What type of adipose tissue tends to decrease as humans age?

While infants possess a high concentration of calorie-burning brown fat, the amount and activity of this thermogenic tissue significantly decrease as humans age. This age-related reduction in metabolically active adipose tissue influences how the body manages energy and temperature, leading to important metabolic changes over time.

Quick Summary

Metabolically active brown adipose tissue (BAT) and inducible beige fat both decline in mass and function with advancing age. This reduction is linked to changes in thermoregulation, energy balance, and an increased risk of age-related metabolic diseases like obesity and type 2 diabetes.

Key Points

• Brown and beige fat decline: The most significant reduction in adipose tissue with age is seen in brown adipose tissue (BAT) and the thermogenic function of beige fat.

• Metabolic slowdown: The loss of calorie-burning brown fat contributes to a lower metabolic rate and decreased energy expenditure in older adults.

• Impaired temperature regulation: With less active BAT, older individuals have a reduced ability to regulate body temperature in cold environments, increasing their risk of hypothermia.

• Increased disease risk: The decline of thermogenic fat and shift toward visceral white fat storage is linked to a higher risk of metabolic and cardiovascular diseases.

• Multi-factor causes: The decrease is caused by a combination of factors, including mitochondrial dysfunction, reduced sympathetic nervous system signaling, hormonal changes, and altered stem cell function.

• Location matters: While total fat mass may increase with age, it's the specific reduction in thermogenic fat and shift towards less healthy visceral fat that poses a metabolic risk.

The Decline of Thermogenic Fat with Age

As humans grow from infancy into adulthood and beyond, the body undergoes a profound shift in its fat composition. The type of adipose tissue that tends to decrease with age is brown adipose tissue (BAT). The activity of beige fat, which is located within white fat depots, also declines. Both BAT and beige adipocytes are categorized as thermogenic, meaning they burn energy to generate heat. This decrease in thermogenic fat has significant consequences for metabolism, body temperature regulation, and overall health as we get older.

The Characteristics of Brown Adipose Tissue

Brown adipose tissue, often referred to as 'good fat,' is highly active and plays a crucial role in non-shivering thermogenesis, particularly in newborns who lack the ability to shiver.

• Mitochondrial Abundance: The brown coloration of BAT is a result of its high density of mitochondria, which are the powerhouses of the cell. These mitochondria contain a unique protein called uncoupling protein 1 ($UCP1$).
• Heat Production: When activated, $UCP1$ uncouples respiration from ATP production, causing the mitochondria to produce heat instead of chemical energy. This process is vital for maintaining body temperature in cold environments.
• Age-Related Loss: While prominent in infants, BAT mass and activity decline steadily throughout childhood and into adulthood. Research shows that active BAT is far less common in adults over 50 compared to younger individuals. The remaining deposits in adults are typically found in the neck, supraclavicular, and paravertebral regions.

The Emergence and Decline of Beige Fat

Beige fat cells are another type of thermogenic fat that emerge within white adipose tissue (WAT) depots in a process known as "browning".

• Inducible Thermogenesis: Unlike classical BAT, beige adipocytes can transition between a thermogenic, brown-like state and an energy-storing, white-like state based on environmental conditions.
• Stimulation: Browning is often triggered by stimuli such as chronic cold exposure, exercise, and certain pharmacological agents.
• Aging's Impact: Similar to BAT, the ability to form beige adipocytes and maintain their function diminishes with age. The reduction in beige adipogenesis is attributed to various factors, including the aging of progenitor cells and changes in the local adipose tissue environment.

What are the Mechanisms Behind Thermogenic Fat Loss?

The age-related reduction in brown and beige fat is a complex process influenced by several biological changes. Key mechanisms include:

• Mitochondrial Dysfunction: The aging process leads to a progressive decline in mitochondrial function within brown adipocytes. This results in reduced levels of $UCP1$ and impaired thermogenic capacity.
• Impaired Sympathetic Nervous System (SNS) Signaling: Activation of BAT is regulated by the SNS. With age, there is a reduction in the sensitivity of adrenergic receptors in adipose tissue, meaning the cells become less responsive to the signals that trigger heat production.
• Hormonal Alterations: Changes in endocrine signals contribute to the decline of thermogenic fat. Decreasing levels of sex hormones (e.g., estrogen in women) and altered thyroid hormone function negatively affect BAT activity.
• Progenitor Cell Dysfunction: Adipose stem and progenitor cells (APSCs) have a reduced capacity to proliferate and differentiate into functional brown or beige adipocytes as they age. This limits the body's ability to regenerate its thermogenic fat supply.
• Inflammaging: Chronic, low-grade systemic inflammation, or "inflammaging," is a hallmark of aging. Pro-inflammatory cytokines can inhibit thermogenic gene expression and impair the function of brown and beige fat cells.

