The Marathon of Weight Loss: Why Does It Take So Long?


As athletes, we often strive to maintain or reach a specific weight for various reasons: to compete in a particular weight class, improve power-to-weight ratios, or to achieve an aesthetic look. The journey to weight loss involves limiting energy intake, increasing energy expenditure, or both, to create an energy deficit (Garthe et al. 2011; Trexler et al. 2014.) But why does it seem to take so long? The answer lies within our bodies' intricate responses to energy deficit and weight loss.

The Physiological Challenges of Energy Deficit and Low Body Fat

Athletes, especially bodybuilders and martial arts competitors, often reach very low body fat percentages, sometimes less than 5%. Elite female gymnasts and endurance runners can have body fat percentages as low as 10-13% (Trexler et al. 2014). Achieving these levels poses unique physiological challenges.

When the energy deficit is large, muscle and adipose tissues are often lost, which in turn reduces energy expenditure (Leibel et al. 1995; Garthe et al. 2011; Trexler et al. 2014). This means the less fat mass you have at the beginning of your weight loss journey, the more likely you are to lose fat-free mass during the process (Forbes 2000).

Hormonal and Metabolic Adjustments

The body's response to weight loss and energy deficit is complex. Hormone balance and energy metabolism adjust in ways that can reduce energy expenditure and satiety (Trexler et al. 2014). For instance, thyroid and sex hormone secretion can decrease, and energy metabolism can slow down after just two weeks of energy deficit (Strauss et al. 1985; Friedl et al. 2000; Du-cet et al. 2001; Johannsen et al. 2012; Rossow et al. 2013; Knuth et al. 2014).

These adjustments are not limited to resting metabolism but also extend to energy expenditure during physical activity. In an energy-deficient state, non-exercise energy expenditure (NEAT) decreases, reducing total energy expenditure. This can have an adverse effect on weight loss and weight management. Even during conscious physical activities, like exercise, energy expenditure decreases. It's been reported that the same exercise consumes 10-20% less energy than before (Doucet et al. 2001; Bravata et al. 2007; Hill et al. 2009; Trexler et al. 2014; Müller et al. 2015). This adaptive response to low energy intake is known as adaptive thermogenesis (Duocet et al. 2001; Trexler et al. 2014; Müller et al. 2015.)

The Role of Adaptive Thermogenesis in Weight Loss

Another crucial factor that contributes to the complexity of weight loss is adaptive thermogenesis. This process involves a greater than anticipated decline in energy expenditure, considering the changes in body tissue masses. Adaptive thermogenesis is most commonly stimulated by the presence of a negative energy balance (Müller et al., 2016). Furthermore, increased aerobic exercise during contest preparation may enhance its development, as demonstrated in a recent study (Isola et al., 2023). There is evidence that long-term increases in exercise energy expenditure can suppress other components of energy expenditure, even while in energy balance (Pontzer, 2018).

Resting energy expenditure (REE) is inevitably reduced after weight loss. This reduction occurs due to metabolic adaptations and the loss of energy-expending tissues such as adipose tissue, skeletal muscle, and other organs (Müller, Heymsfield, and Bosy-Westphal, 2021; Martin et al., 2022). For instance, a study by Martin et al. (2022) found that approximately 40% of the reduction in REE after weight loss was attributed to metabolic adaptations. These adaptations may reflect a decline in immunity, reproduction, and stress response (Pontzer, 2018).

Although Isola et al. (2023) did not measure organ masses directly, they proposed that the decrease in adjusted REE with fat mass and lean mass reflects metabolic adaptation. This proposition was supported by their observation of a decline in resting heart rate in both sexes, previously linked to changes in the autonomous nervous system during adaptive thermogenesis (Rosenbaum and Leibel, 2010). Previous studies reported a temporary suppression of the immune (Sarin, Gudelj, et al., 2019) and reproductive (Hulmi et al., 2017) systems, decreased levels of systemic inflammation (Sarin, Lee, et al., 2019), and using leukocyte transcriptomics, repressed mitochondrial oxidative function and protein translation (Sarin et al., 2021) during contest preparation in an earlier female physique athlete study.

While it's been questioned whether adaptive thermogenesis only exists during an energy deficit (Martins et al., 2020), it's noteworthy that even when participants in the studies referenced had reached energy balance during the last week of competition preparation, many of the fat-loss-induced physiological and molecular mechanisms behind adaptive thermogenesis were still present (Hulmi et al., 2017; Sarin, Gudelj, et al., 2019; Sarin, Lee, et al., 2019; Sarin et al., 2021).

This discussion of adaptive thermogenesis underscores how our bodies are finely tuned machines, constantly adapting to energy intake and expenditure changes. It serves as a reminder that weight loss is not a straightforward process but rather a dynamic interplay of various physiological and metabolic factors. Understanding these mechanisms is essential for effectively managing weight loss and achieving athletic performance goals.

The Thermodynamics of Weight Loss

Weight loss follows the first rule of thermodynamics: energy can't disappear; it has to be consumed to lose body weight. Hence, body weight decreases when less energy is consumed than expended (Hall et al. 2012). This energy deficit is necessary to reduce a significant amount of adipose tissue.

However, the larger the energy deficit, the more body weight tends to come from lean mass (Helms et al. 2014). For this reason, a weight loss rate of 0.5 kg per week is recommended (Fogelholm 1994; Mero et al. 2010; Hall et al. 2012; Helms et al. 2014). This rate allows adequate protein intake and energy to support exercise, minimizing muscle mass loss.

Reducing daily energy intake by 500-1000 kilocalories per day can result in a steady weight loss of 0.5-1.0 kg per week (Schoeller 2009; Hall et al. 2012). But to continue this weight loss, energy intake must decrease continuously as total weight decreases because energy expenditure also decreases as the body becomes lighter (Hall et al. 2012; Müller et al. 2015).

Interestingly, studies show that individuals vary in their metabolic adaptations during and after energy deficits. Body weight does not necessarily fall, even if the calculated energy deficit is sufficient. It appears that the lower the energy intake, the more strongly the body adapts (Helms et al. 2014; Trexler et al. 2014).

Aiming for Optimal Weight Loss

In optimal weight loss, fat mass reduction is maximized without muscle mass loss (Freedman et al. 2001). This delicate balance can be achieved by strength training, maintaining a moderate energy deficit, and following a protein-rich diet. These strategies can prevent muscle mass loss and even increase muscle mass despite energy deficit (Donnelly et al. 1993; Garthe et al. 2011; Churchward-Venne et al. 2013).

Concluding Thoughts

The journey to weight loss is indeed a marathon, not a sprint. It's a complex process heavily influenced by our body's physiological and metabolic responses to energy deficits. As we've seen, large energy deficits and low body fat percentages present unique challenges. Hormonal changes, metabolic adjustments, and reductions in energy expenditure during physical activity all contribute to making weight loss a slower process than we often expect.

But understanding these mechanisms can help us navigate the journey more effectively. By focusing on sustainable practices such as strength training, a protein-rich diet, and maintaining a moderate energy deficit, we can achieve optimal weight loss, minimize muscle mass loss, and maximize fat mass reduction.

Remember, weight loss is not just about numbers on a scale. It's about enhancing performance, achieving personal goals, and promoting overall health and well-being. So, the next time you feel frustrated by the pace of your weight loss, remember the complex journey your body is undertaking. Patience, consistency, and a well-balanced approach will ultimately lead you to your goal.

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