Chronic inflammation, caused by a hyperactive immune system, has been linked to heart disease and strokes, Type-2 diabetes, late-life disability, and a greater risk of dying.
The new research found that adult levels of C-reactive protein (CRP), an inflammation blood marker, rose with shorter durations of breastfeeding in infancy.
Compared with receiving no mothers' milk at all, being breastfed for less than three months reduced CRP levels by a fifth.
Breastfeeding for three to six months lowered CRP levels by 26.7%, six to 12 months by 29.6%, and more than 12 months by 29.8%.
Higher birth weight was also associated with lower CRP for individuals who weighed more than 2.5 kilograms when they were born.
CRP was 9.2% greater for those weighing in at 2.8 kilograms than for those born a kilogram heavier.
The findings, published in the journal Proceedings of the Royal Society B, come from a US study of almost 7,000 US men and women aged 24 to 32.
Dr Thomas McDade, from Northwestern University wrote that a lack of breast milk leads to an "increased risk for cardiovascular and metabolic diseases".
Consumption of breast milk may have lasting effects on inflammation by shaping regulatory biological pathways during sensitive phases of immune development, said the scientists.
The NHS recommends exclusive breastfeeding during the first six months of a baby's life. Thereafter, mothers are encouraged to continue feeding breast milk alongside solid food.
"Efforts to improve birth outcomes, and to increase the initiation and duration of breastfeeding in accordance with current recommendations, may reduce levels of chronic inflammation in adulthood and lower risk for chronic degenerative diseases of ageing," the researchers concluded.