I experienced this. I never needed to lose weight, always been lean. But I wanted to see if I could reduce my blood sugar since it was borderline high. So I went very low carb for a while. Did not intend to but lost 5 kilos right away and felt fine, although it seemed like I was a bit underweight. A few weeks later got a miserable case of flu and couldn't eat for a few days. Lost even more weight, felt terrible, looked like death. It seems like the little reserves I previously had were gone from my change in diet.