The case is about whether the New York City Board of Health has the authority to regulate restaurants in ways that protect the health of the City’s residents, over the fierce objections of some of the world’s biggest corporations, the soda companies. And, more important, whether the Board can protect New Yorkers from many other current and future health hazards, from unsafe conditions on the city’s beaches to toxic chemicals in food or water.

Established in 1805, the New York City Board of Health was, for most of the 1800s, composed of the mayor and the Common Council—the predecessor to today’s City Council. After epidemics of cholera and dysentery between the 1830s and 1860s, it became clear that these local elected officials were unsuited to preventing epidemics; the council was then restructured into a committee of appointed officials led by physicians. It retains that basic structure now.

As laid out in the City Charter in New York State law, the Board of Health has the authority to issue regulations that protect New Yorkers from all health threats, including chronic diseases. Over the decades, the BOH has responded to the emerging health threats of each era, requiring reporting of cases of tuberculosis in 1897, banning lead paint in homes in 1960, and requiring landlords to install window guards in buildings to protect toddlers from falls in 1976.

The Board has regulated restaurants for decades, and its rules can be very intrusive. Just ask any restaurant manager what he thinks about the health department’s inspectors fining them for having a cutting board with deep grooves or a gap under the doorframe big enough to fit a mouse. None of those rules to reduce the risk of food poisoning, though, would prevent nearly as much disease as a reduction in consumption of sugary drinks, which is a key driver of the twin epidemics of obesity and diabetes that are killing some 100,000 Americans a year.

Over the last few decades, soda companies have learned a puzzling but well-established fact of human nature: When beverages are served in larger containers, people just drink more. When McDonald's opened in the 1950s, the only size of sugary drink sold was seven ounces. Now, a “medium” at McDonald’s is three times as large at 21 ounces and a “small” is 16.

Pumping up container sizes is not the only marketing technique soda companies and chain restaurants used to nearly triple the consumption of soda between the 1970s and 2000, but it’s a powerful one. And that’s why they are fighting the portion rule ferociously.

To the surprise of those of us that worked on the rule, the lower courts have ruled with the soda companies, saying that the Board of Health does not have the authority to set limits on container sizes. The lower-court rulings were shockingly broad. The first-level court (called the Supreme Court in New York State) ruled that the Board of Health only had jurisdiction over diseases that were “communicable, infectious, and pestilent.” The Appellate Division decided that the Board could not pass rules in any area unless specifically directed to do by the City Council.