A study of 59,269 adults, published in The Journal of Clinical Endocrinology & Metabolism in October 2026, explains how.

More lean mass, worse metabolic health

Researchers at Pennington Biomedical used UK Biobank body scans to measure muscle and fat. The average participant was about 55. Then they looked at two outcomes: metabolic syndrome and high blood sugar.

Metabolic syndrome is a cluster of warning signs: a big waist, high blood pressure, high blood sugar, high triglycerides, and low HDL cholesterol. Have enough of them together and your risk of heart disease and type 2 diabetes climbs.

The people with more lean mass had more metabolic syndrome and higher blood sugar.

Building muscle after 40 is one of the best things you can do for your health. I've said it a hundred times. So before you cancel your training, look at how they measured it.

"Lean mass" isn't the same as muscle

The study measured body composition with DXA scans. A DXA scan reports "lean mass" as everything in your body that isn't fat. That's muscle, but it's also organs, bone, and water. In most adults, muscle makes up only about half of it.

And as people get heavier, lean mass and fat go up together. A bigger body carries more of both. So a big number on the lean mass line doesn't tell you whether someone built muscle or just got bigger.

The researchers saw this coming. They ran the numbers a second way, measuring lean mass as a share of body weight.

The result flipped.

They compared the same amount of lean tissue, about 6 to 7 pounds, two ways:

• Counted as raw total, it came with about 45% higher odds of metabolic syndrome.

• Counted as a bigger share of body weight, it came with roughly half the odds.

Same tissue, opposite result. What mattered was what came along with it.

The scale and the lean mass number treat all weight the same. Your metabolism doesn't.

Where the fat sits matters

The researchers also used MRI scans to see where people stored fat. Two locations changed the results.

Visceral fat had the biggest effect. That's the deep belly fat packed around your organs, as opposed to the soft fat you can pinch.

Fat inside the muscle mattered too. Muscle can get marbled with fat, like a ribeye. In people with more of it, carrying more lean mass lined up with worse blood sugar and more metabolic syndrome. That effect was small, but it pointed the same direction.

This matters because your muscles are the biggest place your body stores blood sugar. Muscle with fat running through it does that job worse.

The authors' conclusion was simple. More lean mass isn't automatically better if fat comes with it.

One honest caveat: this was a snapshot study. It looked at people at a single point in time, so it shows a pattern, not cause and effect. But the pattern matches what I've seen in years of training people.

The problem with chasing lean mass after 40

Most guys I talk to aren't eating pizza and ice cream to put on size. They're eating clean. Salmon, grass-fed beef, rice, sweet potatoes. Just a lot of it.

But your body doesn't care how clean the surplus is. Eat more than you can turn into muscle, and the extra gets stored as fat. Salmon and sweet potatoes will do it as reliably as a drive-thru.

And after 40, your body tends to build muscle more slowly.

In a 2017 study from the University of Nottingham, researchers put three age groups through 20 weeks of fully supervised lifting: 18 to 28, 45 to 55, and 65 to 75. Every group got stronger at the same rate. But the young group added about 5% to their lean mass, while the 45-to-55 group added about 1%.

It was a small study, and other research has found smaller age gaps. So "slower" is fair. "You can't build muscle after 40" would be wrong.

The same study had good news. The 45-to-55 group still lost about 5% of their body fat and almost 7% of their belly fat, from lifting alone. Training still works in midlife. It just works better on body composition than on size.

Put those together and the problem is clear. When muscle gain slows down and the eating doesn't, the extra has to go somewhere.

The scale climbs and the shirts get tight, and it feels like progress because the food was healthy. Some of that weight is muscle. A lot of it is the belly fat and marbled muscle the UK Biobank study linked to worse metabolic health.

What I do at 47

I eat around 2,500 to 2,800 calories a day. I don't track every day. Every few months I log everything for a couple of days to check, and it always lands in that range.

Most of my meals, 85 to 90% of them, are the same handful of meals I rotate through. When my training load goes up, I add fats, not carbs. When it drops, I pull the fats back a little.

That's enough to fuel Jiu Jitsu and strength training, hold onto my muscle, and stay lean. I don't bulk and I don't cut.

Your number will be different from mine. What matters is the pattern: eat close to what you burn, keep it steady, and adjust in small amounts.

What to do instead of bulking

Gain slowly. Small surplus, plenty of protein, and patience.

Stop judging progress by the scale. Watch two things: your waist and your strength. Measure your waist at the belly button and write it down next to your best push-up or pull-up number. Check both again in 8 weeks. If your waist is growing faster than your strength, you're gaining the wrong thing.

Train with intent. Fewer reps, done hard, beat a pile of junk volume. That's the whole idea behind the Minimum Effective Dose: the least amount of work that still drives progress, done well enough that you recover from it.

You don't need to be bigger. You need more of you to be muscle.

Why 40 is a real turning point

You've seen "40+" slapped on every supplement label and fitness program out there. It's easy to write off as a marketing scare tactic. But there's real science behind that number.

This is Part 1 of a three-part series:

1. More muscle isn't always healthier after 40 (you're here)

2. Why your body stops forgiving you after 40: the evolutionary reason

3. Aging speeds up in your mid-40s: what the research shows and what to do about it

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Stay capable,

JT

Sources

• Devarakonda et al. Relationship between fat-free mass and metabolic syndrome is shaped by ectopic fat in a UK Biobank cohort. The Journal of Clinical Endocrinology & Metabolism, 2026.

• Phillips et al. Physiological adaptations to resistance exercise as a function of age. JCI Insight, 2017.