Clinical Obesity in Adults and Children. Группа авторов

of the interacting components [119]. Systems that are interconnected in this way via feedback loops within and across scales, known as “complex adaptive systems,” are fundamentally different from traditionally engineered systems and often show counter‐intuitive behavior that defies simple cause‐effect expectations. They take on a life of their own that cannot be understood or managed as would an engineering problem.

      The realization that food systems, like ecosystems, are complex adaptive systems [120] could hold the key to reconceptualizing public health nutrition in ways that deliver fundamental change. It could help to explain, for example, why nutrition transitions that are unquestionably linked to immense national, social, and individual cost are so inevitable in their genesis, inexorable in their progression and trenchant in their persistence, despite well‐meaning and often costly policy interventions. One reason is that measures aimed at mitigating the impacts of nutrition transitions can drive adaptation and coevolution that cause unanticipated effects that nullify the measure, exacerbate the problem, or create new problems.

      An example is mentioned above, where the recommendation in the US dietary guidelines to reduce fat intake caused a shift in consumer choice towards high carbohydrate foods. Not only did the food industry respond to meet the demand, but it further intensified it through marketing strategies predicated on the supposed benefits of low‐fat (high carbohydrate) products. The consequence was a rapidly changed food system that did not solve the obesity crisis but rather created further problems.

      One of those problems is the loss of trust in national dietary guidelines [102,115] and, as mentioned above, the emergence of extreme diet philosophies that vilify dietary carbohydrates in whatever form, despite the fact that many of the healthiest dietary patterns, such as the traditional Okinawan diet, are high in carbohydrate [121,122]. Failure to distinguish between healthy and unhealthy forms of carbohydrate has led to blanket demonization of all carbohydrates [123]. This, in turn, paved the way for an even more prolific and lucrative commercial sector marketing alternative diet philosophies, many leveraging off the tarnished reputation of carbohydrates. Since carbohydrate is, with few exceptions (e.g. the traditional Inuit diet), the major source of dietary calories, removing them or reducing them significantly inevitably results in increased fat intake. Whatever the direct health consequences of that, increasing fat to the extent required to maintain protein around healthy levels for humans would involve eating a diet of 80–90% fat, which would not be sustainable for most humans. Consequently, low carbohydrate diets are also usually high in protein [124]. While this might be beneficial in the short term for weight loss or clinical management of obesity and diabetes [122,125], principally because of protein leverage, there is evidence that high protein diets, especially when combined with low carbohydrate, accelerate aging and the onset of associated diseases [126]. Furthermore, as discussed above, high protein diets can in the long‐term cause obesity by exacerbating protein leverage through decreasing protein efficiency, and obesity can itself feed into this process by increasing the breakdown of lean tissue and hepatic gluconeogenesis [53].

Effective management of such complex systems cannot be devised using conventional engineering approaches which assume the interactions among the components are linear – i.e. not characterized by myriad feedbacks. Among the impediments that arise are two common biases that distort the nature of the challenge [127]. The first of these, the “hierarchical bias,” assumes the system has a strong top‐down organization and thus that top‐down interventions will be effective. As discussed in relation to dietary guidelines, this fails to appreciate that the system might adapt to the intervention in complex ways. A complex adaptive systems approach, by contrast, recognizes that outcomes are driven by interactions among the components, emphasizing the need to understand these interactions and identify efficacious control points. The second bias, the “complexity bias,” assumes that the interacting agents in complex systems themselves have complicated properties, whereas the complex adaptive systems approach recognizes that the drivers of complex outcomes can in fact be very simple [127]. Together these insights can help to reframe the public health challenge of nutrition transitions by focussing the search for simple properties of interacting agents that might have disproportionate leverage in shaping food systems and directing intervention strategies accordingly.

      We believe that the relevant properties of interacting agents that explain nutrition transitions have already been identified. In consumers, the central issue is appetite systems, and how these interact with such factors as palatability, pleasure associations, cost, and convenience. In the broader food environment, the key issue is commercial profitability. The protein leverage hypothesis is a model of how these have come together to drive the obesity epidemic [18,70]. This recasts the debate of individual responsibility vs. social determinants by emphasizing that nutrition transitions are an emergent outcome of dynamic interactions between people and food environments. Interventions and policies that assume individuals and corporations will exercise agency in a direction not aligned with appetite and profit are bound to fail.

Conclusions

      The fundamental biological drivers of dietary intake are no different in humans than other species, from insects in laboratory studies to wild primates in natural ecologies. Humans differ, however, in being the only species facing a dietary crisis of its own making. The root cause is that our evolved biological appetites have found expression via cultural means including science, technology, and economics, which have driven transitions towards food environments with which that biology is incompatible. A particularly relevant dimension of this transition is the emergence of corporate agency and its progressive empowerment through transnationalization. Interdisciplinary perspectives are essential for understanding this process and even more so for managing it. There are many productive contact points between the sciences of ecology and public health that can help facilitate progress towards a solution. Ecological perspectives can illuminate human biology through insights from species that are not complicated by the culture‐driven transformations with which humans have become inextricably intertwined. Ecology can also dispel the perception that the engineering prowess that provided the tools on which the food industry and its globalization are predicated is up to the task of managing the nutrition transitions they have facilitated. For that, a different paradigm is needed, which respects that, like any other species, we live in an ecology that assumes emergent properties not inherent in any of the components, whether that be individuals, corporations, or governments. Complexity theory can provide the tools for conceptualizing this situation and the potential for managing it through identifying key control points. Ironically, a complex systems approach might succeed where other approaches have failed by demonstrating the problem is simpler than assumed. Even though humans and corporations are immensely complex systems, the primary drivers of nutrition transitions are deceptively simple: biological appetites for nutrients and the corporate appetite for profit. Protein leverage is a simple model that can illuminate how these appetites have interacted to generate an obesity epidemic. The challenge ahead is to use this information to formulate solutions.

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