You’ve likely known someone (or maybe you are that someone) who has a “fast metabolism” — they seem to be able to eat whatever they please and never gain an ounce of fat. Metabolism is widely talked about particularly in the context weight loss. A healthy metabolism is important for weight control and body composition, but also for optimizing healthspan and lifespan. In fact, recent studies have found that having a “youthful” metabolism is a strong predictor of lifespan and the absence of disease.¹ Understanding what metabolism is and how to maintain it are an important aspect of healthy aging.

BREAKING DOWN METABOLISM

‘Metabolism’ is all of the biochemical reactions that happen throughout all cells in our body.² We take in nutrients through food which is composed of protein, carbohydrates, and fat. These macronutrients are broken down into simpler molecules during digestion, in particular amino acids (from protein), simple sugars (from carbohydrates), and glycerol and free fatty acids (from lipids). These “building blocks” are used to make cellular structures and provide our body with energy. This “energy” is provided in the form of a molecule called Adenosine Triphosphate (ATP).

One way to think about metabolism is how many calories you burn, also known as “total energy expenditure.” Total energy expenditure consists of three major components: basal metabolic rate (BMR), physical activity energy expenditure, and diet-induced thermogenesis.2 BMR — also known as resting energy expenditure — is the minimum energy that we need to exist and maintain bodily processes. Resting energy expenditure makes up about 60-70% of our total energy expenditure. Diet-induced thermogenesis, also known as dietary induced energy expenditure, is the amount of energy we need to digest, absorb, and store the food we consume. This component makes up about 5-10% of total energy expenditure. The final contributor to total energy expenditure is the most variable and accounts for around 20-40% of total energy expenditure. This is physical activity energy expenditure — the energy we use for exercise and non-exercise activity.

Components of Total Energy Expenditure

BMR (60-70% of total energy expenditure): the minimum energy that we need to maintain bodily processes like respiration, circulation, and electrochemical gradients. This is the energy you’d burn if you stayed in bed all day long (no fidgeting!)

Dietary induced energy expenditure (5-10% of total energy expenditure): the energy used to digest, absorb, and store the food that we consume.

Physical activity energy expenditure (20-40% of total energy expenditure): the energy needed for exercise and non-exercise related activity and muscular contraction

‘METABOLISM’ VS. ‘METABOLIC HEALTH’

While calorie burning is one way to describe metabolism, we can also think about metabolism more broadly as “metabolic health.” Metabolic health is a largely misused and confusing term, but in general, being “metabolically healthy” can be defined as having “ideal” levels of blood sugar, cholesterol, blood pressure, and a healthy body composition.³ Even this can be pretty nuanced — as there is likely no single “ideal” for any of these components, only what is “ideal” for each individual.

Being metabolically healthy means that our body can effectively utilize the nutrients we consume and produce the energy we need. It means having healthy mitochondria that are fine-tuned energy factories and that our organ systems function “optimally” for what we need at all stages of life. Proper metabolic health is infinitely more important than just “burning calories.” Optimizing metabolic health is crucial for healthy aging and longevity.

As we get older, resting energy expenditure declines about 1-2% each decade.

WHAT INFLUENCES METABOLISM?

We have already discussed the three main components of metabolism. These three components are further influenced by many lifestyle, environmental, and genetic factors that can affect the way our body handles nutrients and uses energy.

Genetics: Decades of studies have found evidence that how much energy (calories) we use throughout the day is partly determined by genetics. In fact, up to 40% of the variance in total energy expenditure might be due to characteristics that we inherit.⁴

Diet composition: What we eat could be just as important for metabolism as how much. Some research has shown that high-protein diets cause a higher energy expenditure because protein requires more energy to digest and process than a similar amount of carbohydrate or fat.⁵ All macronutrients actually have a different value of energy needed for digestive processes. The energy utilized to digest and absorb each macronutrient (as a percentage of calories provided by each nutrient) is 0-3% for fat, 5-10% for carbohydrate, and for 20-30% for protein.2 This means that we burn about ~.25 calories for each gram of fat, ~1 calorie for each gram of carbohydrate, and ~2-3 calories for each gram of protein we consume.

