Leptin is a hormone involved in the regulation of body weight. Which of the following statements correctly describes a key feature of leptin action in humans?

Introduction / Context:
Leptin is an important hormone that links fat stores in the body to the regulation of appetite and energy expenditure. It has become well known in discussions of obesity and weight control. This question tests whether you understand where leptin is produced and where it acts, an essential part of human physiology and endocrinology.

Given Data / Assumptions:

- The hormone in question is leptin. - Options discuss whether leptin is produced by fat cells or the brain, and whether it acts on the brain or on fat tissue. - Some options incorrectly describe leptin as an enzyme or as simply deficient in thin people. - We assume standard human physiology.

Concept / Approach:
Leptin is primarily produced by adipocytes, the cells that make up adipose tissue. The amount of leptin produced generally reflects the amount of stored fat. Leptin travels through the bloodstream to the hypothalamus in the brain, where it binds to specific receptors. Its main actions are to reduce appetite, increase energy expenditure, and signal that energy stores are sufficient. Thus, leptin is secreted by fat cells and acts on the brain, not the other way around.

Step-by-Step Solution:
1. Identify the source of leptin: it is synthesised and secreted mainly by adipose tissue, particularly white adipocytes. 2. Recognise that leptin levels in blood rise as body fat increases and fall when body fat decreases. 3. Understand that leptin crosses the blood brain barrier and binds to receptors in the hypothalamus, a brain region that regulates hunger and energy balance. 4. When leptin levels are high, it signals that energy stores are adequate, suppresses appetite, and can promote increased energy expenditure. 5. Therefore, the key feature is that leptin is produced by fat cells and acts mainly on the brain to influence feeding behaviour and metabolism. 6. This directly matches option A and contradicts descriptions that say leptin is secreted by the brain or that it functions as an enzyme.

Verification / Alternative check:
Research on genetic leptin deficiency and leptin receptor defects in both mice and humans shows that when leptin signalling is absent, individuals experience extreme hunger, low energy expenditure, and severe obesity. Treatment with leptin in leptin deficient individuals can normalise appetite and weight, confirming that leptin signals from fat tissue to the brain. This real world evidence reinforces the understanding that leptin is produced by adipose tissue and acts on the hypothalamus.

Why Other Options Are Wrong:
- It is secreted by the brain and acts mainly on fat cells to store more triglycerides: This reverses the true direction of signalling. Leptin is not a brain secreted hormone that promotes fat storage. - It is an enzyme that directly breaks down stored fat in adipose tissue: Leptin is a hormone, not an enzyme. It regulates appetite and metabolism rather than directly catalysing fat breakdown. - It is usually deficient in thin people and present in excess only in obese people: Thin people can have normal functional leptin levels appropriate to their body fat. Many obese individuals have high leptin levels but show leptin resistance rather than simple deficiency.

Common Pitfalls:
A common misconception is that obese individuals are always deficient in leptin, similar to how some people with diabetes are deficient in insulin. In reality, many obese people have high leptin levels but reduced sensitivity to the hormone. Another mistake is to think of leptin as a fat burning enzyme instead of a signalling molecule. Remembering that leptin is a hormone produced by fat cells and acting on the hypothalamus in the brain helps clarify its proper role.

Final Answer:
Leptin is secreted mainly by fat cells and acts primarily on the brain to influence appetite and energy balance.