With an incredible number of diet and weight-loss weblogs online, does it really appear sensible to start this market now- supposing you aim for online sales through these blogs? What's your experience with weight-loss niche?
On the other hand, this is a great time of year to promote weight loss products and healthy eating diets. It's after the holidays (and most people eat more then than they usually do). In the next few months, people will want to get into shape for summer time and going to the beach, etc.
I agree with both posters above. On one hand, now is the best time. The post holidays/upcoming spring-season time. On the other hand, it is an over saturated market that's going to be very difficult to get into. If there are other options that you can think of for yourself, I would go for that instead.
It might be a very competitive niche, but it also has plenty of categories and you can specialize in a handful of products you believe in. When you do that, you make it work for you. I'm the same way with software, which also is a very competitive niche.