BODY WEIGHT

Body weight regulation is a complex intertwining of physiological, metabolic, environmental, behavioral, and genetic variables that determine energy intake and expenditure. Body weight is the tangible result of the “ energy balance ” between energy intake and energy expenditure 1 .  

Below are the main reasons that contribute to weight gain:  

Caloric surplus: The energy taken in through food is used by the body both at rest (to maintain vital functions) and during physical activity 1 . A caloric surplus occurs when the number of calories consumed exceeds the number of calories burned by the body 2 . Weight gain is therefore the result of an excess of energy compared to real needs.  

The consumption of energy-dense foods and beverages, combined with inadequate physical activity that does not compensate for the excess calories, contributes to the development of a caloric surplus. It is also essential to consider the role of basal metabolism, or the amount of energy used by the body to maintain vital functions at rest, which represents the largest share of total energy expenditure. Increasing the level of physical activity increases energy expenditure and promotes the development and preservation of muscle mass, which contributes to the increase in basal metabolism 1 .  

Unhealthy Eating Habits: Unhealthy eating habits, such as frequent consumption of highly processed foods, excessive portion sizes , irregular meal times, and emotional eating , can contribute to weight gain. Such behaviors often lead to excessive food intake and consumption of foods that are poor in nutrients, but rich in refined sugars , saturated and trans fats , promoting weight gain and worsening metabolic health over time .  

Behavioral factors: Eating behavior is not only influenced by physiological mechanisms such as hunger and satiety, which balance the energy consumed with that introduced, but also by psychological and social factors. The latter influence food choices, the quantities consumed and the times at which one eats 1 . The pleasure derived from certain foods can lead us to eat more of them , in the same way, eating distractedly or without real hunger can increase calorie consumption and lead to weight gain over time .  

Sedentary lifestyle: A lifestyle characterized by prolonged sedentary behavior and from lack of physical activity , could contribute to weight gain and poor metabolic health. Physical activity effectively contributes to weight loss because, by strengthening muscle mass, it also increases energy expenditure relative to basal metabolism, which is influenced by muscles for a third 1 .  

Genetics: Genetics play an important role in determining an individual's predisposition to weight gain and obesity 2 . Although genes alone do not determine weight status, they can influence metabolism, fat storage, appetite regulation, and response to dietary and lifestyle changes 2,3,4 .  

Medical Conditions: Certain medical conditions, such as hypothyroidism , polycystic ovary syndrome (PCOS), hormonal imbalances, insulin resistance, and metabolic disorders, can contribute to weight gain and difficulty losing weight. These conditions affect metabolism, hormone levels, appetite regulation, and energy balance, making weight management more difficult.  

Stress and emotions: Periods of high stress and/or of anxiety can influence eating behaviors, food choices, and weight management. Chronic exposure to stressful events activates the hypothalamic-pituitary-adrenal (HPA) axis, causing excessive release of glucocorticoids, such as cortisol 5 , 6 . Excess cortisol can elevate plasma glucose levels, reduce adiponectin levels, promote lipolysis and muscle catabolism, and antagonize the anabolic effects of hormones such as growth hormone and insulin 5 , 7 , 8 .  

Animal studies have shown that hypercortisolism caused by chronic stress can cause visceral obesity and insulin resistance, contributing to the development of the obese phenotype 4 . Even in humans, epidemiological research confirms a strong correlation between prolonged exposure to stress and the onset of metabolic disorders 4 .  

Additionally, chronic stress can lead to more eating because it stimulates appetite through the same stress-induced hormones 6 .  

Lack of sleep: Inadequate sleep duration and poor sleep quality can disrupt hormonal balance, appetite regulation, and metabolism, contributing to weight gain 6 . Sleep deprivation affects hormones such as leptin and ghrelin, which regulate hunger and satiety , leading to increased appetite, a preference for high-calorie foods, and an altered metabolism, which promotes fat storage and weight gain.  

The main mechanisms related to weight gain include :  

Insulin Resistance: Insulin resistance occurs when the body's cells become less sensitive to insulin , which plays a key role in regulating blood sugar levels and promoting the cells' uptake of glucose for energy. Insulin resistance promotes fat storage, especially in the abdominal area, and impairs the body's ability to utilize it . to get energy .  

Hormonal Imbalances: Hormonal imbalances, such as elevated cortisol levels (due to chronic stress), thyroid dysfunction, and imbalances in sex hormones (e.g., estrogen, testosterone), can disrupt metabolic processes, appetite regulation, and energy balance, leading to weight gain.  

Leptin Resistance: Leptin is a hormone produced by fat cells that helps regulate appetite, energy expenditure by signaling the brain when energy reserves are sufficient. Leptin resistance occurs when the brain becomes less responsive to leptin signals, leading to reduced satiety, increased appetite and difficulty regulating food intake . Leptin resistance leads to an altered sense of satiety.  

Genetic Factors: Genetic factors play a significant role in determining an individual's susceptibility to weight gain and obesity. Certain genetic variants may predispose individuals to a slower metabolism, reduced caloric expenditure, increased appetite, and a tendency to store fat, contributing to weight gain and the risk of obesity.