Which of the following is a proven effect of ephedra?

The correct response highlights that ephedra has been shown to function both as a bronchodilator and as a weight loss aid. Ephedra, particularly its active compound ephedrine, has been documented to stimulate the central nervous system, leading to increased heart rate and metabolism. This can enhance the body’s ability to burn fat, making it a popular ingredient in weight loss supplements.

Additionally, ephedra's bronchodilator effects have been utilized in the treatment of asthma and other respiratory conditions. It works by relaxing the muscles in the airways, allowing for easier breathing. These attributes contribute to its dual reputation in both respiratory aid and weight management.

While other options touch on relevant topics, they do not accurately represent well-documented effects of ephedra. For instance, while it may affect endurance performance indirectly through increased energy expenditure, there is no strong evidence to suggest it enhances endurance significantly on its own. Similarly, ephedra is not directly linked to increased protein synthesis or to a reliable reduction in blood sugar levels, which further underscores the focus of the correct answer.