by Benjamin Bikman, PhD
Obesity is a global health crisis affecting over 650 million people worldwide, leading to conditions like type 2 diabetes, heart disease, and insulin resistance. For years, weight management has focused on calorie control—eat less, move more—but many people struggle to lose weight despite these efforts. Emerging research suggests that hormones play a critical role in regulating hunger, fat storage, and metabolism, which may explain why some individuals find it difficult to shed excess weight.
This article explores these key hormones—ghrelin, leptin, adiponectin, cortisol, estrogen, thyroid hormones, and insulin—and their significant roles in weight regulation and fat management.
Ghrelin: The hunger hormone
Ghrelin, often referred to as the “hunger hormone,” is produced primarily in the stomach. Given its source, it may not be too surprising to learn that its main job is to signal the brain when it’s time to eat, increasing appetite and food intake. Ghrelin levels rise before meals, driving hunger, and fall after eating.
When you are on a deliberately calorie-restricted diet, ghrelin levels can stay elevated, which can make you feel constantly hungry and lead to overeating, often sabotaging weight-loss efforts.
Research shows that elevated ghrelin levels can persist even after long-term weight loss, which helps explain why maintaining weight loss can be so difficult (Müller et al., 2015).
Therefore, strategies to reduce ghrelin levels—such as eating protein-rich meals, managing stress, and getting enough sleep—are key to curbing hunger during weight-loss efforts.
Leptin: The satiety hormone
If ghrelin is the gas pedal of hunger, leptin is the brake. Produced by fat cells, leptin signals the brain to stop eating by promoting satiety.
However, in people with obesity, a condition known as “leptin resistance” can develop. This means that despite high leptin levels, the brain no longer responds effectively to the hormone, and the individual continues to feel hungry.
Leptin resistance is a major factor many people face when trying to lose weight. It creates a feedback loop where overeating leads to more fat storage, which raises leptin levels even higher, but the brain doesn’t “see” the signal to stop eating (Scarpace et al., 2009).
Lifestyle changes like a balanced diet and regular physical activity can help improve leptin sensitivity, aiding in long-term weight control.
Adiponectin: The fat-burning hormone
Adiponectin is a hormone that plays a crucial role in regulating glucose levels and fatty acid breakdown. Unlike leptin, adiponectin levels are inversely related to body fat. Higher adiponectin levels are associated with better fat metabolism and reduced inflammation, which are both beneficial for preventing weight gain and promoting weight loss.
Increased adiponectin levels have been shown to enhance fat burning and reduce fat storage. Low adiponectin, on the other hand, is commonly observed in people with obesity and is linked to a higher risk of insulin resistance, type 2 diabetes, and cardiovascular diseases (Wang et al., 2016).
Exercise and weight loss can increase adiponectin levels, improving metabolic health and aiding in fat loss.
Cortisol: The stress hormone
Cortisol, known as the “stress hormone,” is released by the adrenal glands in response to stress. While short bursts of cortisol are essential for handling stress, chronic elevations in cortisol can contribute to fat accumulation, particularly around the abdominal area.
High cortisol levels can increase cravings for sugary foods, making it harder to stick to a healthy diet. It also promotes the storage of fat, particularly visceral fat, which is the dangerous fat stored around internal organs that increases the risk of metabolic diseases (Bjorntorp, 1996).
Managing stress through techniques such as mindfulness, yoga, or exercise can help keep cortisol levels in check, supporting better weight management.
Estrogen: The female hormone
Estrogen plays a key role in fat distribution and metabolism, particularly in women. Before menopause, women tend to store fat in their hips and thighs, which is considered less metabolically harmful than abdominal fat. However, after menopause, the decline in estrogen shifts fat storage to the abdomen, increasing the risk of metabolic disorders like insulin resistance and cardiovascular disease (Hetemaki et al., 2017).
In addition to influencing fat distribution, estrogen plays a dual role in fat cell regulation. It can promote the growth of new fat cells (adipogenesis), but it also helps keep existing fat cells smaller and less likely to become inflamed. Estrogen increases the sensitivity of fat cells to insulin, making them better at metabolizing fats and preventing them from enlarging (Park et al., 2017). After menopause, the loss of estrogen can lead to increased fat cell size and inflammation, contributing to the difficulty in losing weight as women age.
