Four Reasons Our Metabolism Changes with Age

Four Reasons Our Metabolism Changes with Age

Aging Metabolism

By Gary Pepper, M.D., and Sam Jeans MSc

It’s tempting to think that our bodies simply ‘slow down’ as we age, which makes it harder to keep weight off, but a study of 6,600 people aged between one week old to 95-years-old revealed that it’s not as simple as one might assume.

In our prior post, we defined what metabolism really is (a mixture of at least 3 different processes), so we can now move on to discussing how these processes change as we age.

A 2021 study, published in Science, found that our total daily energy expenditure is 50% faster in our first year of life than during adulthood. After our first birthday, our daily energy expenditure begins to taper off by some 3% each year until around the age of 20.

But here’s the surprise; instead of our total daily expenditure slowing gradually from say, the age of 30 onwards, it tends to remain very stable until the age of 50 or 60. After then, it decreases only slightly, by around 0.7% per year.

“Some people think of their teens and 20s as the age when their calorie-burning potential hits its peak…But the study shows that, pound for pound, infants have the highest metabolic rates of all.” – Dr. Katzmarzyk, Professor and Associate Executive Director for Population and Public Health Sciences at the Pennington Biomedical Research Center.

In essence, what this study shows is that the metabolism – and total energy expenditure – slows over time because of a mixture of factors, and not just because of age only. In other words, many people gain weight as they age for reasons other than what is often simply described as “the body slows down as you age”.

If you’re struggling more with your weight as you age, there are probably a few different things going on.

Problems and Solutions for Age-Related Weight Gain

1: Your Diet Has Changed

Diets can change in either direction with age, they might vastly improve as we realize the benefits of a healthy diet, or they might slip deeper and deeper into bad habits and indulgence!

What we do know is that a high-fat, low-protein diet lowers the thermic effect of food, which means that more excess calories end up deposited as fat.

A 2016 study from Purdue University found that a high-protein diet can boost the metabolism, boosting energy expenditure by around 100 calories per day. That’s 100 calories you’re losing for free, without moving, simply by eating high protein foods. For how long the increase in calorie burning by high protein diet lasts, is not clear, however.

Grabbing a shelf full of protein supplements in your 50s, 60s, or 70s is probably not required.

The following is a list of natural high-protein foodstuffs to consider:

  • Nuts
  • Beans, peas, and legumes
  • Seeds
  • Eggs
  • Soy
  • Unprocessed dairy
  • Oats and grains
  • Dark leafy vegetables

2: You’re Moving Less

Next up, it’s all too easy to start moving less with age. Rather than low-level exercise, it’s usually the more vigorous activities that start getting neglected with age.

Exercise demands remain quite consistent throughout life, and it’s generally now recommended that older adults up to the age of around 50 to 60 commit to a similar exercise regime as those aged much younger. The older we become however, limitations on exercise duration and intensity are imposed by illness and degenerative conditions such as arthritis, heart and lung disease and whatever injuries have occurred over the years.

Web MD recommends a considerable 150 minutes of moderate cardio spread over 3 days or more, which is what the CDC recommends for all adults who are medically able.

A strong body of research has also found that more rigorous exercise like high-intensity interval training can protect seniors from an array of diseases whilst bolstering their metabolism. It is strongly advised, particularly in older individuals, that before stepping up your exercise regime to always consult a physician prior to starting.

3: You’ve Lost Muscle Mass

Muscle loss (sarcopenia) reduces BMR whilst simultaneously reducing the number of calories burnt during exercise. Harvard Health cites that muscle loss can hit 3% to 5% per year after the age of 30. A higher percentage of lean muscle tissue boosts the level of calories burnt during exercise and at rest, so fighting against muscle loss is a potent way to maintain a steady metabolism.

It’s possible to fight against muscle loss by engaging in strength and resistance training in addition to your standard exercise regime. Follow a strength or resistance training plan approved by a professional, aimed at your age group and skill level and always take things slowly at the start.

