Abstract
The sine qua non of intervention studies in general, and randomized controlled trials in particular, is to define and isolate an exposure of interest that defines the intervention and distinguishes between groups. The isolation of a presumptive cause is a prerequisite to the confident attribution of given effects. In the context of dietary intervention studies, this has historically translated into a unitary intervention diet type, no matter the diversity of preferences, tastes, upbringings, ethnicities, and cultures represented in a given study cohort. To the extent such diversities have been constrained to achieve this aim, generalizability (external validity) has been diminished. To the extent such diversities have been ignored, inattention to them has likely shifted results toward the null and compromised adherence over time. These same liabilities pertain to food service projects and public health nutrition, notably the food-as-medicine movement. We propose a remedy to these issues and an update to the formula for dietary intervention research (and service) that accommodates a multicultural society: the fixed-quality, variable-type (FQVT) nutrition intervention. This method standardizes the objective measure of diet quality and incorporates fixed tolerances for nutrients of particular interest, while allowing for a range of diet types responsive to the variable preferences of study participants/population members. We describe the application of tools to facilitate this methodological innovation, enumerate the expected advantages, and characterize means of empirical testing. We submit the FQVT method as a promising, testable advance in the evolution of clinical nutrition research and food-is/as-medicine programming.