Evolution + economics = obesity crisis

A recent paper I wrote with Steve Simpson and David Raubenheimer explores how our evolved biological mechanisms that control how much we eat combine with modern economic circumstances to create the obesity crisis.

Some time in the last ten years, the number of people who are overweight exceeded, for the first time in human history, the number of people who are undernourished. From the rise in obesity flows a cascade of health and social problems, reducing life expectancies, burdening healthcare services and hobbling workforce productivity. Despite it’s tragic importance, we don’t fully understand the causes of the obesity crisis.

On the face of it, weight gain and obesity seem simple: people are eating too much and exercising too little. But why? What makes some people eat so much more energy than they spend? And what has changed since the mid-20th Century so that so many more people are eating too much and not exercising enough?

It seems that every expert has a different opinion. Energy-dense foods are definitely part of the problem. One hundred grams of broccoli contains only about 150 kilojoules (kJ) of energy and 100g of lean kangaroo meat only contains about 490 kJ, but a similar weight of pizza contains 1000 kJ and French fries contain about 1300 kJ. A lot of good work in tackling the obesity crisis involves trying to take energy-dense – often take-away or pre-prepared – foods off our tables, replacing them with simpler home-made dishes involving less energy-dense ingredients.

But why do we eat these energy-dense foods? Perhaps because they are more affordable. Adam Drewnowski, Professor of Epidemiology at  the University of Washington, and his colleagues have shown that energy-dense foods are also relatively cheap. A consumer gets more kilojoule for their buck from chips and pizza than from broccoli and lean meat. Their results might explain why, in wealthy industrialised nations, poorer people are at far greater risk of being overweight or obese than wealthy people. The rich can afford to eat well, but poor people are trapped into eating unhealthy energy-dense foods. This led Drewnowski and Nicole Darmon to say that “the obesity epidemic is not so much a failure of biological systems but a social and economic phenomenon”.

The amount of protein vs. carbohydrate + fat per dollar spent. Part b is detail od the bottom left corner of part a. Foods below the dotted line contain more than 15% protein. Foods on a solid line contain the same total energy (per $ spent).

Important as the social and economic dimensions of the obesity crisis are, they are not an alternative to biology. Modern science shows that biology, economics and culture are not either-or propositions. We don’t have to fall back on the old trick of polarizing “nature” from “nurture” when we attempt to understand complex phenomena. Our biological bodies get fat, due to biochemical processes in our cells, and it pays to take a closer look at how our evolved bodies interact with our changing economic and social circumstances if we are to understand the unfolding global catastrophe of obesity.

My book Sex, Genes and Rock ‘n’ Roll (to be published by NewSouth Books in June 2011) is all about how our evolutionary past and our economic and social present together shape the modern world. Researching the first two chapters – about the obesity crisis – I noticed how the research on food prices provides immense detail on how much each kilojoule of energy costs in different foods, but it doesn’t consider where these nutrients come from.

Not all kilojoules are created equal. We get our energy from the three macronutrients: carbohydrate, fat and protein. But our bodies handle these macronutrients differently. We use up the energy we get from carbohydrates and fats first. If we eat too much carbohydrate or fat, the excess energy can be stored as fat for later use. Because we can store energy, our bodies can handle short periods in which we eat too much or too little. But our bodies need protein in order to grow and to repair the damage that is always being done to our cells. That protein must come from our diet and cannot be stored, so we need to eat a finite amount of protein every day. For somebody who uses 10700 kJ per day, that amount of protein is 1500 kJ – or 14%.

Professors Steve Simpson (Sydney University) and David Raubenheimer (Massey University, NZ) have pointed out that the amount of protein we eat each day has an enormous effect on how much we eat. According to Simpson and Raubenheimer, at least 14% of our dietary energy should come from protein. If we eat a diet where more than 86% of the energy comes from carbohydrate and fat, we tend to overeat until we get enough protein. In fact, we tend to eat an extra 53 kilojoules of carbs and fat in order to make up for each kilojoule of protein that is missing from our diet. If we eat a protein-rich diet, we ingest 11kJ less carbohydrates and fats for every extra kilojoule of protein we consume.