4: Your Hormone Levels Have Changed

The hormone levels of both men and women progressively change with age.

Whilst men obviously do not experience anything like the level of hormone change that comes with female menopause, they may still experience somewhat of a “male menopause” which involves the loss of testosterone often beginning during the 50’s. Obesity in men is also associated with lower testosterone levels, creating a vicious cycle of increasing obesity causing lower testosterone which then inhibits the ability to lose weight, and so on.

Medical News Today primarily recommends lifestyle changes to support hormone levels, including increasing exercise and decreasing consumption of processed and fatty foods. Limiting alcohol consumption is also recommended not only to reduce caloric intake but alcohol may also suppress male hormone (testosterone) production.

Some herbs and supplements are also suggested to support metabolic hormones – Ginseng, Maca, and Ashwagandha are amongst those that show the most promising preliminary evidence, though large-scale studies are still lacking.

Summary: How Getting Older Affects Your Metabolism

Age does affect the metabolism, but it’s not as simple as your body just “slowing down” as you get older – and we are not powerless to stop it!

We have greater control over our metabolism than generally assumed and making just a handful of small changes might be all it takes to beat the “middle-aged spread”.

Considering all 3 components of the metabolism – BMR, TEF, and TEE – helps target strategies aimed at improving each, such as switching to a high-protein diet to increase calorie burning during eating, increasing exercise, and also supporting hormone function, and increasing lean muscle mass to boost BMR.

A combined, holistic approach provides a much better outcome than focusing on just one part of the metabolism in isolation of the others.

Disclaimer: This and all articles at are for educational purposes only and are not intended as medical advice or treatment. By choosing to view this article you agree to the Terms of Use of Metabolink, Inc which can be found at


What is Metabolism and How Does it Change with Age?

What is Metabolism and How Does it Change with Age?

What is metabolismWhat is Metabolism and How Does It Change with Age?

By Gary M. Pepper, M.D. and Sam Jeans, MSc

Our bodies undergo many transitions as we age, some good and some not so good!

In the latter category of “not so good”, many people tend to struggle more with their weight as they get older. Obesity rates are higher in older populations, and in the United States, more than 30% of both men and women over 60 are obese.

Obesity in older age brings about much of the same risks as it does at any age, including an increased risk of heart disease, diabetes, high blood pressure, stroke, and mobility problems, and the risks climb ever higher with each year that goes by!

Unraveling what happens to our metabolism as we age helps provide strategies for staying healthy and reducing the risks of putting on excess weight. The popular consensus is that, as we age, our bodies ‘slow down’, which makes it tougher to keep weight off, but does that really tell the whole story?

Metabolism Defined

At its most fundamental, metabolism encompasses every chemical process required to sustain life. It’s easy to confuse the entire concept of metabolism with just the basal metabolic rate (BMR) alone, but this is just one component of metabolism.

Many people use the terms interchangeably, whereas, in fact, there are three main components to metabolism:

1: The Basal Metabolic Rate (BMR)

The body is in constant motion, so even when we’re totally sedentary, sleeping, or otherwise at rest, we require energy and nutrients to function correctly.

So long as the heart pumps, the lungs breathe and the body and its organs go about their business growing and repairing cells, we are utilizing energy in one form or another, and this is our basal metabolic rate (BMR).

