Certain chemical messengers, such as neurotransmitters, are formed from dietary constituents, which serve as chemical precursors. Furthermore, studies have shown that one can change his or her consumption of these precursors in order to alter the release of their respective neurotransmitter products. Acetylcholine, the neurotransmitter that serves as a message carrier between nerves and muscle cells, is formed from choline, a common constituent in fish, liver, and eggs. It follows, therefore, that the consumption of these foods, or dietary supplements that carry the same components, will aid the body's production of this chemical messenger.

In 2007, Kingsley et al. published a study demonstrating that oral supplementation of choline increases plasma choline levels and therefore increases the efficiency by which the body creates acetylcholine. Kingsley concluded that, due to the increase in their bodies' production of acetylcholine, athletes who supplemented their diets with artificial sources of choline would exhibit enhanced performance in activities that require extensive muscle contraction. When acetylcholine binds to receptors on skeletal muscle fibers, it opens sodium channels in the cell membrane allowing sodium ions to enter the muscle cell and initiate a sequence of steps that produce muscle contraction. Kinglsey also suggested that, due to its involvement in the formation of acetylcholine, low choline levels would be a contributing factor to the development of muscle fatigue.

However, in 2009, further studies suggested that an increase in a person's consumption of choline has only a negligible impact on his or her exercise performance. The researchers in this more recent study point out that choline is not only a precursor of the production of acetylcholine, but also a critical component of many fat-containing compounds in the cell membrane. This membrane surrounds all body cells, and because its growth is inextricably linked to adequate choline supplies, membrane formation is an alternative to acetylcholine synthesis as a major function of choline. In these studies, increasing plasma choline levels exerted no effect on either brief, high-intensity cycling tests or more prolonged aerobic exercise tasks. This conclusion is supported by a study of marathon runners supplementing their diets with choline who saw no improvement in endurance or athletic performance when participating in strenuous exercise.

According to the passage, conflicting studies have been published regarding which of the following?


(A) The result of increased acetylcholine levels on muscle contraction

(B) The role of choline in acetylcholine production

(C) Consumption of foods that contain choline versus dietary supplements

(D) The impact of choline consumption on athletic performance

(E) The benefit of improved athletic performance as a result of increased choline consumption