Two-thirds of Americans are overweight. One-third is obese. Obesity and a host of illnesses related to weight problems kill more people than any other disease.

Experts are scrambling to find answers for what causes the epidemic and seem to come up with new explanations every day, only to be contradicted by the next study. Unsurprisingly, consumers are confused and stop paying attention.

How is it that we are eating ourselves to death, not just here but increasingly around the world? Does the so-called “Western diet,” consisting of cheap, highly processed, highly caloric foods, make us fat? Or is it sugary sodas? Are portion sizes too big? Does the food industry turn us into addicts? Do we just not exercise enough?

So far, none of the countless studies on these subjects have had much impact in practical terms. Lobbying efforts and political gridlock are oftentimes blamed for the maddeningly slow progress. But that may not be the only reason. Some experts warn that despite of all the research, finding definite answers may prove elusive for some time to come.

“If we can find the causes of obesity, we can try to eliminate or counter them,” wrote Christopher Chabris, a professor of psychology at Union College, together with his colleague Daniel Simons of the University of Illinois in an op-ed piece in the New York Times. “Unfortunately, finding causes is easier said than done, and causes we think we see can turn out to be illusions.”

Hoping for a smoking gun that lets us clearly identify causation may not be in cards, ever. As an example, the authors cite a study that analyzed potential connections between food advertising on billboards and prevalence of obesity in certain parts of Los Angeles and New Orleans. The study results showed that areas with more outdoor food advertisements had a higher proportion of obese people than those with fewer ads. So, there seems to be a direct link.

Other reasons why

Not so fast, say the professors. Studies like these have a significant problem: They can show association between separate phenomena but not causation. In this particular scenario, interpretations of the findings could go in totally different directions. Even if you accept the existence of a correlation between food advertisements and obesity, you still have to consider a wide field of possible explanations. One could be that food vendors tend to invest more of their advertising budget in places where they believe food consumption is especially high. Obesity then may be an indicator that this is a good market for them. Which comes first, the chicken or the egg?

So does this mean there is no proof that food advertising influences people’s eating habits, and not necessarily in a good way? Studies like these can at best provide some indication that there may be a link, but they don’t provide evidence for causation, the professors conclude. Only the gold standard of scientific research, which is the randomized controlled trial, could prove such connections, they say. And these standards are often not achievable in the real world.

The question is what does that mean in terms of taking anti-obesity measures. Do we have to remain indolent in the face of this alarming health crisis just because we can’t pinpoint its causes with the highest standards of certainty? Food manufacturers and restaurant operators support that view. But if no compelling evidence can be had that advertising works, why would they keep spending billions of dollars every year on doing just that?

With all due respect for scientific standards, I can’t help but think that calling for more and more studies on the causes of obesity only delays critical action that ought to be taken now.

We may not know whether food ads influence everyone’s eating habits, but we still can ban them from children’s programs on TV. We may never be sure whether drinking sugary sodas or consuming sweet snacks cause diabetes, but we still can stop offering them on school campuses. It may be unclear whether posting calorie counts in restaurants will make patrons order smaller portions, but we should at least give them the option, so their choices can be better informed.

How many more studies and trials do we really have to conduct before we take at least a few steps in the right direction?

Timi Gustafson R.D. is a registered dietitian, newspaper columnist, blogger and author of the book “The Healthy Diner – How to Eat Right and Still Have Fun”®, which is available on her blog and at amazon.com.  For more articles on nutrition, health and lifestyle, visit her blog, “Food and Health with Timi Gustafson R.D.” (www.timigustafson.com). You can follow Timi on Twitter and on Facebook.