Eating little and not losing weight can be frustrating, and the reasons can be varied and complex.

 Here are some possible scientific explanations:

1. Hidden calories.  Sometimes, people may underestimate their calorie intake.
2. Basal metabolism. Every person has a different basal metabolic rate, which is the amount of energy the body uses at rest.
3. Muscle mass. The amount of muscle mass relative to fat affects metabolism. A person with more muscle mass may burn more calories at rest.
4. Thermogenesis Not Induced by Physical Activity (NEAT). This includes all calories burned from daily activities such as walking, moving and even fidgeting. A low NEAT level may contribute to a reduced overall caloric expenditure.
5. Metabolic adaptations. With prolonged calorie restriction, the body can adapt by reducing the metabolism to conserve energy, making weight loss more difficult.
6. Thermal effect of food. Digestion and metabolism of food require energy. Some foods require more energy to be digested and metabolised than others. For example, proteins have a greater thermal effect than carbohydrates and fats.
7. Hormonal factors. Hormones play a crucial role in metabolism and weight regulation. Problems such as hypothyroidism, polycystic ovary syndrome (PCOS) or insulin resistance can interfere with weight loss.
8. Stress and sleep. High levels of stress can increase levels of cortisol, a hormone that can promote fat accumulation, particularly in the abdominal area. Lack of sleep can also negatively affect metabolism and the hormones that regulate appetite.
9. Water retention. Certain medical conditions, medications or excess salt in the diet can cause water retention, masking fat loss.

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