The study of nutrition is the study of the nutritional basis of disease. If anything is alternative medicine, it is the medical arts that are being practiced by mainstream medical professionals today. Over many centuries, physicians have learned that when symptom A occurs, and the physician gives drug X, performs surgery Y, or gives radiation Z, the symptoms subside. This is not science, it is art. That why doctors today are called practitioners of the medical arts.
Medical science is completely different.
Science is completely different from trial and error. In science, experiments are performed, preferably controlled and double-blind. Double-blind means that neither the patient nor the experimenter knows who is getting the treatment and who is not. This way subjective judgments are reduced to a minimum. For ethical reasons, it is very difficult to do controlled experiments on humans.