Contrary to the argument presented by Bauman, King and Bauer sales data from McDonald’s would reveal little about the impact of the deal between McDonald’s and Weight Watchers on the health of the community.

This deal (so far restricted to New Zealand) allows McDonald’s to put a Weight Watchers logo on foods that meet specific standards for fat and overall kilojoule content. The Weight Watchers-endorsed meals generally have a healthier profile than other McDonalds’ meals.

Bauman, King and Bauer argue that if McDonald’s released its sales data showing the impact of the deal on sales of the Weight Watchers-endorsed meals and other products, it would be clear whether or not this arrangement had an overall positive or negative impact on the diet of the population as a whole.

They illustrate the current likely situation where sales of core (less healthy) meals outstrip sales of non-core (healthier) meals and pose two potential scenarios resulting from the deal. In scenario 2, sales of the (less healthy) core foods grow faster than those of the (healthier) non-core foods as a result of the deal. In scenario 3, sales of non-core foods grow at a faster rate than those of the core foods. They believe that only in scenario 3 would the community be better off, nutritionally speaking.

On this basis they argue that:

“The partnership with Weight Watchers could have a detrimental effect on the population’s diet if the Weight Watchers co-branding brought more people into McDonald’s, and the majority of these consumed ‘less healthy McDonald’s choices’.”

However, this conclusion is based on a groundless assumption that other food consumption patterns remain static while changes occur in the level of consumption of McDonald’s meals. Of course, this is unlikely to be the case as food consumption decisions do not occur in isolation but are the result of complex, inter-related factors, themselves affected by changes such as the McDonalds’ deal. Put simply, if people are eating more McDonald’s then they are likely to be eating less of something else. The health impact of these changes can only be assessed if we know the nutritional status of this “something else” and so can assess whether or not the switch to McDonald’s represents an overall gain or loss of nutrition.

For example, Bauman et al’s scenario 2 could occur as a result of families switching from eating a similar fast-food-style restaurant to accommodate the wish of one family member for a healthier meal. In this case, sales of core McDonald’s meals would grow at a faster rate than those of non-core meals but (assuming the alternative restaurant provided meals of similar nutritional status to core McDonalds’ meals) there would be an overall nutritional gain to the community.

Similarly, scenario 3 could result from people substituting their healthy, home-cooked and nutritionally superior meals for inferior McDonald’s-Weight Watchers meals, including an occasional purchase of products from the core menu. In this case, sales of healthier meals would grow at a faster rate than the core McDonald’s meals but the community’s nutritional status would be poorer overall.

Of course, in reality, the changes in food consumption patterns that result from the McDonald’s deal will be the result of thousands of individual decisions, some of which will be health-promoting and others that will have a negative health impact.

Assessing whether or not the deal leads to an overall nutritional gain or loss to the community requires an understanding of the cumulative health impact of these decisions. Similarly, we cannot assess whether or not McDonald’s makes an overall positive or negative contribution to the nation’s health unless we know what people would be eating in its absence.

Bauman et al’s argument that: “If only we could see which of scenario 2 or 3 actually occurred in this type of ‘partnership’, we could understand whether this was in fact improved public health, or just promoting the corporate [and our] bottom line” is simply wrong. Sales data will not, on its own, enable us to draw any conclusions about the result of changing consumption patterns on our waist — or bottom — lines and whether or not McDonald’s or Weight Watchers benefits commercially from this deal is irrelevant to assessing its impact on population health.