Capsaicin is the compound in chili peppers that makes them spicy. In addition to adding a delicious burn to food, it has been shown to reduce fatigue in mice. In humans, consuming 2.5 mg of capsaicin per meal (7.68 mg/day) has been shown to restore the energy balance in the body.
Capsaicin affects energy because it impacts glucose metabolism in the body. When capsaicin enters the gut, it triggers a vagal response to the brain, thereby regulating appetite by helping hormones from the brain’s appetite regulation center more effectively detect when enough is enough.
The amount of capsaicin varies widely among different types of peppers, and it’s proportional to how spicy the chili tastes. For example, a mild jalapeño: 0.165 to 0.33 mg of capsaicin; serrano chili: 0.396 to 1.518 mg of capsaicin. Spicier chilies like Thai bird’s-eye chilies and habaneros can be an efficient way to get capsaicin (if you can handle them).
Rather than trying to add up capsaicin levels, just try to incorporate more spicy foods into your diet. Use extra cayenne pepper in your cooking, and if you order in Thai, Indian, or other spicy foods, order them one level spicier than you normally would.
Bear in mind that it’s not just generic “spiciness” that matters, but the capsaicin itself. In other words, spicy foods that derive their burn from non-capsaicin compounds, like mustard, horseradish, black pepper, and ginger, do not affect energy balance in the same way.