Twelve weeks of a Western diet enlarged fat cells inside the bone, not just under the skin
In short
Feeding 20-week-old rats a Western diet supplying 42% of energy as fat for 12 weeks raised bodyweight (P = 0.02) and whole-body fat percentage (P = 0.009), and enlarged the adipocytes in humeral bone marrow adipose tissue (BMAT) (P = 0.02) alongside gonadal white fat (P = 0.005). At the transcript level, BMAT responded more robustly to the diet than white fat did, and the two responded differently: BMAT was enriched for lipid transport pathways, white fat for lipid biosynthesis and modification. Marrow fat is not passive filler inside bone — it is a metabolic tissue that moves with the diet.
The real question in a bulk is not how much weight went on but where it went. Most people count subcutaneous and visceral fat and stop there. The body has a third fat depot: bone marrow adipose tissue (BMAT), inside the bone itself.
BMAT has only recently been recognised as a functional depot distinct from white (WAT) and brown (BAT) fat. That it expands in obesity was known; the mechanism was not. This study opened that question at the level of gene expression.
What was measured, and how
First, 20-week-old Sprague-Dawley rats on standard chow (12% of energy as fat) had the transcriptomes of BMAT, WAT and BAT sequenced. At that baseline, BMAT stood out for transcripts related to cellular proliferation and an adipoprogenitor cell population — the signature of a tissue still growing rather than a finished store.
Then came the intervention: 12 weeks of a Western diet supplying 42% of energy as fat, after which femoral and tibial BMAT and gonadal WAT were compared against the chow group.
What got bigger after twelve weeks
- Bodyweight — significantly increased (P = 0.02).
- Whole-body fat percentage — significantly increased (P = 0.009).
- Adipocyte size in humeral BMAT — significantly increased (P = 0.02).
- Adipocyte size in gonadal WAT — significantly increased (P = 0.005).
Nothing surprising so far: change the diet, the fat cells enlarge. What is interesting is that the fat cells inside the bone moved the same way.
Not just a bigger response — a different kind
In the transcriptomes, BMAT responded more robustly to the Western diet than WAT did, and the character of the response diverged. BMAT was enriched for lipid transport pathways; WAT for lipid biosynthesis and modification.
The authors read that as two different jobs. White fat moves toward making and storing lipid; marrow fat moves toward taking up lipid already in circulation. Which raises the possibility that BMAT acts as a nutrient sensor and lipid sink rather than a bystander.
What a lifter takes from this
One thing has to be said plainly: this study did not measure bone density or bone strength. It does not support the conclusion that more marrow fat means weaker bone. What it establishes is that marrow fat is a diet-responsive metabolic tissue. What training does to bone is covered separately in strength training and bone density.
Something practical still survives. A single number on the scale cannot tell you the composition of the weight you added, and in this experiment the fat did not only land where you can see it. What decides a bulk is the size of the surplus and the fraction of it that becomes fat — that ratio is in the fat-to-muscle ratio of a bulk. And because the muscle index normalises for bodyweight, weight added as fat inflates the denominator and cuts the score. If your total is climbing while the index sits still, that gap is the answer.
This is a 12-week rat experiment using a diet supplying 42% of energy as fat — a far higher fat fraction than a typical human bulk, in a different species over a different timescale. Bone density and fracture risk were not measured.
Frequently asked questions
What is bone marrow adipose tissue (BMAT)?
It is fat tissue residing in the marrow inside bone, now recognised as a functional depot distinct from the white fat under the skin and around organs and from heat-producing brown fat. It expands in obesity, and in this study its adipocytes enlarged in response to a high-fat diet.
Does a high-fat diet increase marrow fat?
In rats fed a Western diet supplying 42% of energy as fat for 12 weeks, adipocyte size in humeral bone marrow increased significantly (P = 0.02). Over the same period bodyweight (P = 0.02) and whole-body fat percentage (P = 0.009) also rose.
Do marrow fat and ordinary body fat respond to diet the same way?
Not in this study. The transcriptomic response was more robust in marrow fat, and marrow fat was enriched for lipid transport pathways while white fat was enriched for lipid biosynthesis and modification. The authors read that as marrow fat sensing nutrient status and taking up lipid rather than manufacturing it.
Does more fat in the marrow weaken bone?
This study cannot answer that, because it measured neither bone density nor bone strength. Its findings extend only as far as showing that a high-fat diet changed the size and gene expression of marrow fat cells.
Should I cut dietary fat when bulking?
This experiment fed rats an extreme diet supplying 42% of energy as fat and does not set a human fat intake target. What matters practically is the size of the calorie surplus and the fraction of it stored as fat, and that a single bodyweight number tells you nothing about that composition.
Source: PubMed