Good morning,

How is your Sunday morning going?

This is the 20th issue of this newsletter. Time flies! If you are a new reader, welcome! If you have been here since issue one, thank you for sticking around!

Here’s what’s in today’s issue:

• One big thing. Is your metabolism slowing down with age? Or is something else the cause? This is all you ever wanted to know about metabolism but were afraid to ask. 📉

• You have to check this out. Brutal measures to take back control of your time. Plus, watching TV. 📺

• Tools and tech. Finding the best award flights. ✈️

Let’s get into it.

1️⃣ ONE BIG THING

The metabolism mysteries.

She’s always been lucky. She has a fast metabolism.

I have a slow metabolism. It’s tough for me to lose weight.

I’ve added a few pounds over the years because my metabolism is slowing down.

You’ve heard these sentiments, right?

Metabolism has always been a black box to me. How does your metabolism get a ‘speed’?

For Gen Xers like you and me, it can feel like our bodies are slowly betraying us. A fun Friday night with a few too many beverages didn’t used to cause Saturday to be canceled. A decade ago, I didn’t realize my hip was a joint. Now, it regularly reminds me.

And a few reckless eating days (ok, weeks) didn’t show up so quickly on the scale.

So maybe my metabolism IS slowing down. 👎

Or is it?

For this week’s newsletter, I decided to solve the metabolism mystery. As is the case often with me, the answers came in the form of 2–5-hour podcasts from both kinds of doctors (MDs and PhDs).

I found Dr. Andy Galpin to be the protector of the secret tomb containing the holy grail of metabolism. I first heard him on Peter Attia’s The Drive podcast, a two-part, five-hour podcast on the structure and function of muscles.

Dr. Galpin’s podcast Perform recently covered all things metabolism (it is 2+ hours if that’s your thing, like me). If not, I’ve tried to save you some time with this summary.

Here is the main spoiler: Metabolism does not slow down as you age. If it seems that way, it is because something else is to blame. Let’s solve this mystery!

Metabolism math.

Let’s start with the basics. What is your metabolism?

Simply put, it is the total energy you burn every day. The official term for this is your Total Daily Energy Expenditure (TDEE), which is counted in calories. Everything your body does—from basic functions like breathing and pumping blood to digesting your food to running after your dog in the yard—burns calories.

Your metabolism is simply how many calories your body burns every day.

Here are the four distinct ways you burn calories, with notations for the percent of your total daily energy expenditure each one consumes:

1. Resting Metabolic Rate (RMR): 40-80% of daily calories

The energy your body uses for essential functions like breathing, maintaining your heartbeat, and cell repair—basically, keeping you alive when you're completely at rest.

2. Thermic Effect of Food (TEF): 8-15% of daily calories

Simply digesting and processing your food.

3. Non-Exercise Activity Thermogenesis (NEAT): 5-30% of daily calories

Everyday movements like walking around your home, cleaning, gardening, or even fidgeting.

4. Exercise Activity Thermogenesis (EAT): 0-30% of daily calories

Purposeful exercise, like going to the gym, jogging, cycling, or playing sports.

The first thing of note here is the wide range of percentages for each category. As a percentage, if you don’t exercise (#4) and you are largely sedentary throughout the day (#3), then the percentage of the total for #1 and #2 will go up.

For example, for someone who sits all day for work and doesn't exercise, their resting metabolic rate can be as much as 80% of their total energy expenditure.

Here is a more balanced example for someone that exercises at moderate intensity several times a week and moves around throughout the day:

Did any of this surprise you?

My two discoveries were:

• For most of us, just staying alive (RMR) takes more energy than everything else combined.

• Small doses of basic movement throughout the day equal or exceed planned exercise sessions. I underappreciated this.

Is your metabolism slowing with age?

I certainly thought this was true, and in some ways it is. But if your metabolism seems to be slowing down, this is mostly the byproduct of another aspect of aging: muscle loss.

You’ve seen how much energy you burn just staying alive 🕺. One of the biggest energy consumers you have is muscle. If you weigh 175, have an average build, and are moderately active, you will have about 60 pounds of muscle. It burns energy around the clock, even when you are sleeping.

Muscle mass explains up to 80% of your resting metabolic rate.

The problem? Beginning around 30, you slowly lose muscle mass (about 3-5% per decade). The chart below shows the rise in muscle mass early in life, followed by a slow decline and a sharp drop off in your 70s.

This is the chart for women:

This is the chart for men:

What appears to be a slowing metabolism is mainly a decline in muscle mass. Research finds that when muscle mass remains steady, total daily energy expenditure does not go down significantly as we age.

Now look back at the charts above. The black line is the median value of all the small dots, which represent people in the study.

You do not have to be on the median line. You can be one of the dots well above the line, maintaining (or building) muscle mass as you age. This will keep your metabolic engine burning brightly.

Interlude: Why does this matter?

Let me take a quick step back. Why do we care about metabolism and how much energy we burn daily? The answer is body weight and body composition.

Some people overcomplicate this idea, but ultimately, weight loss or gain is a function of calories eaten compared to calories burned over time. When we eat more calories than we burn (via the four categories above), the excess energy is stored as fat. Yes, there is some nuance here, but much of that nuance (e.g., not all calories are created equal, metabolic adaptation) is accounted for via changes in how the body regulates energy expenditure (metabolism).

Calorie consumption is often the focus of weight management. However, studying metabolic dynamics can help us gain a deeper understanding of the calorie-burning side of the equation (of which exercise is only a small part).

