Why You Don’t Just Lose Fat When You Lose Weight


It’s a hard truth, but one that everyone trying to trim down must reckon with: “Losing weight” and “losing fat” are not the same thing. In fact, when most people say they want to “lose weight,” what they really mean is that they want to be less fat.

However, muscle loss is a common cause of weight loss when dieting. This is unfortunate for two reasons: First, it means the fat is sticking around. Second, it means that the muscle is being lost, which can lead to an even flabbier appearance, despite what it says on the scale. 

There are additional implications for fitness, strength, and metabolism that all those looking to trim down in the new year need to be aware of. 

To achieve weight loss, creating a calorie deficit is necessary. This can be achieved either by consuming fewer calories than the body utilizes or by burning more calories through exercise, though usually a combination of both. 

Initially, in a calorie deficit, the body depletes its glycogen stores, which consist of glucose from carbohydrates. As glycogen is stored with water in the muscles, its utilization results in the release of water weight, explaining the significant early weight loss experienced by some individuals.

Once glycogen stores are depleted, the body turns to metabolizing fat for energy. However, certain tissues, like the brain, cannot directly use fat for energy. In a calorie deficit, the body resorts to metabolizing muscle tissue, as protein stored in muscles can be converted into glucose for energy. This muscle loss not only affects fitness and strength but can also lead to a slowed metabolism, contributing to weight regain after initial weight loss.

Various factors influence the amount of muscle lost during a calorie deficit. Contrary to previous beliefs, both lean and obese individuals can experience significant muscle loss. Ethnicity and genetics may also play a role, with some studies suggesting differences in muscle mass loss among ethnic groups.

The rate of muscle loss is independent of the speed at which weight loss occurs. Instead, the determining factor is the overall amount of weight lost. Generally, if a person loses 10% of their body weight, approximately 20% of this is fat-free mass, including muscle.

While popular beliefs suggest that a high-protein diet helps preserve muscle mass during weight loss, research indicates that muscle loss occurs similarly across various dietary approaches. Combining exercise, particularly resistance and endurance exercises, with a protein-rich diet is the most effective way to mitigate muscle loss during weight loss.

To maintain muscle mass during weight loss, it is recommended that adults consume 0.8g of protein per kilogram of body weight per day. However, increased protein intake, ranging from 1.2-1.5g per kilogram of body weight, or even exceeding 2g for those engaging in intense exercise, may be necessary. It’s crucial to avoid excessive protein intake (over 2.5g per kilogram of body weight) to prevent potential adverse effects on metabolism, kidney, and liver health.

Despite efforts to prevent muscle loss, other metabolic changes, such as alterations in metabolic rate and increased appetite, can contribute to weight regain. Therefore, the sustainability of dietary and lifestyle changes is crucial for long-term weight management.

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