Weight/Muscle Loss

I have been running half marathons regularly but have not lost much muscle in my upper body. I am still at normal body weight.
Will trail running result in an overall reduction in weight and muscle loss?

Trail running typically builds more muscle than road running, at least if we're talking about hilly trails, since all the climbing requires more leg strength. Even just the roughness of trails ends up requiring more strength than a smooth road for example.

So, among professionals, you may notice that road runners tend to be super skinny, while trail runners have more lean muscle (not bulky muscle, just more of it than road runners). That's my observation but I think most people agree.

As far as weight loss, there are a lot of factors. Trail running is similar to road running or anything else, where it's more about the calorie deficit. Exercise often increases your appetite enough to compensate for the energy you're burning so running doesn't always lead to weight loss. Another consideration: if you're actively training for a race, it's not wise to try to reduce calories or your performance will suffer; if calories are kept equal to burn then of course that also means no weight loss. And, if you're gaining some muscle strength, you may even gain weight. All that said, if you're running very high miles every week, like when you're deep into training, weight loss may happen anyway simply because your body just can't keep up with the burn. But that's only if you're running really high miles (and if you don't have an older, slower metabolism).

Yeah, I should start trail running. It is more exciting than road running. I might start with shorter distances, though.