You know the saying, "a little knowledge is a dangerous thing"? That is where you are at right now.

Really, really, really rigorously controlled studies with very carefully designed questions can have smaller numbers as long as there is a robust effect size. That's how statistics work. (And indeed, gargantuanly huge studies can show "differences" that are practically worthless because the sensitivity is so high.) Would a larger study have been better? Sure. But a larger study would, for practicality's sake, have to have been less controlled because you can't lock thousands of people in a metabolic ward for a year, controlling their every move, to see what happens. It would be crazy expensive and you wouldn't find participants because people have jobs and families and shit. And if we send them home and have them report, we have no idea if they actually did what they were supposed to or not. And guess what? We've done that. Many times. This study is meant to complement the larger studies out there that have been done ad nauseam but have shitty control variables.

Where was the control group?

They served as their own controls (which is a completely valid research method). If you read and understood the study, you would already know that.

Where was the double blind study?

This is not a appropriate context for a double blind study. Unless you feed them only DIY Soylent, which brings in a whole new set of potential confounding variables... There is no realistic way to hide the food that they are eating. Many very well-designed studies are not double blind for various reasons.

Where was the peer review?

This is a peer-reviewed study. It's a peer reviewed study in a good journal.

This is dishonest bullshit junk science that hurts people.

It really isn't.