The push hypothesis for obesity

My blog post on the summary of my SIAM talk on obesity was picked up by Reddit.com.  There is also a story by mathematics writer Barry Cipra in SIAM news (not yet available online).  I thought I would explicitly clarify the “push” hypothesis here and reiterate that this is my opinion and not NIH policy.  What we had done previously was to derive a model of human metabolism that gives a prediction of how much you would weigh given how much you eat.  The model is fully dynamic and can capture how much you gain or lose weight depending on changes in diet or physical activity.  The parameters in the model have been calibrated with physiological measurements and validated in several independent studies of people undergoing weight change due to diet changes.

We then applied this model to the US population.  We used data from the National Health and Nutrition Examination Survey, which has kept track of the body weights of a representative sample of the US population for the past several decades and food availability data from the USDA.  Since the 1970’s, the average US body weight has increased linearly.  The US food availability per person has also increased linearly.  However, when we used the food availability data in the model, it predicted that the weight gain would grow linearly at a faster rate.  The USDA has used surveys and other investigative techniques to try to account for how much food is wasted.  If we calibrate the wastage to 1970 then we predict that the difference between the amount consumed and the amount available progressively increased from 1970 to 2005.  We interpreted this gap to be a progressive increase of food waste.  An alternative hypothesis would be that everyone burned more energy than the model predicted.

This also makes a prediction for the cause of the obesity epidemic although we didn’t make this the main point of the paper.  In order to gain weight, you have to eat more calories than you burn.  There are three possibilities for how this could happen: 1)  We could decrease energy expenditure by reducing physical activity and thus increase weight even if we ate the same amount of food as before,  2) There could be a pull effect where we became hungrier and start to eat more food, and 3)  There could be a push effect where we eat more food than we would have previously because of increased availability.  Now the data rules out hypothesis 1) since we assumed that physical activity stayed constant and still showed an increasing gap between energy intake and energy expenditure.  If anything, we may be exercising more than expected.  Hypothesis 2) would predict that the gap between intake and expenditure should fall and waste should decrease as we utilize more of the available food.  This then leaves us with hypothesis 3) where we are being supplied more food than we need to maintain our body weight and while we are eating some of this excess food, we are wasting more and more of it as well.

The final question, which is outside my realm of expertise, is why food supply increased. The simple answer is that food policy changed dramatically in the 1970’s. Earl Butz was appointed to be the US Secretary of Agriculture in 1971.  At that time food prices were quite high so he decided to change farm policy and vastly increase the production of corn and soybeans.  As a result, the supply of food increased dramatically and the price of food began to drop.   The story of Butz and the consequences of his policy shift is documented in the film King Corn.