system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is weight loss a symptom of many deadly illnesses?

Things like HIV and Cancer often have extreme weight loss as a symptom, but what about these illnesses make your body actively lose weight so quickly?
gpt
For cancer at least, the "cancer" are essentially the robbers. They rob your body of nutrients, especially carbohydrates such as glucose. Normal cells can utilize glucose efficiently, compared to cancer cells. Cancer cells can only utilize glucose by glycolysis, which is far more inefficient than aerobic metabolism by healthy cells, therefore requiring more glucose to make the same energy to sustain itself. As a result, a cancer patients body will have less glucose, thus pulling resources like your fat. (Fats can be converted into energy to use for cancer cells).