Ask someone in the know to list the substances in breast milk that make it the ideal food for newborns and you may hear about proteins that guard against infection, fats that aid in the development of the nervous system and carbohydrates that promote the growth of healthy bacteria. But, you may not hear too much about the nitrite and nitrate in breast milk and their contributions to developing gastrointestinal, immune and cardiovascular systems.
In a study published online on Oct. 19 in advance of print in Breastfeeding Medicine, the official journal of the Academy of Breastfeeding Medicine, researchers at The University of Texas Health Science Center at Houston (UTHealth) announced the results of an observational study showing that the levels of nitrite and nitrate in breast milk change during the initial days after birth, which the scientists argue is to accommodate the changing physiologic requirements of developing babies.
"This research shows the essential nature of nitrite in breast milk," said Nathan Bryan, Ph.D., the study's senior author and an assistant professor at the UTHealth Brown Foundation Institute of Molecular Medicine for the Prevention of Human Diseases (IMM). "While the nitrite and nitrate composition of breast milk has been reported, this is the first study to demonstrate the changing levels of nitrite and nitrate early on."
Dietary nitrite and nitrate are part of a normal diet. When people eat nitrate-rich vegetables, the bacteria in their mouths and stomachs converts the nitrate into nitrite, which in turn aids in the production of nitric oxide. Nitric oxide keeps blood pressure levels normal, fights infection and supports the nervous system. Animal studies suggest nitric oxide might even guard against heart attack and stroke.
The scientists measured nitrite and nitrate levels in breast milk during the first three days of birth (colostrum), days three to seven (transition milk) and eight or more days (mature milk). Seventy-nine patient samples were analyzed and they were donated by mothers who were either admitted to Memorial Hermann - Texas Medical Center (TMC) in Houston for childbirth or who were visiting a UT Physicians' clinic in the TMC.
Bryan said colostrum has significantly higher concentrations of nitrite and significantly lower concentrations of nitrate than both transition and mature milk, which he believes may be nature's way of providing nitric oxide to the newborns whose gastrointestinal tract is not yet colonized by bacteria that convert nitrate to nitrite. Nitrite-rich colostrum overcomes this deficit, he said.
Human milk concentrations of colostrum, transition milk and mature milk were 0.08 mg/100ml nitrite and 0.19 mg/100ml nitrate, 0.001 mg/100ml nitrite and 0.52 mg/100ml nitrate, and 0.001 mg/100ml nitrite and 0.3 mg/100ml nitrate, respectively.
To corroborate their findings, researchers analyzed milk samples taken from two women on 14 consecutive days and the scientists observed the same change in the nitrite and nitrate levels.
Some women cannot nurse their children due to health issues. Other women may choose not to breastfeed so the investigators also measured the level of dietary nitrite and nitrate in alternative sources of newborn nutrition: formula, cow milk and soy milk.
Noting that breast milk is considered more beneficial to newborns than these others sources of nutrition, Bryan said the study revealed that colostrum contains the highest amount of nitrite of any of the milk products tested.
"This is another difference that has been noted between mother's milk and formula," said Pamela Berens, M.D., one of the study's authors and a professor of obstetrics, gynecology and reproductive science at the UTHealth Medical School. "Studies like this help us better understand the benefits of breast milk."
The World Health Organization (WHO) and the American Academy of Pediatrics recommend exclusive breastfeeding for the first six months of life. It is good for both mothers in that it can reduce the risk of breast and ovarian cancers and for babies in that it protects against disease and infection.
Bryan said the concentration of nitrite and nitrate found in breast milk calls into question the amount recommended by the Joint Food and Agricultural Organization/WHO Acceptable Daily Intake (WHO ADI) standards. Total daily nitrite intake for nursing infants is 20 times that recommended by the WHO ADI, he said.
Too much nitrite/nitrate or too little nitrite/nitrate can be bad for health, Bryan said. Much of the concern about nitrite/nitrate levels stems from a condition associated with too much nitrite in the blood system called methemoglobinemia (blue baby syndrome). Typically, this is caused by infant formulas made from bacteria and nitrate contaminated well water. The levels of nitrite and nitrate that cause blue baby syndrome are much higher than what is present in breast milk, he said.
"These data, considered together with nitrite and nitrate exposure estimates from foods, show that humans are exposed from birth to dietary sources of nitrite and nitrate. The presence of nitrite and nitrate in breast milk argues for a fundamental role in physiology, which is supported by a number of basic science studies and some clinical trials," Bryan said.
"Contrary to the prevailing scientific opinion about the biological effects of nitrite and nitrate, our data support the view that humans may require these dietary components from birth - from nature's most perfect food," said Norman G. Hord, Ph.D., M.P.H., R.D., the study's lead author and an associate professor of food science and human nutrition at Michigan State University (MSU).
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