The Impact of Reduced Thermogenic Fat on Metabolic Health

The loss of brown and beige fat has significant implications for an aging person's metabolic health.

• Reduced Energy Expenditure: Less thermogenic fat means a lower basal metabolic rate. Since these cells are highly efficient at burning calories, their decline leads to a decrease in overall energy expenditure.
• Poorer Thermoregulation: The ability to regulate body temperature in response to cold diminishes with less active BAT. This can make older adults more sensitive to cold and increase the risk of hypothermia.
• Increased Risk of Metabolic Disease: Studies have found that higher levels of brown fat are associated with a reduced risk of metabolic conditions like obesity, type 2 diabetes, and cardiovascular disease. The loss of this metabolically active tissue contributes to the age-related increase in these health issues.
• Lipid Redistribution: As thermogenic fat decreases, there is often a redistribution of fat from subcutaneous depots to visceral areas. This accumulation of visceral white fat is particularly linked to adverse health outcomes, including insulin resistance.

Comparison: White vs. Brown Adipose Tissue

Feature	White Adipose Tissue (WAT)	Brown Adipose Tissue (BAT)
Primary Function	Stores excess energy as triglycerides.	Dissipates energy as heat (thermogenesis).
Cell Morphology	Large, single (unilocular) lipid droplet, few mitochondria.	Smaller cells with multiple small (multilocular) lipid droplets, abundant mitochondria.
Energy Role	Energy storage and endocrine function.	Energy expenditure to regulate body temperature.
Location (Adults)	Widespread, including subcutaneous and visceral depots.	Primarily in supraclavicular, neck, and paravertebral regions.
Change with Age	Overall mass may increase, especially visceral fat, even as subcutaneous fat decreases.	Mass and activity significantly decrease.

Conclusion

As humans age, the body's composition changes dramatically, marked by a decline in metabolically active brown and beige adipose tissue. This shift contributes to a less efficient metabolism, impaired thermoregulation, and an increased susceptibility to metabolic disorders. The loss of thermogenic fat is a key factor in the physiological changes associated with aging, and understanding its underlying mechanisms opens up potential new avenues for promoting health and longevity. The study of how to activate or preserve these energy-burning fat cells in older individuals is an active area of research for addressing age-related metabolic decline.

For a comprehensive review of the current research on this topic, see the National Institutes of Health (NIH) for an article on brown and beige adipose tissue and aging.

Frequently Asked Questions

What is the primary difference between brown fat and white fat?

The primary difference is their function: white fat primarily stores energy, while brown fat burns energy to produce heat. Brown fat cells are rich in mitochondria containing $UCP1$, which enables them to generate heat, a process known as thermogenesis.

Why do humans lose brown fat as they age?

The loss of brown fat is attributed to several age-related changes, including a decline in mitochondrial function, reduced signaling from the sympathetic nervous system, alterations in hormone levels, and decreased regenerative potential of adipose precursor cells.

Where is brown fat located in adults?

In adults, the primary deposits of brown fat are found in smaller quantities compared to infants. Common locations include the neck, supraclavicular (above the collarbone), and paravertebral (along the spine) regions.

Does having less brown fat lead to a slower metabolism?

Yes, a decrease in the mass and activity of brown and beige fat contributes to a lower basal metabolic rate. Since these cells are responsible for energy expenditure through heat production, their reduction means the body burns fewer calories at rest.

What are beige fat cells, and do they also decrease with age?

Beige fat cells are thermogenic adipocytes that appear within white adipose tissue. They can be activated to burn calories in response to stimuli like cold exposure. Their formation and activity also decline with age, contributing to reduced thermogenic capacity.

What are the health consequences of a decrease in brown fat?

The decline in brown fat can lead to an increased risk of several age-related metabolic diseases, including obesity, insulin resistance, type 2 diabetes, and cardiovascular disease. It is also linked to poorer thermoregulation.

Can adults increase their amount of brown fat?

While the total mass of classic brown fat does not significantly increase, it can be activated. Some evidence suggests that cold exposure, exercise, and certain dietary or pharmacological strategies may enhance the activity of existing thermogenic fat or promote the 'browning' of white fat.