There is no way to make accurate observations about nutrition? Well shit then, I guess we'd better just give up and accept our inevitable future obesity.
Well, maybe. Research into obesity has been going on for decades in many countries without clear cut conclusions. On the other hand, tackling obesity alone is a different matter from finding food that makes us healthy. There might be some hope, yet. While we don't know that food can make us healthy, it's quite probable that it makes us fat or thin, or, at the very least, has a significant effect on weight.
Actually, the warp-drive/teleportation problems might be easier, because we'll probably know when we get there. With nutrition, there's little evidence to suggest that a theoretically perfect nutrition would result in much improved health over sub-par nutrition. We don't even know that nutrition has a strong effect on health at all (other than eating poison, or few other truly well-known facts).