When you examine the data, most likely you will come to the sorts of conclusions that common sense indicated in the beginning: biology matters—some people can eat and not get heavy, others struggle with weight; what you eat matters, say fatty food versus fruits and vegetables; and how physically active you are matters.  One of the few curves thrown in might be obesity is shaped by lower socioeconomic status, but even that is not too surprising.

So, the question is not really what but why?  And that is a much harder question.

Let me first give you a laundry list of factors linked to obesity, based on my impressions of going over the research these past weeks.  They are more-or-less in order of importance: genetics, socioeconomic status, activity levels, early weight (at birth, by age seven), weight gain in early adulthood, restrictive/binge style of eating, calorie/fat dense vs. fiber/micronutrient dense foods, and social network/cognitive associations of eating and weight.  Other factors could certainly be added to this list, some perhaps shifted around, but it more or less represents an expansion on the common-sense model.

So, why?  Here are some ideas: 

-Weight gain canalizes (once you gain it, it is hard to lose it, especially for predisposed individuals)
-Activity moderates (but does not prevent—it pushes back against a tendency to gain weight)
-Early acquired eating patterns are hard to change: family and social environments affect diet habits, whether these environments favor higher calorie and fat dense foods or foods that have more fiber and micronutrients
-Food insecurity heightens excessive calorie intake and favors lower energy expenditure, especially in food-rich environments.  Food insecurity includes: restriction during gestation and abundance afterwards; restrictive/binging pattern; not having access to food, especially desired food; not eating breakfast.

Right now I am playing around with a three-systems approach to weight regulation: (1) a body-brain system that regulates energy expenditure and storage; (2) an appetite system, largely mediated by the brain but with direct influences from the body and environment; and (3) a cultural biology system, mediating things like eating patterns, body image, and expected exercise.

So, weight canalizes, meaning the body/brain system is pushed up.  Appetite matters, in the sense that when the eating system is excessive, you take in more calories than are burned.  Cultural biology mediates through established cultural habits of food and eating, activity levels, and desired or expected body types.

Other things could be thrown in, such as class and inequality or a deeper examination of how eating gets excessive (likely similar in ways to drug use getting excessive… it’s complicated, but see posts in the addiction category).  But not right now.

Here’s my simplest hypothesis: As the body-based and social-based systems push onto the appetite-based system, people are more likely to eat more calories than they burn.  Weight is gained, and then hard to lose.

These gains can happen at many different periods of development (gestation & birth; infant growth to three years; middle childhood growth; adolescent growth; adult weight gain).  Gestation and early childhood appear to be critical periods for establishing body and appetite systems.  Middle childhood and adolescence are more important to cultural biology, which then shapes the appetite system more than the body-based system.  The adult period is important due to vulnerability to weight gain (height is fixed) and a more sedentary lifestyle, favoring increasing BMIs.  This adult dynamic plays out based on already-established developmental vulnerabilities and in the context of present social and food environments.

Does it make any sense?  Enough for me to write it out, but beyond that, I am not so certain.  But certainly some thoughts to share. 

Here are some links/citations to help you make up your own mind:

General on-line summary of obesity research 

Article summarizing risk and protective factors related to obesity 

Article on restrictive/overeating pattern as a risk factor 

Article on food insecurity 

Article on how home environment matters 

Article on impact of early development 

Article on food types, breakfast, and activity 

Article on body dissatisfaction, social class and social mobility