Last week at the European Association of Agricultural Economist's meetings, I saw Louis-George Soler present a keynote talk on food and nutrition policies. The paper-version of the talk, written with Vincent Réquillart is being published in the European Journal of Agricultural Economics.
One of the key points of his talk was that much of the policy analysis on effects of fat taxes, soda taxes, veggie subsidies, etc. only consider consumer responses and ignore how firms will react to the policies. It is often the case that such supply-side responses will substantively reduce the health impacts of the policies.
For example, suppose Congress passed a law banning advertising of sweetened sugared cereal to children. How might Kellogg's or General Mills respond? Given that the firms can no longer use their revenue on promotion and advertising, they might instead re-direct those funds to cost-cutting efforts that reduce the cereals' prices. Competition moves from who has the most compelling ad to who has the lowest price. Lower prices will encourage more consumption: exactly the opposite of what was intended by the ban.
Another point they raise is related to the "pass-through" effect of taxes on firms profits and retail prices. Given the nature of competition between firms and the type of tax (excise or ad valorem), a tax can be "over-shifted" or "under-shifted" to consumers. Thus, tax policies might cause a larger or smaller reduction in consumption than anticipated.
Take another example. Suppose the government requires firms to add "high fat" labels to certain products. The research cited in the Requillart-Soler paper suggests that firms may respond by lowering the price of the high fat items and increasing the price of the low fat items. While the "high fat" label will tend to discourage consumption, the now lower relative price for high fat items will tend to encourage consumption.
None of this is to say that food policies won't have any impact on health, only that studies which ignore food companies' responses to the new policy environment will often overestimate the health impacts of food policies.