My friend and I have been workoing out for awhile now, 4-5 times a week with calistenics on our days off. Sadly, my friend developed a eating disorder and is eating way to little. This made me start tothink. What is the minimum calorie intake before your metabolism begins to slow and shutdown? Also, when does your metabolism begin to slow? If I get good advice I can pass it on to him and hopefully help him.