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