Most of the public health messaging we hear focusses on the risks associated with excess body weight, which is absolutely justified. However, what we don’t tend to discuss is the long-term health effects of being underweight.

Clinically defined as a BMI of under 18.5, ‘underweight’ carries numerous long-term health consequences, several of which are as serious as those linked to obesity. It’s really important to understand that ‘thinness’ i.e., chronically sitting at the low end of the scale, does not equate to health.

The big one. Being underweight is associated with higher all-cause mortality.

This is the most significant finding across research and it’s not just in people who are unwell, but in the general population. The 32-year Swiss National Cohort (SNC) study which included over 31,000 participants, found underweight individuals had a 37% higher mortality risk than those at a healthy weight, driven mainly by external causes of death rather than cancer or cardiovascular disease. Where death happened through injury, the authors suggested that the underweight status may have increased risk directly through frailty and falls, and impaired ability to survive after major accidents.

In populations with existing illness, the risk is even higher; underweight patients with heart failure had a 59% higher mortality risk than those at a healthy weight, and underweight COPD patients had an 88% higher risk.

The relationship isn’t perfectly linear or identical across every group. Some research on adolescent girls did find lower cardiovascular mortality risk associated with underweight status later in life. But the overall pattern, especially in adults and especially where illness is present, clearly shows low body weight as a risk factor, rather than being protective as some people may assume.

Bone mineral density is a big concern.

Adequate body weight, and specifically adequate fat mass (emphasis on the word ‘adequate’, not excess), plays a direct role in skeletal health. Being underweight is one of the strongest independent predictors of low bone mineral density and osteoporosis. Bone remodelling depends on a stable supply of calcium, vitamin D, and energy, and it’s also influenced by circulating oestrogen, a hormone partly produced by fat tissue. In women with eating disorders, low BMI, low fat mass, and amenorrhoea are the strongest predictors of reduced bone density, more so than age or how long the person suffered with the eating disorder (anorexia) for.

It’s not just females that this affects; boys and men with anorexia nervosa show the same pattern of low bone mineral density in the whole body, notably in the spine and hips - low body weight, low muscle mass and low testosterone are the strongest predictors. This is also the case in ‘atypical’ anorexia – when someone may not be “underweight” on a BMI scale, but has lost significant weight from their own baseline. The good news is that boys who restore weight to a healthy BMI can see bone accrual rates that outpace those of healthy peers, but staying underweight, even partially recovered, continues to erode bone.

Impaired immune defences.

When energy or protein intakes are chronically low, the body’s capacity to mount an effective immune response is reduced. Life-span research has shown that being underweight at any age is a significant risk factor for infection. In underweight people with chronic illness, there is s higher a risk of serious infection compared to those with severe obesity. Low energy and protein intakes impair the production of white blood cells and antibodies and impact the functionality of the gut-barrier reducing the body’s ability to defend against pathogens.

Reproductive health is closely linked to energy intake.

The reproductive system is very sensitive to energy status. When a female of menstrual age, loses her period of 3 months or more, the condition is diagnosed as hypothalamic amenorrhoea. It is caused by chronic energy deficiency which disrupts the hypothalamic-pituitary-gonadal axis. This prevents the communication between the brain and ovaries, leading to the suppression of ovulation. The minimum body fat thresholds required for normal reproductive function are around 17% for menarche and 22% for ongoing fertility. This is why chronic underweight, whether from restrictive eating, overtraining, or illness, is one of the most common reversiblecauses of infertility in women.

Ladies. It is important to also be aware that ‘within-day’ low-energy-deficiency (LEA), even with a healthy body fat percentage, can also cause the loss of periods. When energy intake is not enough around intense training, the body will suppress ovulation. Your brain does not wait and assess intake over a 24-hour period, it acts in real-time to respond to changes in the bloodstream. This is why I am so anti- training fasted.

Why a bit of body fat is protective post-menopause.

Spending your whole life believing you should be skinny can really backfire as a female post-menopause – no one talks about why a bit of body fat is a good thing! For postmenopausal women, the relationship between body fat and health outcomes shifts. Because the ovaries stop producing oestrogen after menopause, adipose tissue becomes the body’s main remaining source of it, produced through a process called aromatisation. This is part of why leaner postmenopausal women tend to lose bone density faster and why sitting toward the upper end of a healthy weight range, or even mildly overweight, is associated with better bone density and lower mortality risk in this life stage than being underweight or even at the lower end of “normal.”

As with everything, there is an important caveat: this protective effect depends on muscle mass being preserved alongside the fat. When obesity coexists with muscle loss (known as sarcopenic obesity), the protective effect disappears and is replaced by elevated risk for cardiovascular disease, falls, and metabolic syndrome. So, it’s not a case of “more fat is always better,” it’s that being underweight is a real risk in older women and maintaining muscle mass matters just as much as maintaining a healthy amount of body fat.

A little summary…

The conversation around body weight seems to always focus on the risks of excess weight, but there is clear evidence that being chronically underweight carries risks of its own - higher mortality, weaker bones, a less resilient immune system, reproductive dysfunction, and metabolic consequences that can persist for decades. This does not mean every underweight person is unwell, individual health is much more complex than the number on the scale, but it does mean “thinness” is not synonymous to “healthy.” Body weight deserves to be assessed in both directions, with equal seriousness.