The BMR varies depending on the following factors:

  • Body Size and Muscle: Larger bodies have greater energy demands and higher BMRs.
  • Lean Muscle Tissue: Whilst it’s true that larger bodies have greater energy demands, BMR also increases relative to the proportion of the body that is made up of lean muscle. Muscle burns more energy than most other tissues.
  • Body Fat Percentage: Fat burns energy at a much lesser rate than most cells. A high body fat percentage relative to lean muscle tissue results in a lower BMR, even if the body itself is physically larger.
  • Age: Age does affect the BMR for a few different reasons, which we will discuss shortly.
  • Growth: Younger children have generally higher BMRs due to body growth, and they also need more energy to maintain their body temperatures.
  • Gender: Men likely have higher BMRs than women on average as they’re larger and have higher percentages of lean muscle tissue.
  • Genetics: Genetics do play a role in BMR, but there is debate surrounding quite how drastic the influence of genetics is compared to other factors.
  • Hormones: Hormone levels, particularly those governed by the thyroid gland, also influence BMR. Hypothyroidism (sluggish thyroid function) is associated with a lower BMR whereas hyperthyroidism (excessive thyroid function) is associated with elevated BMR. Metabolism-related hormones are also influenced by diet, for example, iodine deficiency may result in an underactive thyroid that lowers BMR.
  • Activity Level: Whilst physical activity burns energy directly as a result of exercise, it also raises BMR by stimulating the growth and repair of new cells, such as muscle cells.
  • Infection: Infections stimulate an immune response that requires energy, thus boosting BMR.
  • Environment/Temperature: Colder environments may increase BMR slightly as our bodies work harder to maintain a stable core temperature. Conversely, hotter environments may increase BMR as our bodies work harder to cool down.


2: Thermic Effect of Food (TEF)

The consumption of food itself requires energy and some foods are much easier to digest than others.

It takes considerable effort for our bodies to move food through the digestive system, and our body also needs to absorb and transport nutrients from that food, which further requires energy.

A concept that tends to pop up in various diet plans is ‘calorie negative food’ – that is food that burns more calories to digest than they provide as food. Sadly, there is no evidence that we can ‘eat ourselves thin’ by consuming calorie-negative food!

TEF varies considerably depending on the food consumed and its macronutrients content:

  • Fat has a thermic effect of some 0 to 5%, which means that for every 100 calories of fat consumed, the body burns just 0 to 5 calories
  • Carbohydrates have a thermic effect of around 5 to 10%, so for every 100 calories of carbs consumed, the body burns just 5 to 10 calories.
  • Protein has a much higher thermic effect of around 20 to 30%, so for every 100 calories of protein consumed, the body burns 30 calories (Source: Healthline).

Overall, TEF accounts for around 5 to 10% of our daily energy demands. This is increased when we consume a high-protein and low-fat diet.

3: Thermic Effect of Exercise (TEE)

Thirdly, our body requires energy to sustain movement and exercise. This is highly variable and obviously depends on physical activity levels – the more we move, the more energy we require to breathe, fuel our muscles, and repair cells.

Common estimates suggest that physical exercise uses between 15% and 30% of average daily energy expenditure. Engaging in intense or protracted physical exercise ( such as running a marathon) will increase that figure.

The total accumulative energy required for these 3 metabolic components – BMR, TEF, and TEE, makes up our total daily energy expenditure.

In Part 2 of this article, we will cover how aging affects our metabolism and what we can do about it. Check back at for the publication date.

Disclaimer: This and all articles at are for educational purposes only and are not intended as medical advice or treatment. By choosing to view this article you agree to the Terms of Use of Metabolink, Inc which can be found at


Find Out What’s New for Treatment of Hot Flashes

With menopause comes the onset of hot flashes. These are the result of the drop in estrogen in the blood. Replacing estrogen is the most effective way of eliminating hot flashes but because of the health risks, taking estrogen replacement is an unacceptable option for many women. A recent study in the Journal of Clinical Oncology reports on the use of other prescription drugs that can alleviate hot flashes. Watch my video on the subject for a more in depth discussion.

Adopted Children Experience Early Sexual Maturity

Puberty occurs when areas within the brain awaken beginning a cascade of hormone signals which conclude with the gonads (ovaries and testicles) increasing their production of the female and male sex hormones estrogen and testosterone. Under the influence of these hormones a child begins the transition from childhood to sexual maturity. In boys puberty is associated with a growth spurt, the appearance of facial, axillary (arm pit) and pubic hair, acne, deepening of the voice, growth of the testicles and penis while girls undergo a growth spurt, develop breasts, acne, pubic and axillary hair, and growth of the clitoris.

