Is it okay to not consume any calories on the fast day?

In general, we don’t recommend doing a complete fast on fast day because we’ve found it can be very difficult to get through the day. Two short-term studies (2-3 weeks) have tested ‘true’ fasting diets on health outcomes, and dropout rates were very high. Subjects quit the diet because they felt very irritable, constantly hungry, and were not able to concentrate at work. However, the subjects did lose weight (2.3kg), and cholesterol levels decreased. On the other hand, if you find it easier to do SFD by completely fasting on fast day, feel free to do so.