Common logical fallicies used in diet and nutritional arguments are

Poor Definition. On almost every supplement issue, conflicting claims can be found. What is almost never examined is that there is a significant difference between whole food extracts and synthetic analogs. For instance, synthetic vitamins do not work nearly as well as naturally occurring vitamins. Synthetic vitamins and supplements may also become toxic in large doses.

Appeal to Authority. If no authority for a statement is provided or a generic authority is put forward ("most researchers, doctors, athletes, whatever"), the information should be viewed with suspicion. If an authority is used to give weight to the argument, take a moment to look at his or her credentials. It is common for qualified authorities to have different or even opposing opinions. In such a case you must investigate the matter for your self.

False Analogy. The The Aspirin a Day argument is guilty of making a subliminal false apology with an Apple a Day. Aspirins and Apples are completely different because Aspirins are synthetic pharmaceuticals which have destructive side effects and apples are (originally) a whole food with heath building properties.