Biological age

Heavier babies had slightly "younger" organs in middle age, by blood-protein clocks

In about 21,000 to 30,000 UK Biobank participants, a higher birthweight was linked to slightly younger-looking organs in adulthood, judged by blood proteins; having been breastfed was linked to a younger-looking gut. The links were small, the early-life information came from people's own memory decades later, and the study cannot show cause and effect.

Does how we start life leave a mark on how our organs age decades later? A study from Xi'an Jiaotong University in China used blood samples from the UK Biobank, a large British health study, to look for such traces. People who said they had been heavier babies tended to have slightly "younger" organs by a blood-protein measure, and those who had been breastfed had a younger-looking gut. The differences were small, and the study only shows associations.

The idea that conditions before and just after birth shape health for life is old and well supported: low birthweight, for example, has been linked to more heart and metabolic disease in adulthood. What is new is the way of measuring aging. Organs release characteristic proteins into the blood, and from the levels of thousands of such proteins, computer models can estimate a biological age for each organ separately. If an organ looks older than a person's actual age, it is said to be aging faster.

What they did

The UK Biobank recruited about half a million people aged 37 to 73 between 2006 and 2010. For about 45,000 of them, nearly 2,900 proteins were measured in a blood sample (this is called proteomics). From these, the team calculated age gaps for 18 organs and systems, such as the intestine, kidney, liver, brain, immune system and muscles, plus one overall index.

At recruitment, participants had been asked their birthweight and whether they had been breastfed as babies. The analysis included 21,446 people who knew their birthweight and 29,713 who knew whether they had been breastfed. The researchers compared these early-life factors with the organ age gaps, taking into account age, sex, body weight, body size at age 10, neighbourhood deprivation, education, smoking, drinking, exercise and some illnesses.

What they found

  • Birthweight: a higher birthweight went with slightly younger-looking organs in 11 of the 18 systems. The strongest links were for the intestine, esophagus and stomach, followed by the immune system, kidney and liver.
  • Breastfeeding: having been breastfed went with a younger-looking intestine and adrenal glands, but a slightly older-looking thyroid.
  • Both together: people with an above-median birthweight who had also been breastfed showed the most favourable pattern, especially for the intestine, stomach and kidney.
  • Who showed the strongest links: the breastfeeding links were stronger in women, and the links overall were more pronounced in people with better living conditions or healthier habits, and also in those with a higher body mass index.
  • A possible pathway: part of the birthweight link (up to about a third, depending on the organ) went along with allostatic load, a score combining ten blood and body measures of metabolic, cardiovascular and inflammatory strain. An inflammation marker, C-reactive protein, played only a minor role. Nothing similar showed up for breastfeeding.

How large are these effects? Small. Each extra kilogram of birthweight went with an intestinal age gap about 0.07 standard deviations smaller, a fraction of the normal spread between people. Even for the combined group, the difference was around 0.13 standard deviations.

What to keep in mind

The authors list the main weaknesses:

  • Observational and mostly a snapshot. Proteins were measured once, so the study shows links, not cause and effect. People who were heavier at birth or breastfed may differ from others in many ways that started in childhood, such as family income, and these were only partly accounted for.
  • Memory decades later. Birthweight and breastfeeding were reported by participants themselves, as adults. Some did not know, and some will have remembered wrongly.
  • Yes or no only. The study did not know how long or how exclusively anyone was breastfed.
  • Protein clocks are estimates. Organ ages derived from blood proteins may not reflect how organs actually look or work.

The authors' own summary sometimes uses causal language ("shape", "produce"), but their study design supports only associations, as their limitations section says. The thyroid result, they write, is "speculative" in its interpretation.

Why it matters

The study adds an organ-by-organ view to an old idea: that early life leaves traces in adult health. If the links hold up in studies with records collected at birth and repeated measurements over time, they could help explain why organs in the same person age at different speeds. For now, the effects are small and the evidence is indirect. Birthweight in the past cannot be changed, and the study offers no advice on what anyone should do differently.

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