Evidence that food biodiversity and dietary diversity improve nutrition and health outcomes
Evidence from intervention studies
Successive reviews of NSAP studies have found little evidence of an impact of production strategies, including production diversification strategies, on micronutrient intakes, micronutrient status or anthropometric measures. It is likely that methodological limitations in these studies have hampered their ability to demonstrate impacts on nutrition and health outcomes. Programs that have included production diversification strategies and have successfully improved micronutrient intakes have generally included well-designed behaviour change communication components and a strong focus on women's status and empowerment through agriculture (Ruel, 2001; Girard et al., 2012; Ruel et al., 2018).
Evidence from observational studies
Evidence from observational studies on the relationship between food biodiversity and nutrition and health outcomes is similarly weak and inconsistent. The strongest evidence is for a weak association between cultivated biodiversity and child linear growth (using height-for-age Z scores), with the greatest effect size among older and severely stunted children (Kumar et al., 2015; Jones, 2017). However, the magnitude of observed associations is small. No consistent associations have been observed between cultivated biodiversity and other indices of child anthropometric status (weight-for-age and weight-for-height Z scores) (Jones, 2017).
Nonetheless, the importance of dietary diversity to nutrition and health outcomes is supported by evidence from large-scale nutrition epidemiological studies, most of which have been conducted in high-income countries. The most robust evidence available is for a protective effect of fruit and vegetable diversity in the diet against cardiovascular diseases and certain cancers (Jansen et al., 2004; Wirt and Collins, 2009; Buchner et al., 2010; Isa, 2013). There is limited evidence for the protective effect of diversity within other food groups, including cereals and animal-source foods, and for other health outcomes.
There has been growing emphasis on healthy dietary patterns containing a variety of healthy foods, particularly fruits and vegetables, and other plant- based foods (as well as lowering intakes of red and processed meats, other highly-processed foods, and refined grains), rather than on individual foods and nutrients. The most recent exemplar of this is the Planetary Health Diet developed by the EAT-Lancet Commission on Food, Planet and Health (Willett et al., 2019). Dietary patterns that have received the most attention in the literature to-date include the Mediterranean-style diet and Dietary Approaches to Stop Hypertension (DASH), as well as general Dietary Guide lines-related patterns. There is strong evidence for the protective effect of adherence to one of these healthy dietary patterns against cardiovascular diseases including coronary heart disease and stroke (USDA, 2014; Katz and Meller, 2014; Tapsell et al., 2016), and moderate evidence of an association with more favourable outcomes relating to body weight and obesity risk (USDA, 2014). There is some limited evidence of a reduced risk of type II diabetes (USDA, 2014), all-cause mortality (Sofi et al., 2008; Wirt and Collins, 2009), overall or specific cancer risk (McCullough et al., 2000a, 2000b; Hamack et al., 2002; Benetou et al., 2008; Wirt and Collins, 2009; Couto et al., 2011), and development of metabolic syndrome (Meydani, 2005; Serra-Majem et al., 2006). However, it is not possible to distinguish the specific role of dietary diversity to these associations. Overall, given the long-standing emphasis in dietary guidelines on the importance of consuming a diverse diet, it is surprising, and unfortunate, that there has not been greater explicit attention to dietary diversity in nutrition epidemiological studies.