Onions aren't the only source of nutrition. Strict Buddhists live without them entirely, because they are considered one of the "five pungent herbs" thought to arouse desire, and therefore to be avoided. Garlics are also avoided.
So, through this teaching, the devil has his way of toxing out the human body, and making the mind less receptive to truth. A toxic person (and I mean this in relationship to one's health, not demeanor) frequently loses first an ability to reason well, and often becomes more fanatical in his or her beliefs. This is very instrumental, then, in preventing them from being receptive to truth. They tend to become dogmatic. The next level is actual disorders of the mind that cause such things as memory loss, ADHD, personality disorders, narcissism, nervous disorders, and a host of other ailments, depending on the specific toxin levels and toxin combinations involved.
Where I presently work, thankfully, the Buddhists are not strict, and 99% of them eat meat. This helps to provide them a source of sulfur, and their toxic buildup is far less. In some places (other countries), the Buddhists are strict, and some of them do suffer health problems as a result.
The devil has many ways to break us down. Beliefs, eating habits, environmental problems, cultures, and lifestyle factors all play significant roles.
Interestingly, the children of Israel were rebuked for lusting after the leeks and onions and garlic of Egypt. God was providing manna. Onions aren't the be-all-and-end-all solution to health issues. In fact, Mrs. White never once recommended onions for the removal of toxins/poisons from the body in the way that she presented eggs. Eggs are a superior option.
Mrs. White does say this about onions...
Again, there is great virtue in well-cooked onions. Ask Edson White; he can tell you of the good that onions will do. {14MR 339.2}
But she also says this...
The scent of cooking had full access to these rooms with no current of air to take away the nauseating smell of ham, pork, onions, cabbage, and all kinds of scents. If I had not heretofore been most thoroughly disgusted with pork, I should have been [so] now. I could scarcely refrain from vomiting. I became sick and faint, but my good daughter Mary opened the window as far as possible and moved our bed so that the head of it was close by the window, the bed being quite nice. We slept well and felt refreshed in the morning, notwithstanding unpleasant odors. {11MR 141.1}
Apparently, Mrs. White also found onions to be "nauseating." Interestingly, cabbage is also mentioned in that category, and it also happens to be relatively high in sulfur. Durian is "nauseating" and/or "disgusting" to many people, particularly its odor. This is related, again, to sulfur.
Blessings,
Green Cochoa.