Why Eating Less and Training More Makes Women Lose Muscle and Gain Fat

Sims underscores that the healthspan benefits of endurance training are limited when it comes to the musculoskeletal system. Traditional endurance work primarily improves cardiovascular and metabolic fitness but does not provide the necessary mechanical stress to maintain bone density or muscle power. By contrast, heavy resistance training with low volume but high intensity (3–5 RM loads) targets the very systems that atrophy with age. Sims frames it as the "bread and butter" of healthy aging, with endurance acting as "soul food"—enjoyable but insufficient. She emphasizes that the power-based approach (moving the load as quickly as possible) is crucial because it trains the nervous system to react rapidly. This is what differentiates a stumble from a fall in an older adult. The protocol is straightforward, but consistency over decades is key. The mention of brain health and gut health expands the rationale beyond classic strength outcomes, indicating that the systemic hormonal and inflammatory benefits of this training style support whole-body vitality.

Sims acknowledges the immense fear women have when told to eat more, especially when they’ve been conditioned to believe that weight gain comes from overeating. Her approach is to start small: make sure pre- and post-workout meals are covered, which alone can add 200–500 calories without dramatically altering overall meal patterns. This has an immediate positive effect on training adaptations and body composition. She explains that the initial weight gain from carb storage and water can be alarming, but reassures that it balances out. Over time, the reduction in metabolic suppression leads to a body that burns more at rest and supports lean mass gain. She describes working with professional cyclists who were eating only 1,500 calories on rest days—far below maintenance—and how the slow increase in food intake was essential to reversing their deterioration. The protocol is not a crash bulk but a methodical recalibration of the body’s energy sensing. The end goal is a body that can support high training loads and reproductive health without breaking down.

Sims explains that kisspeptin neurons in the hypothalamus are crucial for reproducing and responding to energy status. When energy intake is too low, these neurons are downregulated, signaling to the body that it cannot afford to support reproduction, growth, or adaptation to training. In as little as four days of low energy availability, thyroid output and resting metabolic rate drop. Appetite hormones become disrupted, luteinizing hormone is suppressed, and the body enters a strong conservation state. Because the body is starved for fuel, it breaks down muscle for energy, further lowering metabolic rate. Perimenopausal and menopausal women are especially vulnerable, but the mechanism applies across ages. The result is a vicious cycle: a woman under-eats, loses muscle, gains fat, then restricts more and trains harder, exacerbating the problem. Sims stresses that the only way out is increased, consistent fueling—yet the psychological hurdle is immense because short-term weight gain on the scale reinforces the false belief that eating more is causing fat gain.

This insight reframes the entire under-fueling conversation. Rather than seeing weight gain as a personal failure of discipline, it becomes an expected physiological response to chronic underfueling. Sims’s message is that the female body is not a simple calories-in-calories-out machine when the endocrine system detects scarcity; it will actively conserve and hoard energy by sacrificing muscle and fertility, which is evolutionarily adaptive for survival but disastrous for athletic performance and body composition goals.

Sims details the initial response to refeeding: when a chronically low-fueled woman adds 200–500 extra calories per day, her body begins storing carbohydrate as glycogen in the muscles and liver. Each gram of glycogen binds about 3–4 grams of water, causing a noticeable rise in scale weight. This is misinterpreted as rapid fat gain, triggering anxiety and a retreat back to restriction. However, this water weight is a sign that the body is finally able to recover and top up energy stores. Over a period of two to three months, the metabolic suppression lifts, resting energy expenditure normalizes, and the body composition shifts: more lean mass is built, strength improves, sleep quality rises, and the initial water retention stabilizes. The scale weight often plateaus at a new, higher but healthier set point, while the mirror and performance metrics tell a very different story. Sims emphasizes that the scale is an inadequate tool during this transition; instead, women should track strength, energy, sleep, and how clothes fit. This nuanced guidance is critical for helping women stick with refeeding until the benefits manifest.

Sims explains that the aging process brings inevitable declines in muscle mass, bone density, and neuromuscular coordination if not actively countered. Endurance sports like running or cycling do little to preserve these systems. Heavy resistance training with loads enabling 3–5 repetitions for 3–5 sets, performed in a power-focused manner, stimulates myofibrillar protein synthesis, increases bone mineral density through mechanical loading, and improves neural drive and coordination. This type of training builds what she calls the 'base' for healthy aging: balance and rapid reaction time, which are crucial for preventing falls in later years. She cites broad research showing benefits for brain health, gut health, and metabolic flexibility—areas where steady-state cardio alone falls short. When an 80-year-old missteps off a curb, the difference between a stumble and a hip fracture can be the power and coordination built in decades of resistance training. Her message to endurance athletes is stark: keep the sport you love for the soul, but make resistance training the bread and butter of your longevity strategy.