Natty or Not? What Science Says About Natural Muscle Limits

"Natty or not" debates show up in fitness feeds every few weeks. Someone posts a dramatic before-and-after, commenters argue over whether it was done naturally, and the thread splits fast. That's sentiment, not evidence.

This post won't guess whether any specific person uses drugs. Nobody can tell that from photos. The more useful question is what the research says about natural muscle limits, and what that means for your own training.

Where the "FFMI 25" rule came from

Fat-free mass index, or FFMI, compares your fat-free mass to your height. Fat-free mass includes muscle, but also bone, organs and water, so FFMI is not a direct measure of skeletal muscle. It's still the number people quote when they say no natural lifter can go past 25.

That number traces back to a 1995 study by Kouri and colleagues. They measured 157 male athletes: 83 steroid users and 74 non-users. The non-users' height-adjusted FFMI extended up to about 25. Many steroid users went past 25, and some went past 30. Twenty Mr. America winners from before the steroid era (1939 to 1959) averaged an estimated 25.4.

Read the fine print, though. Drug use in that study was self-reported, the non-user group was 74 men, and the authors called their findings "preliminary." They described FFMI as a useful first screen for possible steroid use, not a hard wall.

What college athlete data adds

A 2019 study by Currier and colleagues measured 209 male college athletes from 10 sports with DEXA scans. The average height-adjusted FFMI was 22.8. Football players averaged about 24.3. The researchers labeled the 97.5th percentile of their sample an "upper limit": about 28.3 for all athletes, 29.1 for rugby players and 25.5 for baseball players.

Those percentiles describe that sample of athletes. They are not a proven natural ceiling. The abstract doesn't establish whether the athletes were drug-free, and college athletes aren't a typical gym crowd. So this study can't tell us how common any FFMI is among drug-free men, or settle where a natural limit sits.

The fair reading: the FFMI 25 line is a rough early estimate, not a law, and no single FFMI number can prove someone is or isn't enhanced. You may also see other cutoffs shared online. We only cite figures we could check against the published paper, so we're leaving those out.

Why drug-assisted physiques aren't a fair benchmark

The clearest controlled evidence on what testosterone adds comes from a 1996 randomized trial in the New England Journal of Medicine. Bhasin and colleagues assigned 43 healthy men to four groups for 10 weeks: placebo or testosterone at supraphysiologic (above normal) doses, each with or without a lifting program.

Among men who didn't train, those given testosterone had larger increases in arm and leg muscle size than those given placebo. The group that got testosterone plus training gained about 13 lb of fat-free mass on average, more than either no-exercise group.

This was a small, short trial in healthy men under medical supervision. It's not a reason to use these drugs. Its relevance here is narrow: a drug-assisted physique isn't a fair yardstick for natural progress. If you measure your first year against an enhanced result, you may call real progress a failure.

The risks are serious. The National Institute on Drug Abuse says anabolic steroids can cause severe, long-lasting and in some cases irreversible damage, including early heart attacks, strokes, liver tumors, kidney failure and psychiatric problems. If you have questions about hormones, talk to a doctor. This article does not give drug or dosing advice.

What the feed may do to your head

Research from 2026 suggests this kind of content may relate to how young men think about drugs.

A survey of 1,515 boys and men in Canada and the US, none of whom had used anabolic steroids, found that more social media time, more frequent exposure to muscularity-focused content, and more frequent body comparisons on social media were each associated with higher intentions to use them. It's a one-time survey, so it shows an association, not cause and effect.

A randomized online experiment from Flinders University showed 282 young men fitness, supplement or travel TikTok content. Fitness and supplement content lowered satisfaction with their own nutrition and raised intentions to use creatine. For steroid intentions, any effect depended on how strong each man's drive for muscularity already was. Comparing their appearance to others' helped explain these effects. These were short-term changes in self-reported satisfaction and intentions after one viewing session, not long-term behavior.

Taken together, these studies point to comparison as one piece worth paying attention to, not the whole story.

What to do with this

The points below are coaching suggestions, not findings from the studies above.

Compare against yourself. Track the numbers you control: training logs, a few measurements and progress photos taken in the same light every few weeks. Your trend over months tells you more than anyone else's physique.

Don't quit over a ceiling you haven't reached. Even among the trained college athletes in the Currier study, the average was about 22.8. For most lifters, consistent training, enough protein and enough sleep deserve attention long before any theoretical limit does. If you're unsure how much protein you need, start with our protein guide.

Train in a way you can keep doing. Our breakdown of how fewer sets fit a busy schedule covers what the research says about volume and effort. If you've taken time off, here's what comes back and how fast.

Curate your feed. You don't have to quit social media. If certain accounts consistently leave you feeling behind, consider muting them.

Be skeptical of shortcut pitches. "Natty plus" protocols, peptide sponsors and miracle stacks show up wherever this debate gets loud. We looked at one of those trends in our BPC-157 and peptides breakdown.

The bottom line

The famous "FFMI 25" line was a rough early estimate from a small, self-reported sample, not a law, and later athlete data doesn't establish a drug-free ceiling either. What the evidence does show is that supraphysiologic testosterone adds fat-free mass on top of training, which is why an enhanced physique is a poor benchmark for natural progress.

You don't need to know who's natty. You need to know whether you're better than you were six months ago.

If you want a place to start, get The Doctrine of Becoming, a free 21-day reset of practices you adapt into your own lifestyle. If you want a coach to build a plan around your schedule and keep you honest, see coaching, $99/month.