A recent study shows that people living on the continent of North America suffer 9 times more chronic fatigue and 31 times more chronic depression than do people living on the continent of Asia. Interestingly, Asians, on average, eat 20 grams of soy per day, whereas North Americans eat virtually none. It turns out that soy contains phytochemicals called isoflavones, which have been found to possess disease-preventing properties. Thus, North Americans should consider eating soy on a regular basis as a way of preventing fatigue and depression.
In this argument, the arguer cites a study showing that North Americans suffer from an amazingly higher rate of chronic fatigue and chronic depression than people living in Asia. From an unknown source, the arguer states that Asians eat much more soy than North Americans, who eat almost none, and that soy contains disease-preventing properties. The arguer then concludes his or her argument by stating that North Americans should consider regularly eating soy as a means of battling fatigue and depression. This argument suffers from at least four critical fallacies.
For the sake of this argument, we will assume that the studies and the statistics about North Americans' and Asians' soy eating habits are correct, and that isoflavones have been found to have disease-fighting properties. Given that, there is still a problem with the arguer directly correlating the eating of soy with the prevention of disease and depression. First of all, simply because soy may have disease-preventing properties, that does not mean that it can therefore fight chronic fatigue and chronic depression. Fatigue and depression may not actually even be considered as "diseases", therefore even given the fact that soy has disease-fighting properties, it would have no effect on the "nondiseases" of fatigue and depression. Secondly, even assuming that fatigue and depression are diseases, they are not specifically mentioned as diseases that soy or isoflavones are able to prevent. Perhaps soy can help prevent osteoporosis (bone loss), mumps or even chicken pox, but that does not mean that it can specifically address the problems of chronic fatigue and chronic depression. These two critical weaknesses alone make the argument unconvincing.
Furthermore, the arguer's conclusion is based on the idea that diet alone can prevent fatigue and depression by comparing the diets of North Americans and Asians. It is highly unlikely that diet alone is responsible for the tremendous difference in the rates of fatigue and depression between the two populations. Other factors such as lifestyles, occupations, residence in city or rural areas and levels of stress may play a much bigger factor than diet. Additionally, the arguer states that soy contains phytochemicals called isoflavones, which supposedly have disease-preventing properties. What is not stated, however, is whether these isoflavones are contained in a form in soy that is usable by the human body. It is possible that the particular configuration of the phytochemicals found in soy products is not usable by the human body, thereby producing no beneficial effects by people eating more soy products. In and of themselves, isoflavones may prevent certain diseases, but perhaps those found in soy are of no benefit to humans. By failing to address these possibilities, the arguer has presented an unconvincing argument.
In summary, the argument fails due to four major flaws in logic. First, "disease-preventing" properties does not mean "fatigue and depression" preventing properties. Secondly, fatigue and depression may not even be considered as diseases. Thirdly, the arguer ignores the probability that diet alone is not the sole reason behind the increased rates of fatigue and depression for North Americans as opposed to Asians. Finally, isoflavones as found in soy may not produce the same beneficial effects as when it is found in other forms. To strengthen the argument and conclusion, the arguer should present evidence that directly links diet to fatigue and depression as well as evidence that shows that soy can specifically prevent chronic fatigue and chronic depression in North Americans.