Materials derived from Citrus plants can be administered orally to humans for the purpose of producing or maintaining weight loss as well as for improving the person's physical performance and increasing the person's lean muscle mass. The Citrus materials include those portions of the plant that are normally considered waste or inedible, such as the leaves, peel, and immature, unripe fruit. The materials contain at least one of the alkaloids from the group consisting of synephrine, hordenine, octopamine, tyramine and N-methyltyramine (1). Two species, Citrus aurantium and Citrus reticulata, are particularly useful. The materials can be administered in their natural form or as extracts, and can be administered in various ways including capsules and tablets. The Citrus materials may also be used as a tea. For weight loss and weight control, the materials can be administered concurrently with caloric restriction or in the absence of caloric restriction. The materials may also be administered for the purpose of increasing muscle mass concurrently with a high protein diet as well as with an exercise program.