Can I increase my metabolic rate?

Yes. Let's examine what Dr. Galpin and the research tell us about what we can do and which actions can make a real difference.

First, some things will increase your energy expenditure acutely (short-term). For example, consuming caffeine will boost your resting metabolic rate by 3-10% for 1-3 hours. It is a non-trivial boost, but one you must repeat daily to maintain this short-lived effect (plus, your body will begin to adapt over time and reduce this impact).

Here are the short-term things that boost your metabolism according to research:

Resting Metabolic Rate (RMR):

• Consumption of caffeine, spicy foods, green tea, and more. 1-10% increase.

Thermic Effect of Food (TEF):

• Eat more protein. If you eat 100 calories each of fat, carbs, and protein, it will take your body 0-2 calories to process the fat, 5-10 calories to process the carbs, and 15-30 calories to process the protein. So, eating 100 calories of protein yields a net of only 70-85 calories going into your system. (Be wary of boosting total calories while boosting protein.)

Non-Exercise Activity Thermogenesis (NEAT):

• Standing desks (100-200 calories a day); treadmill desks (150-250 calories a day); three 10-minute walks a day (200-300 calories a day).

• I think this area is underappreciated in terms of impact. Many small doses of activity add up faster than you would expect.

Exercise Activity Thermogenesis (EAT):

• Aerobic/endurance exercise burns more calories than strength or high-intensity interval training.

• Strength training burns fewer calories but builds muscle, which has a longer-term benefit.

Here are the long-term things that boost your metabolism according to research:

Resting Metabolic Rate (RMR):

Here we find the goldmine for fundamentally “speeding up” your metabolism:

• Sleep. This is a big one. In one study, where calorie consumption was held constant, a group getting inadequate sleep (5.5 hours) lost 2 fewer pounds of body fat and 2 more pounds of muscle than those getting good sleep (8 hours)—in just two weeks. Also, the relationship between sleep and metabolism is bi-directional. Being overweight often impedes quality sleep, and additional muscle mass improves sleep quality (read more here). Another study found people getting under five hours of sleep per night had a 251% increase in their odds of developing type-two diabetes. 🛌

• Adding muscle mass. Every pound of muscle burns about 6-10 calories per day. So, adding ten pounds of muscle can burn 70 calories a day. This may seem modest, but it’s a day-in, day-out benefit. 490 calories every week is like a bonus high-intensity exercise session every week. 💪

• Exercise. Keep in mind that this is the impact of exercise on RMR (not the calories burned doing the exercise itself). Research finds strength training increases RMR by up to 100 calories a day (part of this clearly overlaps with muscle mass), and endurance training boosts RMR by about 50 calories a day. 🚵

• Fish oil. This one is a surprise. This study measured the impact of 14 weeks of taking 3g of fish oil daily and found a on-going increase of 14% in RMR, plus other energy, fat oxidation, and muscle mass benefits. 🐟

• Eating more. Didn’t expect this here, did you? Haha. Indeed, this is the concept of metabolic adaptation. If you consistently consume more calories, your RMR will increase (just as it will decrease under calorie restriction). Of course, it will not increase to match the increase in calories, so you will still gain weight. 😢

Thermic Effect of Food (TEF):

• There isn’t much here. Some research has looked at the gut microbiome and its impact, but in general, enhancing it may slow your metabolism.  

Non-Exercise Activity Thermogenesis (NEAT):

• Not much here.

Exercise Activity Thermogenesis (EAT):

• There's not much here either. Dr. Galpin notes that as you get fitter, your capacity for exercise may increase (good), but you likely get more efficient at the same time (e.g., running gait gets more efficient), which negates the effect. So exercise doesn't have a long-term, ongoing impact beyond the acute benefit.

Assessing your current metabolism.

Understanding your baseline metabolic rate, including the portion of your overall energy expenditure that comes from your resting metabolic rate, can be helpful.

Many testing centers offer RMR tests. By far, the best of these are university labs, which offer these tests affordably ($50-100). Private facilities like Quest and others do this as well.

The best way to get a solid sense of these numbers is to use an online calculator. These are based on extensive research datasets, such as the Mifflin-St. Jeor Equation. Here is a good example of a free online calculator.

Mystery solved.

As the villain always says in Scooby-Doo, “I would have gotten away with it too if it weren't for you meddling kids!".

Thanks to Dr. Galpin's metabolism meddling, I better understand the metabolism mystery.

For me, these are the main things I am going to do:

1. Strength training. On-going improvements to your resting metabolic rate are just one benefit. I will cover the shockingly strong correlation between strength and longevity in a future newsletter.

2. Sleep. I knew this was important generally, but I did not realize its powerful impact on metabolism (or muscle growth). I’m in good shape here.

3. On-going, incidental activity. I need to set reminders to get up and walk around and take several ten-minute walks a day.

4. Eat more protein. For me, this will be more about maintenance, as I consume about 185g of protein a day now.

5. Fish oil. I didn’t see this one coming. Curiously, the research on the cardiovascular benefits of fish oil is not conclusive, but maybe this study means I'll start taking it again.

Enjoy the rest of your day, and I’ll see you next Sunday.

Kevin

P.S. - In case you missed it, here are a couple of my favorite prior newsletters:

• Choosing to begin again. One small choice can change your trajectory—learn how to begin again.

• The shared happy pill. Good relationships are the most important factor in living a happy and healthy life.

• Your life in weeks. If you're in Gen X, you have about 1,000 - 1,900 weeks left to live. How will you use them?