 In the 60s and 70s, a mystery was emerging. Why were childhood asthma rates in the developed world, between 2 and 5%, but in the developing world, as low as 0.007%? So instead of 1 in 20 kids affected, or 1 in 50 kids, it could be more like 1 in 10,000 kids. Extremely rare. And when they moved from a low-risk area to a high-risk area, their risk went up, so it wasn't genetic, what was going on? Were they exposed to something new, or did they leave some protective factor behind? Well, way back in 1938, scientists showed they could stop asthma attacks by lowering children's sodium levels. But this was done with a diuretic drug. But subsequent dietary experiments showed that diets high in salt seemed to increase asthmatic symptoms, and lowering the salt seemed to decrease asthmatic symptoms. But this body of evidence was apparently forgotten until it was picked up again in the 1980s as a possible explanation for why Western countries had higher asthma rates—maybe it was the salt. They graft out childhood death from asthma versus family salt purchases, and it seemed more salt meant more death. But just because a family buys more salt doesn't necessarily mean the kids are eating more. The way you find out how much salt someone is actually eating is you collect their urine over a 24-hour period and measure the amount of sodium since how much salt we eat is pretty much how much salt we excrete. The way you test for asthma is called a bronchial challenge test where you look for an exaggerated response to an inhaled chemical, and indeed there was a strong correlation between how their lungs reacted and how much sodium they were taking in. But look, there's all sorts of food additives like preservatives that can trigger the so-called hypersensitivity reactions, and so maybe high sodium intake was just a marker for high processed food intake. Maybe it wasn't the salt at all. Or maybe it was other components of the diet. For example, the reason sodium may be a risk factor for another inflammatory disease, rheumatoid arthritis, may be that sodium intake is just a marker for increased meat and fish intake, or decreased fruit and vegetable intake. What we needed was a study where we take asthmatics, changed the amount of salt in their diets, and see what happens. And so that's what came next. Take 10 asthmatics, double their salt intake, and in 9 out of 10 their lung sensitivity worsened. No control group, though. Maybe they would have all gotten worse anyway. Which brings us to the 90s, a randomized, double-blind, placebo-controlled trial. Put everyone on a low-salt diet, but then give half of them the sustained-release sodium pills to bring their salt intake back up to a more normal intake. The other half gets placebo. You try that for five weeks, and then you switch them around for another five weeks. That's how you can randomize people to a true low-sodium diet without them even realizing it. Genius. So what happened? Asthmatics on the salt got worse. Their lung function got worse. Their asthma symptoms got worse, and they had to take more puffs on their inhalers. This is comparing asthmatics consuming about 3 teaspoons worth of salt a day to those consuming less than one, so they were effectively able to drop their sodium intake 2 teaspoons of salt worth. If you do a more pragmatic trial and only effectively reduce people's salt intake by a half teaspoon a day, it doesn't work. Even if you are able to cut your sodium down enough to get a therapeutic effect, though, it should be considered an adjunct treatment. Do not stop your asthma medications without your doctor's approval.