BMI for Older Adults: What Changes After 65

As Australians live longer, understanding how health metrics change with age becomes increasingly important. Body Mass Index, while useful across the lifespan, requires different interpretation for older adults than for younger populations. Research suggests that the "healthy" BMI range shifts upward after age 65, and that being slightly overweight by standard definitions may actually offer protective benefits for seniors. This article explores what we know about BMI and ageing, helping older Australians and their families make sense of weight-related health information.

Why BMI Interpretation Changes with Age

Several biological changes that occur with ageing affect the relationship between BMI and health. Body composition shifts naturally over time, even when weight remains stable. Older adults typically have more body fat and less muscle mass than younger people at the same BMI. Height often decreases due to spinal compression and postural changes, which can artificially increase BMI calculations even without weight gain.

Metabolism slows with age, affecting how the body processes nutrients and stores fat. Hormonal changes, including reduced production of sex hormones and growth hormone, influence fat distribution and muscle maintenance. These normal ageing processes mean that the same BMI number has different implications at age 75 than at age 35.

Additionally, the health risks associated with different BMI levels shift with age. Conditions that plague younger adults with elevated BMI—like type 2 diabetes—may already be present or managed in older adults. Meanwhile, new concerns emerge, including the risk of frailty, sarcopenia (age-related muscle loss), and the ability to recover from illness or surgery.

The Obesity Paradox in Older Adults

Perhaps the most striking finding from research on BMI and ageing is what scientists call the "obesity paradox." Numerous studies have found that older adults classified as overweight (BMI 25-29.9) by standard categories actually have lower mortality rates than those with "normal" weight (BMI 18.5-24.9). Some research suggests the optimal BMI for longevity in seniors may be in the 25-27 range.

Several explanations have been proposed for this paradox. Extra weight may provide nutritional reserves during illness or surgery, when the body's caloric demands increase while appetite often decreases. Some degree of body fat might offer protection during falls, reducing fracture risk. The fat tissue itself may have metabolic effects that are more neutral or even beneficial in advanced age.

It's important to note that this protective effect appears limited to the overweight range. Obesity, particularly severe obesity, remains associated with health risks in older adults, though these risks may be somewhat attenuated compared to younger populations. The "sweet spot" appears to be slightly above the traditional healthy range, not dramatically higher.

The Dangers of Being Underweight

If there's one consistent finding across studies of BMI in older adults, it's that being underweight (BMI below 18.5) is particularly risky in this age group. Underweight seniors have significantly higher mortality rates and poorer outcomes from hospitalisation, surgery, and acute illness.

Low BMI in older adults often indicates underlying problems including malnutrition, chronic disease, depression, or frailty. Unintentional weight loss in seniors—losing more than 5% of body weight within 6-12 months without trying—is considered a warning sign that warrants medical evaluation.

For older Australians, maintaining adequate nutrition and preventing unhealthy weight loss is often more important than pursuing weight loss goals. Unless obesity is causing specific health problems, intentional weight loss in advanced age should be approached cautiously and under medical supervision.

Muscle Mass: The Hidden Factor

For older adults, muscle mass may be more important than BMI for predicting health outcomes. Sarcopenia—the age-related loss of muscle mass and strength—significantly increases risk of falls, fractures, disability, and death. Someone can have a healthy BMI while having dangerously low muscle mass, a condition sometimes called "sarcopenic obesity."

Preserving and building muscle becomes increasingly crucial with age. Resistance training is safe and effective for older adults and has been shown to improve strength, function, and quality of life even in people in their 80s and 90s. Adequate protein intake also supports muscle maintenance—older adults may need more protein than younger people to maintain muscle mass.

When evaluating an older person's weight status, measures that account for body composition—like grip strength tests, walking speed assessments, or body composition scans—often provide more useful information than BMI alone.

Practical Recommendations for Seniors

Given the shifting relationship between BMI and health in older age, what should Australian seniors focus on? First and foremost, maintain regular physical activity including both aerobic exercise and resistance training. Physical activity preserves muscle mass, maintains bone density, supports balance and coordination, and provides numerous other benefits regardless of weight.

Prioritise adequate nutrition over calorie restriction. Ensure you're getting enough protein—aim for protein at each meal rather than concentrating it at dinner. Stay hydrated, as older adults often have diminished thirst sensation. If you're struggling to maintain weight or appetite, speak with your GP or an Accredited Practising Dietitian.

Don't pursue weight loss without medical guidance. If you're concerned about your weight, discuss it with your GP who can assess whether weight loss is appropriate given your overall health status and, if so, help you do it safely while preserving muscle mass.

Focus on function rather than numbers. Can you climb stairs? Carry groceries? Get up from a chair without using your arms? Maintain your independence with daily activities? These functional abilities are often better indicators of health than the number on the scale or your BMI calculation.

Working with Healthcare Providers

Older Australians benefit from healthcare providers who understand the nuances of weight and ageing. During appointments, don't be afraid to ask questions about what your BMI means at your age and whether weight management should be a priority given your overall health picture.

For older adults managing chronic conditions, Medicare's Chronic Disease Management plans can provide access to allied health professionals including dietitians and exercise physiologists. These specialists can help develop approaches tailored to your age, abilities, and health status.

Remember that health is multidimensional. A number on a chart doesn't define your wellbeing. Staying active, eating well, maintaining social connections, engaging in meaningful activities, and managing chronic conditions all contribute to healthy ageing—often far more than achieving a particular BMI target. Focus on the aspects of health you can influence, and work with healthcare providers who take a comprehensive view of wellness in older age.