Can having too little body fat damage your bones?

Having too little body fat can indeed damage your bones, but it is not as simple as 'more fat = better bones'. Which fat, where in the body, and how much muscle mass you have makes a big difference.

The clearest example comes from sports research. Among elite endurance athletes who were eating too little relative to their training load, 45% had reduced bone density. A quarter met all three criteria of the so-called 'female athlete triad': insufficient energy intake, disrupted menstruation, and weaker bones. Notably, these women were of normal weight. You do not have to be extremely thin to sustain bone damage.

Muscle mass probably plays a greater role in bone health than fat mass itself. Research in children showed that muscle was the only factor that predicted bone density. Greater fat mass in obese children was even associated with lower bone density in the spine. In postmenopausal women with diabetes, the group with insufficient muscle mass fared worst in terms of bone density, while excess weight without muscle loss appeared to actually protect the bones.

Not all fat does the same thing. Fat around the hips and thighs appears to protect the bones, even after correcting for other factors. Abdominal fat around the organs is associated with lower bone density in multiple studies, although causality has not yet been established here. The overall fat percentage therefore says little: the location of the fat matters.

At older ages, muscle loss and bone breakdown often occur together, sometimes combined with an increase or redistribution of fat mass. This interplay can negatively affect bone health. Low-grade chronic inflammation is considered a possible explanation, but the precise mechanisms have not yet been fully mapped out.