Malcolm Gladwell argues that "what's the proof?" is often used as an excuse for those who profit from a harmful activity not to fix it. It was obvious for 50 years that breathing coal dust gave you black lung disease, but mining companies resisted change, claiming, "There's no proof!" Nowadays, evidence is mounting that football may severely harm the mental health of the people playing it, but the leagues refuse to fix it, claiming, "There's no proof!"
Last year I read a bunch of books about the Tobacco Wars of the 1990s. Everyone knows that tobacco companies still claim "there's no proof" that cigarettes cause lung cancer and emphysema. The most surprising thing I found in those books was that the tobacco companies are basically correct. To this day, scientists still don't understand the exact mechanisms and pathways linking cigarette smoking and lung cancer.
What we do know, though, is that people who smoke get lung cancer and emphysema way more than people who don't. This is consistent across age, race, wealth, age, location, religion, left- vs. right-handedness, you name it. Cigarettes make you more likely to get sick and die. We don't know precisely how this works -- molecules are reportedly involved -- but that's irrelevant. We know enough to tell people they'd be better off if they never smoked.
I'd argue that we're at basically the same place with soft drinks. People who drink more soda have higher rates of obesity, diabetes and heart disease. People who stop drinking soda see their risk for all these things drop.
And yet, the mechanism whereby soft drinks uniquely contribute to obesity isn't fully understood. Maybe soft drinks mess with your satiety signals, making your body "forget" that it's just consumed 300 calories. Maybe all that sugar leads to tolerance, or even addiction. Maybe liquid sugar is converted to fat more efficiently than food. Maybe a vengeful God has cursed mankind by making everything that tastes good slowly kill you.
These unanswered questions aren't an excuse not to act. A 20-ounce bottle of Coca-Cola contains 16 teaspoons of sugar. A kid that drinks an extra soda every day has a 60 percent higher chance of becoming obese. Kids shouldn't be drinking soda, and neither should adults. Period.
Starting in the 1990s, governments around the world started taking tobacco prevention seriously. They removed vending machines, taxed cigarettes, banned smoking in bars and prevented marketing anywhere kids might see it. These steps weren't driven by incontrovertible new proof of tobacco's perniciousness. They were just our actions catching up to our common sense.
I think in the next 10 years you'll see the same thing with soda. Cities are already banning soft drinks from schools and daycares. Soda taxes are appearing on ballots. Bloomberg's large-cup ban is spreading to other cities.
There's still no proof soda causes obesity. Or cigarettes cause cancer. Or football causes CTE. But some things are so obvious, proving them is what you do after you fix them.
 Kessler, David. A Question Of Intent: A Great American Battle With A Deadly Industry. New York: PublicAffairs, 2002.
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