A 2001 review found that VLCD has no serious harmful effect when done under medical supervision, for periods of 8–16 weeks with an average weight loss of 1.5-2.5 kg/week. However, VLCD may increase the risk of developing gallstones if the fat content of VLCD is not sufficient, but data is lacking to know the precise amount of fat that is necessary to avoid gallstones formation. Indeed, dietary fat stimulates gall bladder contraction, thus, if following a fat-free VLCD, the bladder does not empty. Another potential side effect is constipation (depending on the fiber content of the diet).