The number of confounding factors isn't that important if it's possible to do controlled experiments that control for them. Nutrition science has the problem that you usually can't do good controlled experiments or those are very expensive.
Obviously if you can control for a confounding factor then its not an issue, I was simply stressing that the nature of human sciences means that it is effectively impossible to control for all confounding factors, or even be aware of many of them.
Jason Mitchell is [edit: has been] the John L. Loeb Associate Professor of the Social Sciences at Harvard. He has won the National Academy of Science's Troland Award as well as the Association for Psychological Science's Janet Taylor Spence Award for Transformative Early Career Contribution.
Here, he argues against the principle of replicability of experiments in science. Apparently, it's disrespectful, and presumptively wrong.
This is why we can't have social science. Not because the subject is not amenable to the scientific method -- it obviously is. People are conducting controlled experiments and other people are attempting to replicate the results. So far, so good. Rather, the problem is that at least one celebrated authority in the field hates that, and would prefer much, much more deference to authority.