Fat type, storage location and ageing: beyond the scales

Three different meanings of body fat

Dietary fatty acids, the amount of body fat, and the location of that fat are different measurements. A biopsy can describe the proportion of fatty acids in a small sample of subcutaneous tissue. Imaging can estimate fat under the skin, around abdominal organs, or within the liver. A change in one measurement does not establish the same change in another.

Saturated and polyunsaturated fat in overfeeding trials

In the seven-week LIPOGAIN trial, participants gained similar weight when overfed muffins enriched with palm oil or sunflower oil. Saturated-fat-rich overfeeding produced more liver fat and approximately twice the increase in visceral fat compared with polyunsaturated-fat-rich overfeeding. This suggests that dietary composition can influence storage patterns even when weight gain is similar. [1]

LIPOGAIN-2 extended the comparison to adults with overweight or obesity. Saturated-fat-rich overfeeding again increased liver fat more than the PUFA-rich intervention, but there was no differential effect on visceral, pancreatic or total body fat accumulation. The liver finding was more consistent than a universal difference across all fat depots. [2]

Does PUFA go under the skin instead of around organs?

The trials support lower liver-fat accumulation with a linoleic-acid-rich intervention compared with saturated-fat-rich overfeeding under the conditions studied. They do not establish that all consumed PUFA is directed away from organs or that every saturated fatty acid preferentially goes to visceral tissue. They compared diets and subsequent fat accumulation, not an exclusive destination for each molecule. [1,2]

A meal-tracer study in 12 men and 12 women found regional and sex differences in dietary-fat storage. Women stored a greater fraction of meal fat in subcutaneous tissue; the concentration of meal-derived fatty acids was higher in abdominal than thigh subcutaneous tissue in both sexes. Distribution depends on the person and the depot, as well as the diet. [3]

Can the PUFA proportion increase while body mass falls?

Yes: the DIOGENES study reported changes in the relative fatty-acid composition of adipose triglycerides during weight loss and maintenance, including increases in several polyunsaturated fatty acids. This concerns composition, not proof of an increase in the absolute amount of PUFA stored throughout the body. It was not a study demonstrating age-driven PUFA accumulation. [4]

As a simplified illustration, 10% of a 20 kg fat store is 2 kg, whereas 12% of a 15 kg store is 1.8 kg. The percentage has risen while the amount has fallen. This arithmetic is illustrative, not a calculation from DIOGENES, and actual quantification requires appropriate tissue and whole-body measurements.

Ageing does not move every fatty acid in the same direction

The Scottish Heart Health Study examined adipose samples and dietary data from 4,357 adults aged 40–59. Some longer-chain polyunsaturated fatty acids increased with age, while linoleate and gamma-linolenate decreased after adjustment. The cross-sectional design compared people of different ages; it did not follow the same people losing mass as they aged. It therefore cannot establish a general rule that PUFA accumulates during age-related weight loss. [5]

Separately, longitudinal work by Arner and colleagues found that adipose lipid removal slowed with ageing. This concerns lipid turnover, not selective accumulation of PUFA. Lower turnover and changes in storage are useful research questions, but they should not be represented as direct proof of the proposed PUFA mechanism. [6]

Disease risk in older adults requires outcome evidence

PREDIMED’s corrected report found fewer major cardiovascular events among older high-risk adults assigned to Mediterranean diets supplemented with olive oil or nuts than among those receiving reduced-fat dietary advice. It tested dietary patterns and included corrections for allocation irregularities. It argues against treating total fat percentage alone as a sufficient explanation of disease risk in ageing. [7]

A prospective study of older adults linked certain circulating trans-fat measures with incident diabetes, but estimated dietary trans-fat associations were not statistically significant after fuller dietary adjustment. This also illustrates why a blood biomarker, reported intake and a causal dietary effect cannot be treated as equivalent. [8]

The relevance to EPK

EPK research should track function and body composition rather than interpreting every kilogram lost as benefit or every kilogram gained as harm. Dietary fat type, carbohydrate intake, protein, training and age may modify outcomes at a given energy intake. The evidence does not demonstrate that a particular total-fat threshold prevents disease or that stored PUFA is inherently harmful.

For EPK–003, these findings motivate measurements of substrate use and storage location. For EPK–004 and EPK–006, they motivate separating lean-tissue change, fat distribution and functional ageing. Testing age-related accumulation would require repeated measurements of fatty-acid composition and absolute depot mass in the same people, alongside diet and health outcomes.

Related working papers

Read the relevant PDFs: EPK–001 to 007 are the revised version 1.2 papers; EPK–008 to 010 are version 1.0 supplements. Earlier PDFs and DOI records remain on the paper archive.