Is diversity important? The impact of plant food diversity on gastrointestinal and cardiometabolic health

The UK does not eat enough plant foods despite significant strides in understanding their impact on health. Barriers to increasing plant food consumption are complex and interventions to increase consumption have shown limited long-term change. Diversifying plant food intake may be a more achievable option, but the health effects of such an approach are unknown, a challenge our research aims to address.

Evidence for the benefits of plant foods, in particular the quantity of fruits and vegetables is convincing. In contrast, there is a lack of evidence investigating the impact of plant food diversity on health and disease. Despite this, the World Health Organisation (WHO) recommends increasing "the variety of fruits and vegetables consumed" to "ensure a better general dietary pattern" , which although theoretically increases odds of achieving nutrient requirements, has not been extensively tested.

Recommendations focus on fruits and vegetables, excluding other nutrient-dense plant foods with proven health benefits (e.g. grains, legumes, nuts, seeds). Observational data shows associations between plant food diversity and gut microbiota diversity, a common marker of gastrointestinal health and researchers have begun elucidating mechanisms by which diet-related changes in the gut microbiome benefit cardiometabolic health.

Still, there is lack of intervention studies and high-quality evidence for the role of plant food diversity on gastrointestinal and cardiometabolic health.

Despite two decades of public health campaigns, the WHO recommendations for quantity of fruits and vegetables (at least 400g/d = 5 portions/d) remain unmet. Failure to meet this recommendation contributes to health inequalities. Unfortunately, interventions to increase the quantity of fruit and vegetable consumption have variable success.

The most effective method to change behaviour is to limit the need to change behaviour. Focussing on diversity of plant food intake is a strategy potentially requiring less behaviour change than increasing quantity. Research into the health effects of plant food diversity stands to inform future public health strategy and food industry focus. For example, focussing on diversity via reformulation of food products or multi-buy food offers.

Our research will address a crucial gap in scientific evidence by establishing whether increasing plant food diversity in a population that a) consumes below UK average quantity and diversity of plant foods and b) does not achieve recommended fibre intakes, will improve dietary intake, and confer gastrointestinal and cardiometabolic health benefits. The project will address the following objectives: first, we will design two test diets, both providing the same quantity of plant foods (UK average), and either a) high diversity of plant foods (intervention) or b) UK average diversity of plant foods (control); next, we will conduct a randomised controlled parallel-design feeding study comparing the intervention and control test diets on health outcomes in healthy adults; we will then assess the impact of test diets on dietary intake; and finally we will measure the impact of plant food diversity on the gut microbiome, physiology and symptoms, and cardiometabolic outcomes.

This will be the first RCT to assess the impact of plant food diversity on human health, paving the way towards enhancing the efficacy of current public health messages. Low intakes of fruits and vegetables are associated with poor health outcomes, thus by conducting the trial in this population, we aim to determine the potential health gains that can result from focussing on diversity, rather than quantity, in a cohort that stands to benefit most.

The results will provide direction for future research into the benefit of plant food diversity for the maintenance of health, informing public health messaging, food industry guidance and educational strategies to improve dietary intake and limit the burden of disease.