Modified fasting can be done up to weekly without fear of lowering your metabolism.4 For those with significant health challenges or who have a lot of weight to lose, you can safely fast once per week (meaning three days out of every seven).  Our ancestors were incredibly familiar with food scarcity, and most humans around the globe, until about 100 years ago, had long periods of time with only one or two meals per day. (In fact, as strange as this may seem to North Americans, many indigenous cultures around the world currently eat one or two meals a day.) Our bodies know just how to take advantage of those conditions! While modern society offers us food 24-7, making the choice to fast regularly is one of the smartest things you can do for your health.