Healthnotes Newswire (February 15, 2001) — Are breast-fed infants less prone to high blood pressure or heart disease later in life than are formula-fed infants? A new study suggests the answer may be yes.¹ But the study, published Saturday in The Lancet, raises as many questions as it answers.

Researchers at the Institute of Child Health in London sought to discover whether nutritional factors in early life could, in some way, “program” the body to develop cardiovascular problems, including elevated blood pressure, later in life. Although this is not the first study to examine the effect of breast milk versus formula on blood pressure in later life,^{2 3} it is the best-designed trial of its kind and provides further insights into the potential benefits of breast-feeding.

Over 900 premature infants were enrolled in the study. They were randomly assigned to one of two groups. The first group received breast milk (from a human milk bank), and the second group received a nutrient-enriched formula designed for premature infants.

The groups followed this program for the period of their hospital stay as premature babies—an average of 30 days. Within each group, infants were again divided into those who received additional breast milk from their mothers and those who did not. One-quarter of the participants were followed-up at age 13–16 years, at which time their blood pressure was measured. The average blood pressure was found to be significantly lower (4.2 millimeters of mercury on a blood pressure gauge) in the 66 children assigned to receive breast milk compared with those assigned to receive formula.

A Little Blood Pressure-Lowering Goes A Long Way

Although a decrease in blood pressure of just a few millimeters of mercury (mm Hg) may seem inconsequential, it actually has substantial implications for the prevention of future hypertension and other cardiovascular diseases. In adults, a population-wide lowering of diastolic blood pressure (the lower number on a blood pressure reading) by just 2 mm Hg would reduce the prevalence of hypertension by 17%, the risk of coronary heart disease by 6%, and risk of stroke and transient ischemic attacks by 15%.⁴ (Transient ischemic attacks are episodes of numbness, paralysis, speech difficulties, or other neurological symptoms that arise suddenly and resolve over several hours.) Moreover, because blood pressure tracks from childhood into adult life with amplification of early elevations or reductions,^{5 6 7} the blood pressure difference between breast-fed and formula-fed populations is likely to be much greater in adulthood than the 4.2 mm Hg difference among 13–16 year olds would suggest.

Limitations of the Study

The researchers studied only premature infants, so the extent to which their results are relevant to babies born after a nine-month pregnancy is not known. Moreover, although elevated blood pressure is a risk factor for high blood pressure as an adult and, therefore, the risk of stroke, heart failure, and early death, it is only one of the factors contributing to cardiovascular disease. The effects of breast milk or formula on other cardiovascular risk factors (e.g., cholesterol levels, body fat) are needed to complete the picture. However, little information of this type is available from existing clinical trials.

Only 23% of the original group participated in the follow-up. Loss of participants can skew the results of a study, because people may drop out of studies for health reasons that are highly relevant to the study outcome. In the case of the current study, however, other explanations may account for the high attrition rate for follow-up: the duration between the initial study and the follow-up was very long (13–15 years) and the follow-up involved an unpleasant procedure (blood draw) that could have dissuaded participants from returning to complete the study.

How Could Breast Milk Lower Blood Pressure?

Why formula-fed, premature infants are more likely to have elevated blood pressure as teenagers compared with those given human milk remains a mystery. After adjusting for differences in salt and total fat intake between the formula-fed infants and those given human milk, the differences in blood pressure remained. In an accompanying editorial,⁸ Dr. Susan Roberts suggests some possibilities: the human milk contains fatty acids and other growth factors that could be responsible for the difference in blood pressure. Alternatively, she suggests that the different milks could have programmed infants’ food preferences in different ways, resulting in food choices among teenagers that affected their blood pressure.

Although the new trial raises many questions, it nevertheless adds to the large and ever-growing body of evidence that supports breast milk and breast-feeding over the use of infant formulas for all infants.