Historical data shows the average age of puberty today is many years sooner than in previous generations. Most experts attribute earlier puberty to better nutrition. A recent article in reviewed how “over-nutrition” accelerates obese children into puberty sooner (referred to as precocious puberty) than normal weight children. The latest studies on causes of precocious puberty suggests that a child’s social environment also exerts an important influence on the timing of puberty. Researchers in Madrid publishing in The Journal of Clinical Endocrinology and Metabolism 95:4305 2010 analyzed the age of puberty in normal children, adopted children and children whose families immigrated (children not adopted but subject to high levels of personal stress) to Spain. Adopted children were 25 times more likely than other groups of children to undergo precocious puberty (breast development before the age of 8 years in girls, and boys under 9 years of age with testicular growth). Over-all girls were 11 times more likely than boys to demonstrate precocious puberty.

Researchers speculate that socio-emotional stresses early in life of children who are later adopted result in changes in the brain that cause premature maturation of vital nerve pathways. This early brain maturation later results in stimulation of the pituitary gland, turning on the hormone pathways that cause puberty. This seems strange to me because various forms of deprivation in childhood can also delay puberty. For example, girls who have anorexia remain child-like in their body development and may fail to menstruate even into their late teens. A decade ago I studied hormone levels in adults during the stress of illness and surgery and found this lowered the sex hormone levels in their blood. This makes sense from an evolutionary point of view because during stressful conditions nature wisely cuts off the reproductive hormones. Why make babies if the environment is hostile in some way? Why the opposite occurs in children under stress of adoption is an interesting but unanswered question.

Gary Pepper, M.D.,

Growth Hormone Deficiency Tied to Obesity and Accelerated Aging in Young Adults

Due to the potential for abuse and high cost, growth hormone treatment in adults is the subject of much controversy. I believe that treating adults with growth hormone deficiency is many times an appropriate and beneficial choice. Firming up my conviction for treating adult growth hormone deficiency is a recent study conducted in the Netherlands and UK published in the Journal of Endocrinology and Metabolism (JCEM 95:3664-3674, 2010). The researchers compared Body Mass Index (BMI), waist circumference, triglycerides, and HDL (good cholesterol), between normal adults and those with low growth hormone levels due to deficient pituitary function (hypopituitarism). All measurements of obesity and lipid metabolism were significantly worse in the young adults (younger than 57 years) with growth hormone deficiency compared to normal adults of a similar age.

As I pointed out in previous articles at, growth hormone levels naturally decline as we get older. The authors of the present study note that growth hormone levels decline 14% per decade in adults. I conceive of this as one of the ways nature gets rid of us after we complete our biological/reproductive functions, since without growth hormone our muscles, immune and nervous systems, decline, leading to death. It’s planned obsolescence… what is typically referred to as aging. In the recent study senior citizens have equivalent levels of obesity and abnormal lipid metabolism as young adults with growth hormone deficiency. The authors note the effect on the body of growth hormone deficiency in young adults is equivalent to 40 years of aging. The theory that growth hormone functions to preserve our tissues during youth and aging results from its absence, appears confirmed by these results.

Most normal young adults aren’t growth hormone deficient and the population that would qualify for growth hormone treatment from this group is small. What about older adults with low growth hormone who are troubled by the “natural” decline in their body function? Should or could we treat this much larger population with growth hormone? It is my experience that private and federal insurers will not pay for this treatment regarded as “cosmetic”. On the other hand, there will be physicians who will comply with a request for growth hormone treatment from individuals who possess enough cash and motivation. Less affluent or determined individuals will have to contend with natural aging just as our ancestors have done for thousands of years.

This information is for educational purposes only and is not intended as medical advice or treatment.

Gary Pepper, M.D.

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