Hormonal therapy or lifestyle adjustments aimed at balancing estrogen levels may help mitigate these effects.
Thyroid hormones: The metabolic regulators
Thyroid hormones, particularly triiodothyronine (T3) and thyroxine (T4), regulate metabolism at a cellular level. They control how fast the body uses energy, burns calories, and metabolizes fat.
People with hypothyroidism, a condition where the thyroid is underactive, often experience weight gain due to a slower metabolism. Conversely, hyperthyroidism, where the thyroid is overactive, can lead to rapid weight loss.
One lesser-known aspect of thyroid hormones is their role in insulin sensitivity and fat cell metabolism. Thyroid hormones can affect the expression of insulin receptors on fat cells, making them a critical factor in regulating how the body stores and burns fat (Nannipieri et al., 2009). When thyroid hormone levels are optimized, they can improve insulin sensitivity and support effective weight loss.
Research has shown that thyroid hormone receptor activity in fat cells is crucial for regulating fat-burning and overall metabolic health. This interaction between thyroid hormones and insulin sensitivity highlights the need for thyroid function to be in balance for effective weight management.
Insulin: The most critical player in fat storage
Insulin is arguably the most important hormone when it comes to fat storage and overall weight regulation. Produced by the pancreas, insulin helps cells absorb glucose from the bloodstream, converting it into energy or storing it as fat when there is excess.
Insulin serves as a gatekeeper for nutrient storage, particularly fat. When insulin levels are high—often due to excess carbohydrate consumption—more glucose is stored as fat, which can lead to weight gain. Even more, when insulin is elevated, it actively prevents the fat cell from breaking down its stored fat, effectively keeping any fat locked in the fat cell. Conversely, when insulin levels are low or absent, the body is unable to store fat, and it starts breaking down fat for energy, leading to fat loss.
This regulatory function becomes even more critical in conditions like type 1 diabetes, where the pancreas cannot produce insulin. Indeed, type 1 diabetes allows us to determine the absolute dominance of insulin in body fat regulation. In individuals with type 1 diabetes, without insulin, glucose cannot enter the cells and the fat can’t help but release its stored fat, and the body is forced to burn fat for energy, regardless of food intake or the levels of other hormones like leptin, ghrelin, or cortisol. This can lead to rapid weight loss.
An extreme example of this is seen in “diabulimia,” a dangerous condition where individuals with type 1 diabetes deliberately underdose their insulin to lose weight. Without sufficient insulin, even though all other hormones that regulate fat—such as ghrelin, leptin, and thyroid hormones—might be in balance, the body cannot store fat, leading to rapid fat loss. This illustrates the critical role insulin plays in maintaining fat mass. Without it, no matter how balanced the other hormones are, the body cannot sustain fat stores.
Conclusion: Hormones matter more than calories alone
While calorie intake and physical activity are relevant parts of any weight-management plan, hormones play a pivotal role in how our body stores and burns fat. Ghrelin and leptin regulate hunger and satiety, adiponectin and thyroid hormones boost fat burning, and cortisol and insulin control fat storage. Understanding the complex interactions between these hormones can provide more personalized and effective strategies for weight loss and overall metabolic health.
For those struggling with weight, focusing solely on “calories in, calories out” may not be enough. A better understanding of hormonal balance, stress management, and diet can create a more holistic and sustainable approach to weight management. Of course, the most prudent and controllable variable also happens to be the most effective—namely, keeping insulin under control. With simple steps, such as controlling carbohydrate consumption and frequently fasting, both hormones, including insulin, and calories will work in our favor.
About the author: Dr. Benjamin Bikman is the director of the Unicity Scientific Advisory Board. He earned his doctorate in bio-energetics and specialized metabolic disorders as a postdoctoral fellow of the Duke-National University of Singapore. As a professor and scientist at Brigham Young University, Dr. Bikman’s focus is understanding chronic, modern-day diseases with an emphasis on the origins and consequences of metabolic disorders, including diabetes and obesity. He frequently publishes his research in peer-reviewed journals and presents at international science meetings.