The mechanisms that make energy balance complex include:
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Adaptive thermogenesis and NEAT vary from person to person: This means that individuals have different responses to changes in diet and physical activity, which can influence their overall energy expenditure.
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Metabolic rates differ: Variations in basal metabolic rate (BMR) and total daily energy expenditure (TDEE) can significantly impact how much energy a person burns at rest and during activity, contributing to differences in weight management.
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Hormone levels influence weight loss: Hormones such as insulin, leptin, ghrelin, and cortisol can affect appetite, metabolism, and fat storage, making weight loss less predictable and more individual.
The statement "calorie deficits always result in a specific amount of fat loss" is not selected because it's not accurate. The relationship between calorie deficit and fat loss can be influenced by many factors, including those mentioned above, which can lead to variations in fat loss among individuals even when they are in a calorie deficit.