Muscle-to-fat ratio when bulking: 1.8 kg of fat per 1 kg of lean above 104 kg
In short
In 27 college football players tracked by DXA over four years, the linemen — sitting above 104 kg of total mass — gained 1.8 kg of fat for every 1 kg of lean mass. Below that threshold the non-linemen showed no such coupling at all, with total mass and lean mass correlating at r = 0.98, close to one for one. Total four-year lean gain averaged 3.3 kg (4.1 percent) and bone mineral density rose 3 percent, with no difference between the groups.
Twenty-seven college football players were scanned by DXA across four years, and in the linemen — the group carrying more than 104 kg of total mass — every 1 kg of lean mass gained came with 1.8 kg of fat. Roughly two parts fat to one part muscle. Among the non-linemen, changes in lean mass and fat mass were not correlated at all over the same span (linemen only: r = 0.54, p = 0.004).
The gains themselves were small. Across four years lean mass rose 3.3 ± 2.4 kg (4.1 percent) and bone mineral density rose 0.04 g/cm² (3 percent), with no statistical difference between the two groups. Four full years of collegiate training buys three to four percent more muscle. The catch is that not all the added bodyweight over those years is that three to four percent.
At what bodyweight does fat start winning?
The two groups' regression lines for lean mass against total mass intersect at a total mass of 104 kg. Below it, the non-linemen's total mass and lean mass tracked each other at r = 0.98 — near one for one, weight gained was muscle gained. Above it, the linemen loosened to r = 0.80, and the slack went to fat. The baseline gap is wide too: linemen carried 124 ± 15 kg of total mass with 31 ± 9 kg of fat, non-linemen 89 ± 15 kg with 15 ± 5 kg of fat.
What does a 5 kg bulk actually add?
Take someone already past 104 kg who adds 5 kg. Applying the 1.8-to-1 ratio, that is roughly 1.8 kg of muscle and 3.2 kg of fat — two thirds of the number on the scale. The same 5 kg added below 104 kg tilts far more toward the lean side.
How does this show up in a relative strength score?
A relative strength score normalises your Big 3 total against bodyweight, which means fat inflates the denominator without touching the numerator. The 3.2 kg of fat in that example contributes nothing to squat, bench or deadlift while it quietly shrinks your bodyweight coefficient. The one-rep-max gain produced by 1.8 kg of muscle has to outrun that coefficient loss before the score moves at all — and above 104 kg the arithmetic keeps getting worse. Run your pre- and post-bulk bodyweight through the calculator to see how far the score slides on an unchanged Big 3 total.
The sample is 27 players in total: 13 linemen and 14 non-linemen. No data were collected in 2020 because of COVID, so the timeline runs Year 1, Year 3 and Year 4. The authors flag the modest sample size and call for confirmation in larger cohorts. The 104 kg figure is where these 27 athletes' regression lines meet, not a physiological boundary that applies to everyone.
How much fat you are willing to accept during a bulk is ultimately a diet decision. For picking one that outlasts a few weeks, see the diet you can sustain.
Frequently asked questions
What ratio of muscle to fat do you gain when bulking?
Above 104 kg of total mass, players gained 1.8 kg of fat for every 1 kg of lean mass — the figure comes from four years of DXA scans on 27 college football players. Below that mark the coupling did not appear at all, and total mass tracked lean mass at r = 0.98, close to one for one.
At what bodyweight does bulking become inefficient?
In this study the dividing line was 104 kg of total mass, where the two groups' lean-mass-versus-total-mass regression lines intersect. Above it the correlation loosened from 0.98 to 0.80 and the difference went to fat. With only 27 athletes in the sample, the individual threshold will vary.
How much muscle can you gain in four years?
In trained collegiate athletes, lean mass rose an average of 3.3 kg over four years, or 4.1 percent. Bone mineral density rose 3 percent over the same period. Linemen and non-linemen gained at the same rate.
Does bulking lower your relative strength score?
It does if bodyweight climbs while the Big 3 total stays put, because the score normalises the total against bodyweight and added fat only enlarges the denominator. The one-rep-max gain from the muscle you added has to exceed the loss in your bodyweight coefficient for the score to rise.
Source: PubMed