Insulin primarily controls the concentration of which substance in the blood?

Insulin is a hormone produced by the pancreas that plays a crucial role in regulating blood glucose levels. Its primary function is to facilitate the uptake of glucose from the bloodstream into the body's cells for energy production or storage as glycogen in the liver and muscles. When food containing carbohydrates is consumed, blood glucose levels rise. In response, insulin is released to help lower these levels by promoting cellular uptake of glucose.

This process is essential for maintaining homeostasis, as excessively high concentrations of glucose in the blood can lead to serious health issues, including diabetes. Therefore, among the provided options, glucose is the substance that is primarily controlled by insulin in the blood. This understanding of insulin's role highlights the importance of monitoring blood glucose levels, especially in individuals with insulin sensitivity or diabetes.