While leptin levels are highly associated with body fat mass, the more important determinant is energy balance. Long-term usage of a diet that is calorically deficient will cause leptin levels to decrease. One way, and ultimately the easiest way, to prevent this is to incorporate refeed days. Depending on what the current body fat mass is, a person can integrate a day or two of higher than maintenance calories with ample carbohydrates. Insulin has a transient up-regulation effect on leptin levels. A day or two of refeeding will increase leptin levels and ‘reset’ the metabolism. As well, assuring adequate zinc and vitamin E have been shown to increase leptin levels in humans.
The question has been asked: at what level of body fat does this switch take place? There is no single answer to that. Each individual probably has their own 'switch' level, depending on many factors such as age, gender, etc. A range may exist with a threshold for the average healthy population subsets, but this has not been investigated thoroughly. Plasma leptin concentrations have been presented, but not specific body fat levels. Perhaps the best method to determine when it is time to integrate reefed days is when the individual becomes chronically and extremely hungry. However, avoiding this point is optimal.
So, the lower the bodyfat, the more often I would need to refeed. Right?
That is correct. For a female in the 12-14% body fat range, a one or two-day reefed every 5 days; and for a male in the 8-12% body fat range, a two-day reefed every 4-5 days. Experimentation will be necessary to determine the correct frequency intervals, durations, and caloric intake.