With many areas of medicine most of us laypeople know how little we know. We may have an opinion on cancer or leukemia but unless we have personal knowledge we tend to leave those to the experts. For much of what medicine covers we leave things to the experts. After all would you trust just anyone to look after your heart problems?
There is one area in which people have more than just opinions. An area where people are sure they know what's right and they know what's wrong. That's health, fitness, and weight loss.
If you think about it for a second those are areas that medicine, or at least biology, could be authoritative on. Want to know how to lose weight? Consult the current best practice guidelines given to physicians and health professionals. Want to know how to achieve a fit body? Consult similar guidelines for the type of body you want. Unfortunately there is no such set of best practices and guidelines. Or at least none that I've found that covers these areas authoritatively.
Which isn't to say that people haven't written on these topics at length. Some with more rigor than others. For example there's the Beginner's Health and Fitness Guide which is one of many such guides and sites on the web. Even in publications that specialize in these sorts of guides the current best practice changes all the time. The article Interval Training Doesn't Work from T Nation is a good example of this.
I'm not sure if I have an opinion on weight loss or fitness. All I know is what worked for me when I lost a great deal of weight. Like anyone who was successful (at least for a time) I like the approach I took. Unlike many I don't go around evangelizing it because I'm not sure it will work for anyone else. If you're interested I lost weight using a personal variation on The Hacker's Diet.