Newsletter #315: The Truth About Metabolism As You Get Older 🔥
Starting from adolescence all the way to old age, people commonly observe some characteristic changes in their body weight.
This is, well, obvious to most of us, but it is also backed by representative population data. The weight trajectory of American adults usually follows a reverse U-shaped curve, wherein body weight rises from age 20 up through their 50s, with some variation between genders and by race/ethnicity. Then it falls as people advance through old age (I suspect there is some survivorship bias here but I digress).
So, what jumps out here, of course, is that weight seems to reach a peak during middle age. The question is why. A lot of people attribute this trend to a slowing metabolic rate, which generally seems plausible. But is that true?
Herman Pontzer at Duke has conducted some fascinating studies in bioenergetics and metabolism using state-of-the-art methods of measurement. (We actually interviewed him on humanOS Radio a few years back!)
Recently, he and his team performed a comprehensive study, using energy expenditure data from thousands of subjects from around the globe, to figure out how metabolism changes with age. And some of the findings might surprise you. 👀
But first: Dan reunited with old friend and colleague Greg Potter on the Reason & Wellbeing podcast, where they had a wide-ranging conversation about the meaning of health, which you won’t want to miss.
Media Featuring humanOS
🎙️ Dan Pardi: Health and Healthy Ageing: What Are They, Really? Via Reason & Wellbeing.
This week, Dan had the opportunity to join Greg Potter on his podcast to address a surprisingly complicated question: What is health? Dan is currently writing a book that explores this, so it’s a subject that he is well equipped to examine.
On the show, he tackled the following topics:
- Why we need to learn (or relearn) how to be healthy, and why this cannot be fully outsourced to clinicians and experts
- How our concept of health has evolved over time in response to technology, philosophy, and other forces
- How aspects of the modern world might get in the way of achieving optimal health
- The difference between a complicated vs a complex system
- Why certain forms of stress ultimately make you healthier and more resilient
- Dan’s own definition of health, based on his extensive study
This Week’s Research Highlight
Energy expenditure throughout adulthood is largely stable until age 60.
Pontzer and colleagues analyzed data from 6,421 subjects from 29 countries around the world. Importantly, they also represented a very wide range of ages, from 8 days old to 95 years old.
This database contained measurements of energy expenditure acquired using the doubly labeled water method, which is considered the gold standard for this purpose. This method involves administering a dose of “heavy” water in which both the hydrogen and the oxygen have been replaced with unusual isotopes of both elements (deuterium and oxygen-18 respectively) – meaning that they can be readily traced. Then, samples of urine, saliva, or plasma are collected at intervals for up to two weeks thereafter to figure out how much of the isotopes have been excreted at a given time period. The difference in the turnover rates of each isotope can be used by researchers to determine the rate of production of carbon dioxide, and that in turn enables them to calculate the rate of energy expenditure for that individual. Pretty cool but unfortunately very expensive, which is why studies like this are so valuable.
Anyway, after adjusting for body size, as well as lean body mass, the researchers found that patterns of energy expenditure across lifespan fell into four main phases.
In the first year of life, energy expenditure rapidly increases, reaching an all-time peak at around 9-15 months, where adjusted energy expenditures are around 50% higher than adults. Not super surprising, given the tremendous growth and development that occurs during infancy.
Next is the juvenile phase, between 1-20 years of age. Absolute energy expenditure does increase as children get bigger, but the rate after adjusting for body mass actually goes down, hitting a plateau at around age 20.
Now, at age 20, we hit the phase of adulthood, and here is the part that might raise some people’s eyebrows. Total and basal energy expenditure remained largely stable on average from ages 20-60, with no differences between men and women (after adjusting for mass). In other words, there wasn’t a significant decline in metabolic rate going from age 20 to 30, or 30 to 40, or 40 to 50. Even pregnancy didn’t result in a significant bump in metabolic rate. That having been said, if you look at the dots in the figure above, it’s clear that there was substantial variation between individuals.
Finally, at around age 60 and beyond, metabolic rate really does start to decline, by around 7% per decade. For the oldest subjects, in their nineties, adjusted total energy expenditure was 26% below that of middle-aged adults.
