hi is it better to do one signle niche in a particular subject or multiples niches on sub-subjects related to this niche ? for exemple, is it better to do a signle niche about Food suplements, or multiple little niches like whey protein, creatine, vitamins, zinc, magnesium.. etc ?
If you plan on doing any SEO for traffic, then sub-niches are the best because you can find tons of long-tail "buyer" keyword phrases to rank for easily in search engines.
sub-niches are always best. Also with sub-niching you can find affiliate programs for products and ebooks to sell on your sub-niche sites. It's the difference between restaurants vs coffee shops and locally owned pizzerias.