I find it interesting that you talked about nutrition supplements for athletes. I am very close to launching such a product for enhancing aerobic exercise performance in women (ie. improved times in a 3 mile run).. The product contains no stimulants or exotic herbs. In fact three of the five ingredients are well known minerals, but in forms not found in most multi-vitamin-mineral supplements. The research behind the product comes from me. The credibility behind the research is that I am a professor of human nutrition with over 100 research papers. Now, the trick will be to use my connections and credibility in a business savvy way.
While many shorter keywords may come up in your keyword research, it’s important to also pay particular attention to the long tail keyword opportunities. Often, these keywords are more specific. As such, they typically have less search volume and, as a result, less competition. That means it’s not only easier to rank for relevant long tail keywords, but it’s also easier to guess the searcher intent.
There are many ways to get people onto your list. Lead magnets are one such resource. For example, you can build ebooks, checklists and cheat sheets. But you can also do content upgrades, such as PDF versions of an article with added resources in them, four-part video training series, and more. Think about your audience and what you can offer them to better serve them, then treat them with some respect and you'll eventually reap the rewards.
The topic you choose must have enough depth that you can create a lot of content for it. This is important for building an authoritative site, for search engine optimization, and most importantly, for the end user. If you don't have enough content about a topic, you're not going to be taken very seriously as an authority on the topic and it's unlikely you can convince someone to make a purchase from you.