Fasting and exercise can boost the longevity hormone FGF21, but what can we eat—or avoid eating—to get similar effects? Over a century ago, fasting was hailed not only as a means of combating “cerebral lassitude,” but also for the “prolongation of healthy longevity.” If that turns out to be true, FGF21 might be a missing link. FGF21 is characterized as a “systemic enhancer of longevity.” It can be boosted through prolonged fasting, but thankfully, there are other, less drastic measures, such as more carbs or less protein. Give people lots of starchy foods, and their FGF21 levels shoot up. The...