Читать книгу Clinical Obesity in Adults and Children - Группа авторов - Страница 119

Linking public health to the science of ecology is important, because it provides a bridge for the transfer of concepts and methods that could potentially enrich both fields. Nutritional geometry is one example, demonstrating how approaches from animal studies, both in the laboratory and in the wild, could provide fresh insight into major public health challenges, such as the obesity epidemic. Another example is the link between the “ecosystem” concept from ecology and the “food systems” concept in public health, mentioned above. We believe that the combination of these approaches holds considerable promise for reconceptualizing and potentially reversing the obesity epidemic.

As noted previously, one defining feature of the concept of the ecosystem is that it provided a framework for elevating ecological thinking beyond organisms to the broader system that includes both biological and non‐biological aspects of the environment [78]. This integrative approach inevitably directed the attention of ecologists towards questions of how the interactions among the component parts of the system influence the properties and behavior of the system, a quest that has yielded many powerful ecological insights that are relevant also to public health.

One key insight is that the properties of interacting components of ecosystems (e.g. individuals, populations, species) can be changed as a result of their interactions with other components through a process known as “adaptation”. Those changes can reverberate through the system, eliciting adaptations in other interacting components, which can drive further changes in the first, producing a dynamic reciprocal process known as “coevolution” [118]. Important properties of ecosystems, such as the degree of stability and productivity, “emerge” from these adaptive and coevolutionary dynamics among its components. The ecosystem‐level properties can, in turn, feed back to influence the properties 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.