My new favorite term from epidemiology: J-Curve.
There’s a lot of things that increase your mortality in a more-or-less linear way. The more you smoke, the greater your all-cause mortality risk, for example. This isn’t to say you increase your chance of death by 100% moving from one pack a day to two. But on average, your mortality goes up for each additional cigarette you smoke a day. Ten cigarettes is not going to be better for you than five, ever.
Some things, though, don’t work like that. It’s harmful to be overweight, but it’s harmful to be underweight too. Some studies claim alcohol is like this — having no alcohol correlates with a higher mortality than having a drink or two a day, but once you get past a drink or two a day mortality climbs again. The curve is shaped like a “J”, hence the name.
Understanding that things can work this way is important. Vitamin E deficiencies have been correlated with increased cancer mortality, so a lot of people take vitamin E supplements, assuming it’s a linear relationship. But vitamin E supplements have been correlated with increased cancer risk.
Likewise, a lot of health gurus today will point to the harmful effects of over-consumption of sugar, gluten, or dairy (or heck, even fat/oils), and act as though this proves elimination of this thing will dramatically increase your health. It might — if it is a linear relationship. But if it’s a J-curve, you could end up doing as much harm as good.