What makes you lose body fat?

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So what makes you lose body fat when you are already skinny?I have a friend who owns a fitness club.And last year she measured my body fat at 30% but this year it is 23% so how did I lose 7% body fat in a year.I don't exercise, but I did have the flu twice in feb. there was an epedemic here, and everyone who got it lost weight.Almost everyone got it twice,some people got it three times.And at the time I was at a very stressful job. I can understand the muscle I have lost but not the body fat.