A medium peach 75 g (2.6 oz), has 30 Cal, 7 g of carbohydrate (6 g sugars and 1 g fibre), 1 g of protein, 140 mg of potassium, and 8% of the daily value (DV) for vitamin C.
As with many other members of the rose family, peach seeds contain cyanogenic glycosides, including amygdalin (note the subgenus designation: Amygdalus). These substances are capable of decomposing into a sugar molecule and hydrogen cyanide gas. While peach seeds are not the most toxic within the rose family, that dubious honour going to the bitter almond, large doses of these chemicals from any source are hazardous to human health.
Peach allergy or intolerance is a relatively common form of hypersensitivity to proteins contained in peaches and related fruit (almonds). Symptoms range from local symptoms (e.g. oral allergy syndrome, contact urticaria) to systemic symptoms including anaphylaxis (e.g. source: urtica http://www.mypyramid.gov/pyramid/vegetables_why.html ria, angioedema, gastrointestinal and respiratory symptoms). Adverse reactions are related to the "freshness" of the fruit: peeled or canned fruit may be tolerated.