The digestion of dietary carbohydrates is initiated by amylase, an enzyme present in saliva. This enzyme breaks down long carbohydrate molecules into shorter units such as disaccharides. Lipase breaks down fats and is not present in the saliva, rather it is secreted by the pancreas into the duodenum. Pepsin is a protease - an enzyme that breaks down proteins. This enzyme is secreted by the chief cells of the stomach in its zymogen form (pepsinogen). It is activated by the product of the parietal cells (hydrochloric acid). Note that amylase is also secreted by the pancreas to complete the chemical digestion of carbohydrates into monosaccharides so that they can be absorbed by the brush border of the small intestine.

Grapefruit can decrease the activity of cytochrome P450, one of the body's most important detoxification pathways, by as much as 30%. Consumption of grapefruit while on certain medications, including atorvastatin, can lead to toxic levels of the drug. All other foods listed are safe for patients to consume while taking atorvastatin.

Essential amino acids are amino acids that can not be synthesized within the body, but must be consumed through the diet. Tryptophan is the only essential amino acid listed. In all, there are 9 essential amino acids.

Kwashiorkor is a condition of sufficient caloric intake with severe protein malnutrition, often seen in impoverished areas where individuals must survive on carbohydrate-based sources of nutrition. This results in a severe deficiency of albumin, with symptoms such as edema, ascites, fatty liver, and dermal ulceration. Marasmus is a condition not specifically of protein deficiency but of general starvation, which may be due to lack of food security or to intentional fasting or extreme caloric restriction. Pellagra is a condition caused by niacin deficiency, and beriberi is a thiamine deficiency syndrome with neurological, gastrointestinal, and cardiac manifestations.

Vitamin B3 and vitamin B6 are required to convert glutamic acid into glutamine. Biotin (vitamin B7) plays roles in fat and amino acid metabolism, and though there has been no conclusive evidence of its effect on the growth of hair and nails, it is commonly taken as a supplement for these purposes. Folate (vitamin B9) is required during synthesis of red blood cells, and during DNA replication. Folate cannot be synthesized de novo, and thus must be ingested from the diet. Vitamin C is a powerful antioxidant, and is a cofactor to many biological processes. Cobalmin (vitamin B12) is involved in cell metabolism, particularly in that of fatty acids and amino acids. The source of B12 is from the natural flora of the gut. Vitamin D enhances the absorption of minerals such as calcium, magnesium, and zinc from the gastrointestinal tract. Vitamin E is a fat-soluble antioxidant.

Children ages 1-18 are at the highest risk for physiological changes due to malnutrition. Significant nutritional deficiencies at this age can cause failure to thrive, stunted growth, long-term brain damage, and increased incidence of cardiovascular and metabolic disease later in life.

L-arginine is a precursor for the production of nitric oxide, essential to vasodilation. Other precursors to nitric oxide include NADPH, oxygen, and nitric oxide synthetase (NOS) enzymes. These other amino acids do not have essential roles in the nitric oxide synthesis pathway.

The most common cause of copper deficiency in the United States is gastric bypass. This is due to malabsorption from food. Copper is fairly ubiquitous in soil and in the average American diet. Wilson's disease is a disorder of excess copper, rather than copper deficiency, resulting in a variety of neurological symptoms.

Patients who are taking potassium sparing diuretics must be counseled to eat a diet low in potassium. This is due to the fact that consumption of potassium-rich foods, such as orange juice, can lead to dangerously high potassium levels. Excess potassium levels can cause heart arrhythmia and sudden cardiac death.

Vitamin D is a fat soluble vitamin, and so will be better absorbed when taken with a meal that contains fat. The other vitamins listed may be better absorbed with food, but do not require fat for transport across intestinal membranes, as they are water-soluble.