Weight loss due to HIV infection comes years after infection and the cause is certain HAART medication.
That's a good question that i don't know the answer to, but i will say that there are several factors that i do know can cause weight loss to occur. The first of them is that those in the primary stages of infection CAN experience weight loss associated with the initial stages of hiv infection. I've also read reports that also mention the route of transmission may play a major role in determining this-some say those infected via a needles are more prone to experience recurring muscle wasting / weight loss. It's also thought that the disease may have certain built in programs it activates as it progresses that act as a template through out the course of the sufferers illness trigger these.
Weight loss associating with HIV takes years after infection. If you are referring to ARS, weight loss is not a specific symptom.