By Dr. Adrian Holliday

The “obesity epidemic” is a term with which we are all very familiar. However, the prevalence and health consequences of low weight and undernutrition are less well documented and communicated. This is of particular relevance for older adults.

Somewhat paradoxically, we are at an increased risk of both excess weight and low weight as we enter later life. Perhaps surprising to many of us, is that the latter carries the greater risk to health for older adults. In fact, what many would consider as excess weight is protective as we age. A study in the USA followed a cohort of over 4000 over 65s for an average period of 10 years¹. Using World Health Organisation definitions of healthy weight (18.5 – 24.9 km/m²), overweight (25 – 29.9 km/m², and obesity (30 km/m² and above), those in the “overweight” category had a 20% lower risk of all-cause mortality than those in the “healthy weight” category. In fact, those classified as living with obesity were 22% more likely to pass away than those deemed healthy weight. At the other end of the BMI spectrum, those with a BMI of less than 18.5 km/m² were over three-times more likely to pass away than those not classified as underweight. Given that unintentional weight-loss is experienced by 15-20% of older adults, awareness of the health detriments of low weight should be raised.

One reason for older adults having an increased risk of losing weight is a decline in appetite. We may have observed this in loved ones: parents or grandparents who have a small appetite and struggle to eat what we would consider to be a “normal” portion. This age-related reduction in appetite and food intake – termed “anorexia of ageing” – effects approximately 30% of free-living older adults and over half of those in residential care.

Low appetite can be the result of numerous changes experienced in later life. These include the loss of a loved one and loneliness, reductions in physical activity, reductions in smell and taste, and the onset of disease. Recently, our research group has shown that another possible contributing factor is a change in the way our gut senses and responds to the nutrients that we eat. We recruited younger adults and older adults, who we identified as exhibiting either a healthy appetite or low appetite². All participants ate the same porridge breakfast. Before, and for 4 hours after breakfast, we took blood samples to measure the concentration of some appetite-related hormones that are released from the gut in response to feeding. We measured the hunger hormone, ghrelin, and two satiety hormones that signal fullness: PYY and GLP-1. Compared with younger adults, older adults with low appetite had a greater suppression of the hunger hormone ghrelin, while PYY and GLP-1 concentrations increased to a greater extent and stayed elevated for longer. This was not seen in the older adults with healthy appetite, who showed a very similar hormone response to food that was seen in younger people. Consequently, the older adults with low appetite ate less at lunch.

These data suggest that some, but not all, older adults exhibit a degree of hypersensitivity to nutrients, causing an amplified hormone response to cause greater feelings of fullness and suppression of hunger.

Why this occurs in some older adults but not others is not currently known. But, continuing our research in this field, we hope to find out. We suspect that it could be a result of changes in the way the gut digests and absorbs nutrients; changes in the gut environments, such as altered gut microbiome; or changes in the behaviour of the cells which secrete hormones³.

Identifying the cause of this heightened hormone response to feeding may help us design nutritional and pharmaceutical interventions to help appetite-suppressed older adults eat well for health, wellbeing, and longevity in later life.

1. Cheng et al., 2016. Body mass index and all-cause mortality among older adults. Obesity, 24(10), 2232-2239. doi.org/10.1002/oby.21612
2. Dagbasi et al., 2024. Augmented gut hormone response to feeding in older adults exhibiting low appetite. Appetite, 201(3), 107415. doi.org/10.1016/j.appet.2024.107415
3. Dagbasi et al., 2024. The role of nutrient sensing dysregulation in anorexia of ageing: The little we know and the much we don’t. Appetite, 203(6), 107718. doi.org/10.1016/j.appet.2024.107718