There's a thesis somewhere in the mid-to-late-20th-century obsession with "energy" as a by-product of eating. I suspect it has to do with the progressively decreasing levels of motivation due to poor diet, increasingly sedentary activities, and socially-induced malaise.
My personal experience of "energy" is that if you eat for nutrition and satiety only when you're hungry, exercise or move your body regularly through the day, sleep enough and nap when tired, and avoid substances that throw off equilibrium, like coffee, tobacco, and alcohol, you'll have plenty of it. That's only anecdotal of course.
via bOINGbOING