Which nutrient is essential for carrying fat-soluble vitamins and part of cell membranes?

Fat is essential for carrying fat-soluble vitamins, which include vitamins A, D, E, and K. These vitamins cannot be efficiently absorbed or utilized by the body without the presence of fat in the diet, as they require it for proper digestion and absorption in the intestines. Additionally, fats are integral components of cell membranes, contributing to their structure and functionality. They provide rigidity and flexibility, which are crucial for maintaining cell integrity and regulating what enters and exits the cell.

In contrast, carbohydrates and proteins serve different roles in the body, such as providing energy and supporting tissue repair, but they do not have the same function in relation to fat-soluble vitamins or cell membranes. Sugars, a form of carbohydrate, are primarily involved in energy provision rather than serving any role in fat transport or membrane composition. Thus, fat's unique properties make it vital for these specific physiological functions.