Having worked with Steve and David on a variety of projects involving insects, I was well aware of their Protein Leverage Hypothesis, an idea I believe is poised to reshape the study of obesity and human nutrition. Nutrition scientists have known for some time that we feel satiated more quickly when we eat high-protein foods than when we eat less protein-rich items. I started to think how changes in the relative costs of protein, carbohydrates and fat might have altered what people eat. The convenience of online shopping meant I didn’t even need to leave my desk to answer the question. I simply looked up the prices of 106 common items at Coles supermarket in Sydney, Australia and Safeway in Seattle, Washington. We published our results in the journal Obesity Reviews.

It turns out that each Mega Joule (1000 kJ) in a food item costs an average of 17 US cents1, but the effects of carbohydrates, fats and protein could not have been more different. Each MJ of protein raised the cost of a food by $3.26. But surprisingly, each MJ of carbohydrate actually reduced the cost of a food by 38 cents. The energy from fat had a negligible effect.  This doesn’t mean that big bags of sugar or flour cost less than smaller bags, but rather that foods that are full of sugar and starch tend to be cheaper than those that are packed with protein.  As a result, meeting your protein needs can be expensive. Consumers with a tight budget might find themselves buying bigger quantities of foods that are dense with carbohydrates and smaller amount of more expensive high-protein and low-carbohydrate foods like dairy, fish, meat and fresh vegetables.

Fighting Obesity for less than $1 a day

From our food price data, we estimated that it would cost less than 72 US cents to reduce energy intake by 1600 kJ per person per day, the kind of reduction that would bring energy consumption back to 1970’s levels. This adds up to an annual cost of $262 per obese person, or less than one fifth the estimated additional medical costs that are spent on each obese person in America. The US Centre for Disease Control estimates that medical spending on each obese individual in the USA is $1,429 higher than the spending on each person of normal weight, and half of this expenditure is borne by the tax payer.

According to the latest official estimate by the World Health Organisation there are 400 million obese people and 1.6 billion overweight people worldwide. If our estimate of 72 cents per day for Americans to switch from cheaper carb-dense foods to more expensive foods with fewer carbohydrates and more protein hold for other obese people around the world, then reducing energy intake by 1 600 kJ for each of the 400 million obese people worldwide would cost less than $105 billion US dollars per year. These costs are dwarfed, once again, by the estimated healthcare costs of obesity in the USA alone ($147 billion in 2008). Switching to healthier, more protein-rich foods appears to be a highly cost-effective intervention in terms of the medical costs of obesity alone.

Our estimate of 72 cents a day is not really a projection of how much a realistic intervention will cost, but it is a start. But there are a number of possibilities, from taxing sugar or high-carbohydrate foods to subsidizing vegetables and lean meats, that might slow the spread of obesity.

Nutritionists often talk about “empty carbohydrates”, but “cheap carbs” might be a more literal description. Throughout history, carbohydrates have become more plentiful and – in line with the laws of supply and demand – cheaper. Our hunter-gatherer ancestors relied on lean meat and high-fibre plant foods. Agriculture, which first began no more than 12,000 years ago, brought abundant starchy carbohydrates like wheat, rice and corn. And since the 18th Century, sugar has rocketed from a rare delicacy to a major source of dietary energy, propelled first by industrialization and slavery in the Carribean sugar colonies, and then by continuous increases in the efficiency with which sugar cane can be farmed, cut and processed2.

Adam Smith witnessed the start of these changes and the harm that they did, as sugar and the rum that was distilled from Carribbean sugar began to flood the European market. In 1776, he wrote in The Wealth of Nations:

‘Sugar, rum, and tobacco are commodities which are nowhere necessaries of life, which are become objects of almost universal consumption, and which are therefore extremely proper subjects of taxation’

In the last few decades, many societies have recognized the enormous social and healthcare costs that alcohol and tobacco generate. Via taxes and the awards of legal damages, those societies have begun to push some of those costs back onto the suppliers. Yet economists agree that the price of sugar is artificially low: government subsidies and trade tariffs make sugar one of the most distorted commodities on world markets.

