Evolution and obesity


Jason Collins


June 10, 2011

As I indicated in my recent post on Rob Brooks’s Sex, Genes and Rock ‘n’ Roll: How Evolution Has Shaped the Modern World, Brooks devotes some time to the issue of obesity. Rob has also blogged about obesityand published a paper with Steve Simpson and David Raubenheimer on it (although the book covers more ground).

First, why is obesity an evolutionary problem? In his book, Brooks set out why:

Some researchers use the fact that the obesity crisis emerged so recently as a reason to reject or ignore evolutionary or biological explanations for the crisis. After all, they argue, evolution takes thousands of generations. While they are certainly correct that we are not evolving to become fatter, obesity certainly has deep evolutionary roots among its causes. …

The combinations of genes that we inherited from our ancestors are adapted to the environments where those ancestors lived and bred, not the environments we inhabit today.

For most of our evolutionary history, humans ate a diet high in protein and fat, with some complex carbohydrates. With the neolithic revolution, some populations increased the proportion of their diet that consisted of carbohydrates. Over the last 50 or so years, the level of simple carbohydrates in diets has soared. Humans are not adapted to this modern diet and populations that have only very recently been exposed to this modern diet are more likely to face problems. This can be seen in the high rates of obesity on many Pacific islands or the high rates of diabetes and alcoholism among indigenous populations.

One of the arguments as to why the modern diet poses a problem is the protein leverage hypothesis. The basic idea is that humans have a stronger propensity to regulate protein intake than non-protein calories. As humans have a basic daily protein need - we eat until we satisfy our protein requirements. If the food we are eating has low protein content, we need to eat more before hitting that satiation point. These extra calories are what make us obese.

Brooks and his paper co-authors then took this question into the modern supermarket and looked at the prices of protein and carbohydrates. In his book, Brook’s summarises their findings:

I did a quick assessment of the costs of 111 common foods in my local supermarket and takeaway outlets. I was amazed that every megajoule (1000 kilojoules or 239 calories) of energy from protein adds US$3.26 to the average price of a food, but every megajoule of carbohydrate actually reduces the cost of food by 38 cents. …. Because sugars and starches are cheaper relative to protein than at any other time in human history, economic costs are likely to bias the foods we buy and eat toward energy-rich yet protein-poor diets. Within industrialised societies this effect is likely to be most extreme for poor people who have access to a wide range of foods but who are constrained in which foods they can afford to buy.

This led them to estimate that to cut kilojoule intake by 1,600 kilojoules (the estimated jump in calorie intake since the early 1970s) would require a subsidy of US$0.72 per person per day. This is around $262 per obese person per year, or around one-fifth of the estimated annual medical spending due to obesity.

It provides a case for government action, with the benefits well excess of the cost. Brooks puts it as follows:

One possibility is to subsidise high-protein foods such as lentils, lean meat and fish. Another is to reduce subsidies or tariff protection on sugar and cereal staples. An alternative to the politically perilous business of intervening in commodities markets is to tax products that clearly generate a large part of the public health burden. Reductions in carbohydrate intake might more effectively be achieved by raising the price of carbohydrate energy than by lowering the price of protein. Products like soft drinks, cordials, fried potato products and ice cream constitute a large proportion of the energy intake of adults and children at risk of obesity but contain little or no protein. Special taxes on cheap carbohydrates could well prove to be particularly effective.

Getting rid of subsidies and tariffs is an excellent idea, but I am not sure this is a one-way street for obesity reduction. Sugar quotas in the United States raise the price of sugar. Or take high-fructose corn syrup. Corn prices are artificially inflated by the ridiculous ethanol related subsidies in the United States. Would a truly unsubsidised, free market in corn raise or lower the price of corn syrup? I’m not sure, although that is one experiment I would like to see.

On the subsidisation of high protein foods, I am not sure whether this would succeed. Assuming the subsidisation occurred at the point of sale, we would see an increase in demand for these products and some substitution from carbohydrate heavy foods to these subsidised foods. However, we would also see an income effect, whereby the person could use the extra income freed up by the lower price of protein to buy more ice cream. We might also see underlying protein prices increase, with increased demand leading to much of the subsidy going to the fixed factor - the primary producer of the protein.

I’d be more optimistic about the effect of a tax on high-sugar products. There is reasonable evidence that soft-drinks and the like have a high price elasticity (demand is responsive to changes in price). However, there is also likely to be an income effect, whereby reduced income caused by the higher price of sugar might cut protein consumption. I would not expect the reduction in protein purchases due to reduced income to be larger than the incentive to substitute protein for sugars, but we should consider it.  This is particularly the case where obesity is most concentrated in the low socio-economic income groups.

A good step would be to test this by running some randomised trials (if they haven’t already been done) to see if the tax converts into reduced obesity. The evidence on food labelling and calorie disclosure is that it is ineffective, while there is a long history of sin taxes successfully reducing “sinful” consumption. However, I’m wary of these sorts of ideas as there are always unintended consequences. Sin taxes hit a wide number of people who are using the product sensibly. I’d like to see an estimate of the cost of their loss of enjoyment. Even among those who are obese, a substantial part of their life enjoyment might come from eating high-sugar foods. Perhaps they have decided it is worth the cost of obesity (and if you consider that they should not be allowed to impose the costs of their decision on the health system - don’t let them).

Thankfully, I’d be reasonably immune from these taxes as I like to stick to the edges of the supermarket. I’d encourage anyone to do this. However, this makes me even more reluctant to support a sugar tax. One should be wary of advocating intrusive actions when the intrusion is not on yourself.