What If the Traffic Light Doesn’t Work?

The concept of a traffic light diet is a mainstay for clinics that treat childhood obesity. It seems to be useful for shaping healthy childhood eating behaviors. In its Evidence Analysis Library, the Academy of Nutrition and Dietetics says it’s effective. In fact, it even says the evidence is strong for it. But if you look closer, you will find that the traffic light itself has never actually been tested for clinical effectiveness.

A Familiar Fixture

At first glance, the traffic light diet makes perfect sense. It classifies food into three groups – green, yellow, and red. The idea is to encourage kids to eat all they want of the really healthy stuff, like fruits and veggies. Those are the green foods. Then for the yellow foods, kids have permission to enjoy in moderation. Finally, the red foods – for example, ice cream or french fries – would be something to eat rarely.

This is a simple system that codifies the way many people think about food – good or bad, with a big yellow zone in the middle. Leonard Epstein and colleagues developed it at the University of Buffalo in the 1980s.

Digging Deeper

Michelle Bohan Brown, Colby Vorland, Michelle Cardel, and Andrew Brown decided to dig deeper into the evidence for this familiar fixture. Vorland recently presented their findings in a poster at the Nutrition 2020 Live event of the American Society for Nutrition. They concluded:

There is insufficient evidence supporting TLDs as a unique, isolatable factor in improving weight-related outcomes in children. This does not necessarily mean that TLDs are ineffective, as they have been incorporated into successful interventions.

What’s more, it turns out that definitions of stop, go, and caution foods are over the place. In one program, avocado might be in the green group – enjoy all you want. But other programs have lumped it with the yellow or even the red foods. Look out!

Familiarity and Assumptions

We take the findings of Brown et al as a cautionary tale about the bias of familiarity. When we hear a concept often enough, we come to accept it as valid. We stop asking questions.

And yet only by asking questions can we discover the false assumptions we’re making. The traffic light diet dates back to the very early days of a still growing problem with childhood obesity. Perhaps it’s perfectly valid, but Brown et al make some excellent points about the need to validate and standardize it.

Perhaps even more important, they are showing us why we must never stop asking questions. Curiosity might have killed the cat, but it is what brings science to life.

Click here for the abstract from Nutrition 2020, here for the poster, and here for the working paper. For perspective on how traffic light coding for foods might have unintended consequences, click here.

Citrus Traffic Light, photograph © lost places / flickr

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June 20, 2020