Eat five portions of fruits and vegetables a day. Walk 10,000 steps. Drink eight glasses of water. Sleep eight consecutive hours a night. These magic numbers are meant to keep us healthy. In order to optimize our well-being, smart apps track our hydration levels, our intake of greens, our sleep and, of course, our every step. Yet, supposed benefits aside, these targets have one thing in common: They all have their origins in marketing strategy—and in one case, a workers’ rights campaign—rather than science.

The five-a-day slogan, championed by the British government in 2003 and embraced worldwide to encourage healthier eating habits, does not suggest the ideal daily amount of fruits and vegetables we should have. Rather, it is based on the 400 grams recommended by the World Healt...