Impact of the North American Free Trade Agreement on high-fructose corn syrup supply in Canada: a natural experiment using synthetic control methods

Abstract

Background: Critics of free trade agreements have argued that they threaten public health, as they eliminate barriers to trade in potentially harmful products, such as sugar. Here we analyze the North American Free Trade Agreement (NAFTA), testing the hypothesis that lowering tariffs on food and beverage syrups that contain high-fructose corn syrup (HFCS) increased its use in foods consumed in Canada.

Methods: We used supply data from the Food and Agriculture Organization of the United Nations to assess changes in supply of caloric sweeteners including HFCS after NAFTA. We estimate the impact of NAFTA on supply of HFCS in Canada using an innovative, quasi-experimental methodology - synthetic control methods - that creates a control group with which to compare Canada's outcomes. Additional robustness tests were performed for sample, control groups and model specification.

Results: Tariff reductions in NAFTA coincided with a 41.6 (95% confidence interval 25.1 to 58.2) kilocalorie per capita daily increase in the supply of caloric sweeteners including HFCS. This change was not observed in the control groups, including Australia and the United Kingdom, as well as a composite control of 16 countries. Results were robust to placebo tests and additional sensitivity analyses.

Interpretation: NAFTA was strongly associated with a marked rise in HFCS supply and likely consumption in Canada. Our study provides evidence that even a seemingly modest change to product tariffs in free trade agreements can substantially alter population-wide dietary behaviour and exposure to risk factors.

Figure 1:

Trends in the supply of sweetener syrups including high-fructose corn syrup (HFCS) in Canada and the synthetic control, 1985–2000. Note: Vertical line shows the implementation of the North American Free Trade Agreement on Jan. 1, 1994.

Figure 2:

Ratio of post–North American Free Trade Agreement (NAFTA) root mean squared prediction error (RMSPE) to pre-NAFTA RMSPE: Canada and control countries. Note: RMSPE is the square root of the sum of the squared difference between the observed value in a country and its synthetic counterpart across a specified period. Here the RMSPE is estimated separately in the pre- and post-NAFTA periods, and the ratio shown above is the post-NAFTA RMSPE divided by the pre-NAFTA RMSPE.