Understanding Resting Energy Expenditure (REE)
Resting energy expenditure (REE) is the amount of energy (calories) your body burns at rest to perform basic functions like breathing, circulating blood, and maintaining body temperature. It accounts for the majority of your total daily energy expenditure. For decades, it was commonly accepted that REE steadily and significantly declined with each passing decade after our twenties, a belief that made weight gain seem inevitable with age.
The Surprising New Research on Age and Metabolism
Recent, large-scale studies have painted a more nuanced picture of how metabolism changes throughout the lifespan. A groundbreaking 2021 study published in Science analyzed data from thousands of individuals and revealed a more complex timeline:
- Infancy: Metabolism is at its highest, with infants burning calories 50% faster than adults, relative to their body size.
- Adolescence (1-20 years): After the initial spike, metabolism steadily declines by almost 3% per year until age 20.
- Adulthood (20-60 years): Contrary to popular belief, metabolism is remarkably stable during these decades. This means that middle-aged weight gain is not caused by a metabolic slowdown, but is instead linked to factors like dietary changes, reduced activity levels, and stress.
- Older Adulthood (60+ years): A gradual, but definitive, decline in REE begins around age 60, decreasing by about 0.7% per year.
This new understanding highlights that the significant metabolic decline doesn't happen during midlife, but rather begins later in life and is more gradual than previously thought. The reasons behind this post-60 slowdown are multifaceted.
Why Resting Energy Decreases After 60
The decline in REE in older age is not just about getting older; it's a consequence of specific physiological changes. Two main factors contribute to this phenomenon:
Loss of Fat-Free Mass (FFM) and Muscle
The primary driver of the age-related decrease in REE is a reduction in fat-free mass (FFM). FFM includes muscles, organs, and bone, and is far more metabolically active than fat mass. As we age, we experience sarcopenia, the gradual loss of skeletal muscle mass and strength. This loss of metabolically active muscle tissue directly leads to a lower overall REE. Since muscle burns more calories at rest than fat, having less muscle mass means fewer calories are burned throughout the day while doing nothing.
Changes in Organ Metabolic Activity
While FFM loss is the most significant factor, there is also evidence that the metabolic rate of individual organs and tissues may slightly decrease with age. Highly metabolic organs like the liver, brain, and kidneys account for a large portion of resting energy expenditure. Even after accounting for changes in body composition, studies show a slight age-related reduction in overall REE, suggesting that cellular-level changes also play a role.
Mitigating the Decline: What You Can Do
Even with the natural metabolic shift after age 60, individuals are not powerless. Lifestyle choices can significantly influence and mitigate the extent of metabolic decline. The key is to address the primary cause: muscle loss.
1. Prioritize Resistance Training
Strength or resistance training is the most effective way to counteract sarcopenia. By lifting weights, using resistance bands, or performing bodyweight exercises, you can build and maintain muscle mass. More muscle tissue means a higher REE, helping your body burn more calories even at rest. The Department of Health and Human Services recommends muscle-strengthening activities at least two days a week.
2. Increase Protein Intake
Since older adults are at a higher risk of losing muscle mass, their protein needs are often higher than younger adults. Adequate protein intake provides the building blocks necessary to repair and build muscle, which is crucial when combined with resistance training. Lean meats, fish, eggs, dairy, and beans are excellent protein sources.
3. Stay Hydrated
Drinking enough water is vital for overall metabolic health. Studies show that drinking water can temporarily boost metabolism through a process called thermogenesis, where your body expends energy to warm the water to body temperature. Staying hydrated also supports nutrient absorption and athletic performance.
4. Get Sufficient Sleep
Poor sleep quality and duration can disrupt hormones that regulate metabolism and appetite. Prioritizing consistent, restorative sleep is a simple yet powerful strategy for supporting metabolic function as you age. Aim for 7-8 hours per night by establishing a calming pre-bed routine.
Comparing Metabolic Profiles: Young vs. Older Adult
The following table illustrates the key metabolic differences between a younger and an older adult, assuming similar body mass index (BMI) and health status to highlight the effects of age and body composition.
| Feature | Young Adult (e.g., 25 years) | Older Adult (e.g., 70 years) |
|---|---|---|
| Resting Energy Expenditure (REE) | Generally higher due to greater muscle mass and higher cellular metabolic rates. | Lower, primarily due to sarcopenia (loss of muscle) and lower organ metabolic activity. |
| Fat-Free Mass (FFM) | Higher muscle mass composition, which is more metabolically active. | Lower muscle mass; proportion of fat mass increases, which is less metabolically active. |
| Dietary Needs | Higher total calorie requirement to support a more active lifestyle and higher REE. | Lower total calorie requirement but similar or higher nutrient needs per calorie to prevent deficiencies. |
| Thermic Effect of Food (TEF) | Slightly higher TEF; the body expends slightly more energy to digest and absorb food. | Slightly reduced TEF; the digestive process is marginally less energy-intensive. |
| Physical Activity Expenditure | Often higher due to a more active lifestyle, leading to more calories burned through exercise and daily movement. | Typically lower activity levels, but exercise is still a powerful tool to boost metabolism and burn calories. |
The Takeaway for Healthy Aging
The key takeaway is that the traditional narrative of a failing, rapidly slowing metabolism in midlife is a myth. The more gradual decline that occurs after 60 is largely a function of body composition changes, particularly the loss of muscle mass. By proactively building and maintaining muscle through regular resistance training and adequate protein intake, individuals can significantly influence their REE and support a healthy metabolism well into their senior years. This empowers a more optimistic and actionable approach to weight management and overall health in later life. While a complete reversal of the aging process isn't possible, a healthy, active lifestyle can substantially mitigate its effects.
For more information on nutrition for older adults, the National Institute on Aging provides helpful guidelines: NIH News in Health.
Conclusion: A Proactive Path to Metabolic Health
Understanding the true nature of metabolic changes with age is the first step toward effective healthy aging strategies. Instead of accepting an inevitable decline, we can focus on actionable steps. By combining resistance training with a nutrient-dense diet rich in protein, staying hydrated, and prioritizing sleep, seniors can build a robust foundation for metabolic health. It's not about fighting an unbreakable force, but rather working with your body to maintain muscle and energy levels. The science is clear: a proactive approach to fitness and nutrition is the most powerful tool for influencing your resting energy expenditure and promoting vitality as you age.