I can’t imagine that governments will easily defy sugar lobby groups and remove protections and subsidies for sugar growers. But perhaps they will tax the sugary, starchy and fatty products that are causing the most harm. People at greatest risk of obesity tend to be heavy consumers of soft drinks, cordials, fried potato products and ice cream – foods that are full of sugar, starch and (sometimes) fat but not much protein. Taxing products like these might be a helpful first step.

Consumer advocacy groups in the USA, Australia and elsewhere have long called for high-sugar drinks to be taxed, and two American states have already enacted substantial taxes of this kind.  Soft-drink consumption drops quickly when prices rise: a beverage industry newsletter reports that as Coca-Cola prices increased by 12%, sales dropped by 15%3. This tendency for demand to drop dramatically when prices increase (demand elasticity, in economic jargon) is typical of products consumers do not need. Sugary drinks are definitely not necessities, and the dramatic elasticity of soft-drink consumption suggests that taxing sources of cheap carbohydrate energy may help reduce energy intake where that help is most needed.

The sugar in sodas, fruit juices and energy drinks is artificially cheap, which might account for the fact that consumption of these drinks continues to rise. New York’s mayor, Michael Bloomberg, infuriated civil liberties groups and the sugar and soda lobbies last October when he asked the U.S. Department of Agriculture to allow New York City to ban the 1.7 million citizens who receive food stamps from using them to buy soda. Those Americans who are poor enough to receive food stamps are precisely the people most at risk of obesity: people with enough access to food that they do not starve, but not enough money to eat a healthy diet with plenty of protein.

Serious evolutionary biologists care deeply about issues like disadvantage, sexism, alienation and poverty. But biologists have far too seldom and far too sporadically wielded the full force of evolutionary argument in seeking to understand and resolve many problems that our societies face. Hopefully our analysis of food prices have helped advance, in some small way, our understanding of how our evolved biology clashes headlong with modern economic circumstances to create the tragic obesity crisis.


1           I report only the US prices here, but the Australian results were very similar.
2           I give a fuller account of the history of food in my book. Tom Standage’s gives a much fuller account in his excellent book An Edible History of Humanity. Eric Schlosser’s Fast Food Nation and David Gillespie’s Sweet Poison also give shocking insights into the effects of industrialised carbohydrate production. Read Gillespie’s book and visit the Sweet Poison website for practical help in quitting sugar.

3           Elasticity: big price increases cause Coke volume to plummet. Beverage Digest November 21, 3 (2008).

Rob Brooks

I am an evolutionary biologist who thinks about sex for a living. Things I have thought and written about include the evolution of mate choice, the costs of being attractive, the reason animals age and the links between sex, diet, obesity and death. Follow @Brooks_Rob on Twitter.

5 thoughts on “Evolution + economics = obesity crisis

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  • 21 February, 2011 at 8:43 am

    Glad you liked it Sarah. You might enjoy my new book, Sex, Genes & Rock ‘n’ Roll, due out in June – the first two chapters are on exactly this topic, but in more depth. Rob

  • 18 February, 2011 at 2:12 am

    Thanks for this great article (I followed the link from your comment left in the SMH Management Line blog). I am a little overweight and have known for some time that it’s because of my overindulgence in sugar and flour. I don’t eat the traditional junk foods you list like soft drinks, chips and ice cream, but I do love cake!

    The linking of protein kJ intake to overeating was a bit of a lightbulb moment for me, especially because we eat limited meat in our household (my husband does not abide red meat), and I know that while in many households the main meal tends to revolve around what meat is being served, ours tends to revolve around what carb is being served.

    From now on, I think we’ll have to move it to focus more on what fish / chicken / tofu is being served!


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