And the longer the period of breastfeeding, the lower the blood pressure reading, shows the research published in Archives of Disease in Childhood.
The findings lend further weight to the argument against formula milk.
The research team assessed the impact of breastfeeding on elements of the metabolic syndrome in over 2000 randomly chosen children from Denmark and Estonia, aged between 9 and 15.
The metabolic syndrome refers to a combination of conditions that raise a person's risk of coronary artery disease. These include high blood pressure, diabetes, high insulin levels, high triglyceride levels, and low HDL cholesterol levels.
Breastfeeding did not have an impact on every element of the metabolic syndrome. But children who had been exclusively breastfed had lower systolic blood pressure than those who had not, said the researchers.
This was the same for children in both countries, despite the different social, cultural, and dietary factors at play.
Systolic blood pressure refers to the maximum arterial pressure during the contraction of the heart. It is the first number given in any blood pressure reading, such as 120 over 80.
After adjusting for all the other factors likely to influence the results, such as weight, height, and puberty, the average difference between those exclusively breastfed for at least six months, and those who had not, was 1.7 mm Hg.
And the longer a child had been exclusively breastfed, the lower was his or her systolic blood pressure.
The effect may increase as a child ages, because the results showed that the greatest impact was on the older children, although the figures were not statistically significant.
The extent of the impact and the strong effect of duration of exclusive breastfeeding led the authors to conclude that breastfeeding is directly related to lower systolic blood pressure.
"The magnitude of the effect we found with blood pressure is comparable to the published effects of salt restriction and physical activity on blood pressure in adult populations, suggesting that it is of public health importance," they conclude.