Health experts examine how density of fast-food outlets associated with prevalence of obesity

Many studies have linked the meals served at fast-food outlets to obesity, but is there a relationship between the number of restaurants in a country and the girth of its population?

To answer this question, an international team of health experts looked at the number of Subway restaurants per 100,000 people in 26 economically advanced countries. They also considered other factors, including the number of men and women over 15 with a high body mass index, gross national income, a country's Gini coefficient (an indicator of income inequality), urbanisation, motor-vehicle and internet use.

The conclusions, published in the journal Critical Public Health (De Vogli, Kouvonen & Gimeno, 2011), are clear: the density of Subway's outlets is positively associated with the prevalence of obesity across 26 advanced economies in both men and women. Even after adjusting for the other factors, countries with the highest density of Subway restaurants (such as the United States and Canada) have a higher prevalence of obesity than countries with a low density (like Norway and Japan).

The recent explosion in the number of fast-food outlets is down to more than just their 'special sauce'. The authors suggest that the rapid global market integration and trade liberalisation promoted by organisations such as the World Trade Organization - which contribute to an increase in exports of domestic goods, imports of foreign products and the opening of markets to foreign investment - have also played a large part in expanding waistlines. The growth and power of transnational food companies, supermarkets and fast-food restaurants encouraged by such policies has had a dramatic impact on global diets.

The authors are quick to acknowledge the limits of their study. They do not state that the density of fast-food restaurants causes obesity - merely that it is associated with it. They also acknowledge that their study doesn't take into account the time lag between the appearance of such restaurants and a given country's obesity epidemic. The Critical Public Health study was also limited to just 26 advanced economies and one fast-food chain, albeit the largest: Subway now has more outlets than McDonald's.

Nevertheless, this study is an important contribution to the understanding of obesity as a global problem - what the authors themselves call 'globesization'. They also recommend further research into the connections between obesity, fast food and trade liberalisation policies, and call for co-ordinated political action to stop the spread of 'globesization'.