strong benefit of this model, which underpins the protein leverage hypothesis of obesity, is that it uses human biology as a guide to identifying the most important ecological questions around the global obesity epidemic. Importantly, there already are public health frameworks in place with which to integrate the protein leverage model and help answer those questions to illuminate the nutritional ecology of human obesity.
Some relevant frameworks from public health nutrition
An increasingly prominent framework in public health research is that of the “food environment.” This concept first emerged in ecology to refer to the abundance and nature of foods in the environment in which an animal forages [e.g. 74]. In public health, the term was adopted in the context of socio‐ecological theory, around the realization that health‐related behaviors are determined by the interaction of personal and environmental factors [75,76]. It continues to be defined in this way, referring not just to foods available, but also to the factors within the environment that influence the accessibility and choice of foods: “Food environments comprise the foods available to people in their surroundings as they go about their everyday lives and the nutritional quality, safety, price, convenience, labeling and promotion of these foods” [75].
Food environments can be thought of as the consumer interface with a broader system, the “food system,” which comprise “all the elements (environment, people, inputs, processes, infrastructures, institutions) and the activities that relate to the production, processing, distribution, preparation, and consumption of food, as well as the output of these activities, including socioeconomic and environmental outcomes” [77]. In this respect they are closely parallel to the concept of the “ecosystem,” developed in ecology as a holistic framework for understanding the influence of both biological (such as competition and predation) and non‐biological (such as climate and soil nutrients) factors on organisms [78,79].
A second relevant public health framework is the “nutrition transition” concept [80,81]. This framework focuses on the stages through which human societies transition in terms of food and beverage consumption and patterns of physical activity. Although conceived within a public health context, it has an ecological‐evolutionary underpinning in that it emphasizes mismatch between evolved human biological traits, such as preference for sweet tastes, and changes to human food environments that in terms of health outcomes are incompatible with these traits, such as abundant availability of sweet foods in obesogenic food environments.
Both of the above concepts are highly relevant to the protein leverage hypothesis because they frame the important question this hypothesis highlights around the ecological drivers of obesity: what has changed in human food environments concurrent with the rise of obesity that might dilute protein, causing the strong protein appetite to leverage over‐consumption of energy? There is now widespread agreement that this is related to the industrialization and globalization of food systems, in which diets rich in legumes and other vegetables, fruits, and coarse grains are replaced by diets low in fiber and micronutrients, and rich in highly processed oils and carbohydrates, and increased consumption of animal‐sourced food [82]. In the United States and many other higher‐income countries, this transition began in the 1960s, and has happened more recently in many lower‐middle‐income countries.
A third public health framework that can play a significant role in understanding how nutrition transitions create obesogenic environments is the NOVA food classification system. The NOVA system classifies foods into four categories according to the extent and purpose of industrial processing [83]. Group 1 foods are unprocessed or minimally processed, including natural foods that have been altered by, for example, drying, roasting, boiling, pasteurization, or the removal of unwanted parts. Group 2 comprises processed culinary ingredients, which are seldom consumed alone but combined with group 1 foods in the preparation of meals, for example salt, vegetable oils, honey, sugar, starches, and butter. Group 3, “processed foods,” are relatively simple products made by adding group 2 items, such as sugar, salt, or oil to unprocessed (group 1) foods, with the main aim of increasing their durability or enhancing their sensory qualities. Group 4, “ultra‐processed foods,” are highly processed industrially manufactured products typically formulated from many ingredients. Their ingredients include those also used in group 3 foods, such as salt, sugar, and fats, and substances rarely used in cooking, such as additives that mimic the sensory qualities of group 1 foods.
The NOVA system has been criticized for being “rather simple and crude” compared to other classifications that are based primarily on nutritional composition [84]. In our view, however, the fact that it does not focus specifically on nutrients but on processing is a significant strength of the NOVA system, which enables it to integrate diet and health with the food environment and nutrition transitions frameworks to the extent that nutrient‐based classification could not. One reason for this is that the healthfulness of foods is determined not only by their nutritional content but the physicochemical matrix within which the nutrients occur [85]. Processing fundamentally alters the food matrix, and the NOVA system captures this in a way that nutrient‐based classifications do not. Another more fundamental reason is that the composition of a particular category of food does not in itself influence health, but it does so only in relation to the proportion it contributes to diets. The probability that a food category will contribute excessively to diets is determined not only by its nutrient composition but also by social factors including price, convenience, marketing, and hedonic manipulations such as the addition of flavorings [86]. Since these factors are strongly associated with the corporate strategies and commercial success of the ultra‐processing of foods [1], the NOVA system is particularly well suited for examining the ecological potential for foods to influence diets and health.
Protein leverage and nutrition transitions
An urgent priority, increasingly gaining policy attention, is to understand better how people interact with transitioning food environments to shape their diets [75]. This is a complex issue for many reasons. Food environments can be characterized across different scales, including the home, work, school, and neighborhood food environments, and there are challenges in defining and delineating them at the different scales [87]. Alternatively, they can be framed according to constructs likely to influence eating outcomes, including the community environment (e.g. type and location of food outlets, opening hours), the organizational environment (home, work, school, etc.), the consumer environment (nutrition information, available healthy options, food pricing, etc.) and information environment (media, advertising, dietary guidelines), which themselves can be challenging to define and measure [88]. Understanding the routes via which food environments influence consumer behavior adds an additional level of complexity. There are multiple factors involved, including both direct influences of food environments on diets as well as influences that act indirectly via psychological mediators such as perceived control [89,90], and various factors can interact synergistically to influence behavior [91].
Recent research suggests that the protein leverage hypothesis might provide a new approach for integrating with existing public health frameworks to understand how human biology interacts with transitioning food environments to generate epidemics of obesity and associated disease. It has become apparent that a significant dimension of the global nutrition transition to obesogenic food environments is the displacement of traditional diets by a dietary pattern in which NOVA category 4, ultra‐processed foods, are a staple source of calories [92]. These products already dominate the food supplies of high‐income countries, for example comprising 61% of food energy purchased by US households [93], and their consumption is rapidly increasing in middle and lower‐income countries [94,95]. Additionally, several studies have established links between ultra‐processed foods and obesity [92,96–99].
To test whether protein leverage is a biological mediator that links ultra‐processed foods to obesity, Martínez Steele et al. [68] used nutritional geometry to analyze NHANES data (2009–2010) in relation to their contribution to the US diet (
