The Myth of Decreased Metabolism

"Slow metabolism" has become a buzzword, often used as a default explanation for excess weight or the inability to lose it. Many see it as a hopeless condition, leading to the assumption that fighting weight gain is futile. But is this really the case? Modern physiology dismisses the idea of a "slow metabolism" as outdated, and researchers caution against relying on this concept. Despite this, many still cling to the notion that a sluggish metabolism is responsible for weight gain. Let’s clarify the reality.

In some parts of the world, the idea of slow metabolism is often labeled as "broken" or "damaged metabolism" (metabolic damage). However, most scientists agree that this concept is largely a myth, exploited by those seeking to profit from weight-loss products and services. To date, as of February 2018, there is no scientific data supporting the existence of a consistently reduced metabolism as the cause of weight gain.

Misconceptions Around Metabolism

There are several factors that people mistakenly attribute to a slow metabolism. These include hormonal imbalances and physiological reactions to dieting and exercise, which complicate weight loss efforts, but are not evidence of a broken metabolism.

Cortisol

If you’re following a calorie-restricted diet and engaging in high levels of physical activity, cortisol can become problematic. Here’s why:

Cortisol causes water retention.
Caloric deficits raise cortisol levels.
Cardio exercise increases cortisol production.
Stress, particularly the kind that comes from dieting, also boosts cortisol levels.
When cortisol levels are too high, they contribute to water retention, causing your weight to stagnate or even increase. In response, many people trying to lose weight make the mistake of further slashing their calorie intake and increasing cardio. This strategy backfires, as elevated cortisol levels make water retention worse, and the vicious cycle continues.

Often, when dieters relax, increase their caloric intake, and reduce cardio, their cortisol levels stabilize. As cortisol levels normalize, the body sheds excess water, and the scale drops. This sudden weight loss isn't fat loss; it’s simply the body releasing retained water once cortisol levels decline.

Leptin

Leptin is another key hormone involved in regulating body weight. It signals the brain about the body's energy reserves and the amount of food consumed. When leptin levels fall, which often happens on a diet, it triggers several metabolic responses commonly referred to as the "starvation response" or "metabolic damage." However, this is a misinterpretation. What’s actually happening is a natural metabolic adaptation to the reduced calorie intake.

When leptin levels drop, a range of physiological changes occurs:

Metabolism slows down.
Hunger increases.
Energy levels decrease.
Thyroid function declines.
Testosterone levels dip.
Interestingly, cortisol interferes with leptin's ability to signal the brain. High cortisol levels lead to leptin resistance, meaning that even when leptin levels are adequate, the brain receives fewer signals from leptin. This leptin resistance is a significant reason why weight loss can stall despite ongoing efforts.

Understanding Metabolism

Metabolism is not a singular entity but a combination of several processes:

Basal Metabolic Rate (BMR) or Resting Metabolic Rate (RMR) – the number of calories the body needs to maintain basic functions at rest.
Thermic Effect of Food (TEF) – the energy required to digest, absorb, and process food.
Thermic Effect of Exercise (TEE) – the calories burned during physical activity.
Spontaneous Physical Activity (SPA) or Non-Exercise Activity Thermogenesis (NEAT) – the energy expended through everyday movements and activities outside of exercise.
Each of these components responds to changes in food intake and body weight. For example, as soon as you begin a calorie-restricted diet, TEF naturally decreases because you're consuming less food. BMR also drops as you lose weight, since a smaller body requires fewer calories to maintain its functions. Additionally, your calorie burn during exercise decreases as body weight declines.

NEAT, which represents all the energy burned through non-exercise activities (such as walking, fidgeting, or household chores), is another factor that decreases on a diet. The body, sensing a caloric deficit, reduces unnecessary movements to conserve energy. This often leads to feelings of lethargy and less spontaneous activity throughout the day, further reducing daily calorie expenditure.

The Verdict on Slow Metabolism

So, does a "slow metabolism" exist? The answer is no. If someone isn't losing weight, it’s because they are not in a caloric deficit, even if they believe they are. While factors such as cortisol, leptin, and inaccurate tracking of physical activity can make the situation more complex, the fundamental issue is that the individual is not burning more energy than they are consuming.

People who claim to have a "slow metabolism" often overestimate their physical activity and underestimate their food intake. Unintentional cheat meals, overeating, or poor tracking of calories can all contribute to a misunderstanding of why weight loss isn't happening. Coupled with water retention caused by cortisol, the situation may appear worse than it is, leading them to believe they have a broken metabolism.

If these individuals rigorously and accurately track their calorie intake—consistently hitting a target of 1,250 calories, for example—they will lose weight at a noticeable rate. The key is consistency and precise control over food intake, not blaming a "slow metabolism" for their weight-loss struggles.