Physical activity: You might think that physically fit individuals have a higher resting metabolic rate than non-fit individuals, but this might not be the case. While exercise training increases total energy expenditure as a result of a higher physical activity level, it may not affect resting energy expenditure.⁶ However, exercise like high-intensity interval training (HIIT) will elevate your metabolic rate for a period of time even after exercise ends, meaning you’ll have a higher calorie burn throughout the day.⁷

Muscle mass: We need energy to power our muscles, and this means that the more muscle mass you have, the greater your energy needs. Individuals with a greater fat-free mass have a higher resting energy expenditure.⁸ Furthermore, increased muscle mass helps to control blood glucose and insulin levels, leading to better overall metabolic health.⁹

Hormones: Hormones can control how much energy we use and how efficiently we use it. The “appetite hormones” leptin and ghrelin, along with insulin, can dictate energy intake and how efficiently we use glucose.¹⁰ Thyroid hormones also control energy expenditure by regulating body temperature, growth processes, and glucose and fat metabolism.¹¹ Lower thyroid hormones with age may contribute to a drop in metabolism and a relative increase in body fat.¹²

HOW AGING CHANGES METABOLISM

As we get older, resting energy expenditure declines — about 1-2% each decade.¹³ This could mean that the energy you burn might drop by ~400 calories per day between the age of 20 and 70. What’s the reason for this?

One cause is a decline in muscle mass. Losing muscle — a large source of energy consumption — means that overall metabolic rate will drop. Fat mass also increases with age, causing unfavorable alterations in body composition.¹⁴ Since fat consumes less energy than muscle, this leads to a drop in resting energy expenditure. It may not only be muscle that declines — some studies have even found that the metabolic rate of individual organs falls with age.¹⁴

Along with a change in body composition, overall physical activity levels are generally lower in older individuals. Being “less active” in old age contributes to a lower energy expenditure due to physical activity from exercise and general movements.

How we use nutrient sources for energy also changes with age. Aging is associated with reductions in insulin sensitivity, glucose uptake, and fat oxidation.¹³ Our mitochondria and other energy-producing pathways become worse at taking up and using fuel sources. Studies have shown that aging causes impaired glucose and fat oxidation in the mitochondria, along with an inability of the mitochondria to “switch” between using glucose and fat as fuel sources — what is known as “metabolic flexibility”.^15,16

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BOOST AN AGING METABOLISM 

Metabolic dysfunction and energy dysregulation likely contribute to a more rapid aging process and reduced quality of life. Luckily, reinvigorating a slowed and less efficient metabolism is possible using the right lifestyle strategies.

Exercise: One of the best ways to improve metabolic health at any age is exercise. Lifting weights might be key! Resistance training improves muscle strength, power, and builds lean, calorie-burning muscle mass.¹⁷ Aerobic endurance exercise (i.e. “cardio”) can improve mitochondrial capacity, enhance insulin sensitivity, and boost overall metabolic function.¹⁷ A consistent exercise routine that integrates both resistance and aerobic training several times each week can keep your energy expenditure high and promote metabolic flexibility.

Intermittent fasting (IF): Fasting isn’t just for weight loss. During a fast, fatty acids and ketones are used for energy production, promoting a “metabolic switch” from the use of carbohydrates for energy.¹⁸ This can help build the metabolic machinery for burning fat and glucose efficiently. Both intermittent fasting (for instance, the 5:2 fast) and daily time-restricted feeding (TRF) are science-backed fasting strategies to promote metabolic switching and maintain metabolic flexibility.

Increase protein intake: To counteract the effects of a decline in muscle mass with age, eating a higher amount of protein may be crucial. After exercise, 40 grams of protein should be consumed to optimally stimulate muscle protein synthesis.¹⁹ That’s about 6-8 ounces of meat. Eat an adequate amount of protein at each meal throughout the day, and supplement with the amino acid leucine (or consume leucine-rich foods like meat and cheese) — since this amino acid stimulates protein synthesis by activating Mechanistic Target of Rapamycin (mTOR).²⁰ Pairing resistance training with a high-protein meal is the single best way to maximally stimulate muscle protein synthesis.

Enter ketosis: Ketogenic and low-carb, high-fat diets force the body to use fatty acids and ketones for energy due to a lack of available carbohydrates, promoting metabolic switching.²¹ Since impaired glucose metabolism with age can contribute to neurodegeneration, the ability to use fat and ketone bodies ensures energy availability to all tissues in the body. Using exogenous ketone supplements or ketone drinks is a more recently discovered strategy to induce ketosis without a low-carb diet or fasting. Exogenous ketones can increase healthspan in part, by promoting metabolic switching and may even counteract the effects of age on memory performance.²²

Conclusions

A declining metabolism doesn’t have to be the inevitable consequence of aging. Managing your health through diet, exercise, and constant physical and cognitive engagement can promote healthy and successful metabolic aging. A healthy metabolism is the key to a long, prosperous life. And luckily, it’s largely under your control.