Sullivan points us to an USDA study suggesting that a 20% soda tax could cause the average child to loose 4.5 lbs. In particular the study suggests that such a tax would work by reducing caloric intake by 43 calories a day.

Now before we get into the details of the study lets run this by the well established “Hooey Limit Test.” I say to you, “hey looking to loose that last stubborn five pounds, just consume 1/3 less of a soda per day. Not a whole soda mind you, that’s worth a full 15 lbs. Just leave the bottom third in the can.”

Does that sound reasonable or like a bunch of Hooey to you?

If that’s the world we live in then how are we to make sense of the endless struggles that millions of women (and men) go through to loose five pounds, say nothing of a greater battle against obesity.

As a side note, this is what makes me skeptical of explanations like “sugary drinks” for obesity. Wouldn’t someone have noticed by now that people who don’t like sugary drinks are never fat? Wouldn’t that be everywhere? Its not a hard observation to make.

Whatever the secret to weight loss is, it must be elusive. It must be something that is either difficult to find or difficult to execute. Otherwise people would have succeeded by now.

Now, on to the study. What it does is pretty straight forward. They look at consumer behavior. They then estimate the change in consumer purchasing behavior that would come from a change in soda prices. How less soda would consumers buy. How much more bottled water, milk and juice.

They then net out the calories. Since, soda is more calorie dense on average, substituting away will cause a reduction in calories. They then turn caloric reduction into a equilibrium reduction in weight.

Regular readers will note that the immediate problem is that they assume consumption is exogenous. That humans choose their caloric intake for some set reasons unrelated to body composition. They then independently choose their caloric expenditure. We subtract one from the other and composition pops out as the residual.

Now, obviously no one, not even the authors of the study believe that this is accurate in real life. At a bear minimum humans are both aware of and care about their body composition. Getting fatter is costly and so, as we observe, will cause people to take actions to loose weight.

Indeed the FDA waives all these concerns away in typical academic fashion

Assuming that everything else remains equal (e.g., constant physical activity level and no shift to foods other than beverages), a reduction of 3,500 calories leads to a 1-pound loss in body weight

Which is akin to saying that assuming no insurgency the wars in Iraq and Afghanistan should go swimmingly.

Personal attempts to control body composition obviously affect the results in a feedback mechanism that is familiar to economists. However, even deeper than that body composition is critical to the survival of all animals from nematode worms to Blue Whales. In response animals have evolved mechanisms for regulating that composition. One of the most powerful we are all familiar with, its called hunger.

But hunger is not the only one. There is satiation or fullness, restlessness, exhaustion, and all the variants there of. These emotions are balanced through a complex set of hormones which in a healthy adult causes calories-in to be matched to calories-out with a less than .5% error.

When we see obesity, something must have gone wrong with that mechanism. Now Bob Lustig suggests that drinking soda is what goes wrong, but because of the effect of sugar on insulin levels. Gary Taubes says carbs generally. Seth Roberts says our physiological association of calories with taste. David Kessler says food industry generated hyper-palatability. Until repeated large scale studies showed otherwise the official government line was high percentage intake of fat.

My readers know that while I am sympathetic to the carb, read insulin, based theories I am not even convinced that the culprit is nutritional. Every possibility from endocrine disrupters to biological agents to autoimmunity needs to be taken seriously.

What we do know, however, is that something has gone wrong metabolically. And the key to ending obesity is finding out what that something is and fixing it.

In any case its not as simple as adding up beverage changes and then netting out calories as if a change in beverage calories has no feedback mechanism against composition or calories expended.

Note: A previous version of this post attributed the report to the FDA when in fact it was from the USDA