To the Editor.
—Dr Pahor and colleagues1 report the intriguing association between habitual physical activity and reduced gastrointestinal hemorrhage (GIH) and discuss the potential for a direct causal relationship. An alternate possibility is an underlying causal factor common to both. A potential common factor is a reduced proportion of arachidonic acid in membranes of the gastrointestinal mucosa and muscles. Membrane arachidonate is the substrate for mucosa-protective prostaglandins, and reduced muscle membrane arachidonate is associated with insulin resistance (ie, reduced enpheral glucose disposal).2 Insulin resistance results in reduced muscle glycogen deposition, and the level of muscle glycogen correlates with improved endurance performance.3 Thus, it is possible that the elderly, for whom walking may represent a high level of endurance performance, choose to exercise because membrane composition enables them to do so, and the same membrane composition protects against ulceration.We have reported considerable variability in phospholipid arachidonate among