This phenomenon is actually quite interesting. It has been suggested that the drop in metabolism in old age may be driven by age-related deterioration in the function of metabolically active organs, such as the heart and liver, which account for most of the basal metabolic rate. This makes a ton of sense, since age 60 is around the time that we start to see a steep increase in the emergence of chronic diseases, which could certainly be a reflection of declining function of organ systems. Indeed, longitudinal research suggests that advanced chronic diseases are commonly accompanied with a decline in resting metabolic rate, which you can kind of imagine might represent the wheels falling off the wagon. With that in mind, it’s possible that a higher-than-average resting metabolic rate later in life might be a sign of greater resilience and maintenance of repair systems.
Researchers at Duke analyzed energy expenditure data using the doubly-labeled water method from more than 6,000 subjects of a wide range of ages. They found that energy expenditure, after adjusting for body size, goes through four distinct phases. In the first year of life, energy expenditure increases rapidly, peaking at 9-15 months. Then, between 1-20 years, the adjusted metabolic rate gradually declines. From ages 20-60, total and basal energy expenditure remains stable. After age 60, the metabolic rate declines by about 7% per decade, possibly due to age-related deterioration in metabolically active organs like the kidneys, heart, liver, etc.
Random Trivia & Weird News
🔥 Hummingbirds have such a fast metabolism, they have to consume the human equivalent of 155,000 calories per day to survive.
Hummingbirds have the highest metabolic rate of any vertebrate. This is, in part, because hovering is the most energetically expensive form of locomotion in the natural world, but also because of their very high surface-to-volume ratio. Hummingbirds are also unusual among birds in their lack of downy feathers, which means that they are less insulated. To stay warm and fuel their lightning-fast wing beats, these birds have to consume 2-3 times their body mass in nectar every day.
Fortunately, these itty bitty energy-burning machines have evolved a way to conserve precious calories when times are tough — they can rapidly enter an energy-saving mode known as torpor, which is not dissimilar to hibernation. During this time, their metabolic rate slows down by around 10-fold.
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Photo credit: Ken Jones
Podcasts We Loved This Week
- Pamela Maher: Discovering phytochemicals that may protect the brain against aging. Via Brain Ponderings with Mark Mattson.
- Elizabeth Johnson: What does milk do for babies? Via The Joy of Why.
Products We Like
Capsaicin Extract
I’ve always been intrigued by research on capsaicin. A lot of the evidence in this area is observational, focusing on associations between chili pepper consumption and health outcomes. For instance, regular intake of chilis was found to be associated with 23% lower risk of all-cause mortality and 34% reduced risk of death from cardiovascular disease.
But a new randomized trial really caught my attention (and happens to be somewhat relevant to the subject of this newsletter). In this double-blinded experiment, 28 days of supplementation with capsaicin led to increased energy expenditure, increased fat oxidation, reduced body weight, and increased time-to-exhaustion on a treadmill exercise test.
That trial used 100-200 mg of a sustained-intestinal release red chili extract known as Capsifen. I searched and found that the good folks at Life Extension offer a supplement containing this patented formulation. I just started using it myself, since I would very much like to improve my stamina. Each capsule contains 100 mg, so if you are trying to emulate this trial, you will want to take 1-2 pills daily.
humanOS Catalog Feature of the Week
The FLASH Diet
This week, in light of our discussion of metabolism, we’d like to highlight a course from the Ideal Weight Program, developed by our good friend Stephan Guyenet, a researcher who specializes in the neuroscience of eating behavior and obesity.
The Ideal Weight Program is an evidence-based system for sustainable weight management which teaches you the science of body weight regulation and eating behavior, and translates it into simple, practical strategies for weight management. This program offers three different diets based on your weight management goal. For those who need to lose body fat relatively rapidly (but safely), the FLASH Diet is the plan for you.
FLASH stands for Fat Loss and Sustainable Health, and is modeled after protein-sparing modified fast diets that were developed by researchers in the 1970s and remain the most effective fat loss plan ever studied. In this course, Stephan explains the background behind FLASH diet, where it fits in the sequence of the Ideal Weight Program, what to eat and how to prepare food to adhere to this plan, and more.
Wishing you the best,
