Birth weight and breastfeeding are linked to how quickly organs age decades later, according to an analysis of 44,8 thousand UK Biobank participants
Birth weight and breastfeeding are linked to how quickly organs age decades later, according to an analysis of 44,8 thousand UK Biobank participants
A study published on 9 October in Aging Cell used data from UK Biobank, Britain's largest biobank, to build separate “aging clocks” for 19 organ systems from measurements of nearly three thousand blood proteins in 44,8 thousand adult participants. The researchers compared these clock estimates with birth weight in 21,4 thousand participants and breastfeeding history in 29,7 thousand. The effect spans multiple organ systems and has been partly explained: it involves more than half of the systems assessed, and for some, the researchers provided the first statistical evidence of a mechanism connecting early-life exposure to aging decades later.
Protein-based organ aging clocks were first developed in 2023 using small cohorts. In 2025, researchers applied the method to the much larger UK Biobank dataset and built models for all 19 systems. On 1 October, a week before this study, a Scottish evaluation of the same clocks found that they predicted death less accurately than brain MRI and three other methods. Chinese epidemiologists at Xi'an Jiaotong University used these clocks to examine a longstanding question: whether conditions around birth leave a trace in the pace of aging half a century later. Earlier studies had separately linked these conditions to individual diseases; this study was the first to examine the association across 19 organs at once.
The associations were widespread. After adjustment for lifestyle and existing diseases, higher birth weight was associated with slower aging in 11 of 19 systems. The associations were strongest for the intestine, esophagus and stomach, with notable associations also found for the immune system, kidneys, liver, salivary glands, thyroid gland, adrenal glands, skin and muscles. These results agree with earlier findings concerning the intestine, kidneys and muscles in people with low birth weight, bringing them together for the first time in a single analysis.
Breastfeeding had selective effects: it slowed aging in the intestine and adrenal glands but accelerated it in the thyroid gland. The authors describe the thyroid finding as unexpected. They suggest that fluctuating levels of iodine and thyroid hormones in breast milk may place a burden on an infant's still-developing thyroid gland. In people with both favorable factors, the effects exceeded the sum of their individual contributions: for the intestine, the combined effect was almost twice that of birth weight alone.
For birth weight, the authors also identified a partial mechanism: “allostatic load,” the cumulative wear on the body from chronic stress over a lifetime, measured using ten metabolic, cardiovascular and inflammatory indicators. This wear accounted for up to 35% of the association between birth weight and accelerated skin aging and up to 30% of the association for the adrenal glands. It explained considerably less of the associations for the intestine and stomach, where a different, local mechanism appears to be involved. For breastfeeding, allostatic load explained none of the association in any organ. Its effects appear to develop through a different pathway, most likely earlier in life through milk composition and the establishment of the gut microbiota, rather than through decades of stress.
Both effects were stronger in women and in people with more favorable socioeconomic circumstances: the strength of an early-life advantage depends on the course of a person's later life.
Participants reported their birth weight and breastfeeding history from memory when they were already middle-aged or older. This kind of recall error generally weakens an association toward zero rather than creating one where none exists. Nine of the eleven birth weight associations persisted even after simultaneous adjustment for both factors.