According to the research team, the sugar is likely to originate from the mother’s diet and may have a role in determining the baby’s weight in later life.
It was possible, said the researchers that fructose was instructing pre-fat storage cells to become fat cells, raising the baby's risk of one day becoming overweight.
"Lactose is the main source of carbohydrate energy and breast milk is very beneficial, but it's possible that you can lose some of that beneficial effect depending on maternal diet and how that may affect the composition of breast milk," said Dr Michael Goran, lead author and professor of Preventive Medicine, Physiology & Biophysics and Paediatrics at the University of Southern California.
"Other studies have shown that fructose and artificial sweeteners are particularly damaging during critical periods of growth and development in children. We are beginning to see that any amount of fructose in breast milk is risky."
Fructose is not a natural component of breast milk and is normally found in fruit, processed food and soft drinks.
The proof-of-concept study enrolled 25 mothers and infants at one and six months.
Infants were exclusively breastfed for six months and sugars in breast milk (fructose, glucose, lactose) were noted.
Infant body composition was also assessed at these age intervals as was the associations between breast milk sugars and infant body composition at six months of age.
Despite its very low concentration, fructose was the only sugar significantly associated with infant body composition.
Findings revealed that more than one microgram per millilitre (μg/mL) of breast milk fructose was linked with a 257 gram (g) higher body weight, 170 g higher lean mass, 131 g higher fat mass and 5 g higher bone mineral content.
“We know very little about why some children eventually become overweight or obese," said Goran.
"It's important that we study what may be taking place in the earliest times of their development to determine whether anything could be done just after birth to lower their risks."
Although evidence investigating age-related changes is generally lacking, the results here are consistent with a paper that identified a high fructose diet led to increased skeletal thickness and increased length in young rats.
It is well known that the first year of life is an important time for building brain networks and cementing the foundation of a baby’s metabolic system.
“It may well be that minute amounts of fructose have detrimental effects on infant metabolism,” added study co-author Tanya Alderete, postdoctoral research scholar at the Keck School of Medicine.
Dr Goran’s team did not collect mothers' dietary data for this study, so were unable to determine if the trace amounts of fructose found in breast milk was positively associated with regular consumption of fructose-rich foods and drinks.
“It is important to note that these observed associations do not necessarily imply causation,” the study concluded.
“Further work is needed to examine the possibility that even these very small amounts of fructose can affect musculoskeletal development as well as adipose tissue development in infancy and early life, which is a rapid growth period where significant changes in muscular and skeletal growth are occurring.”
The study also highlighted that although women were instructed to exclusively breastfeed it was possible that food introduction may have occurred in some infants during the study period.
This may have contributed to growth and body composition, particularly if infants were given access to fructose-containing food products.
Published online ahead of print: doi:10.3390/nu9020146
“Fructose in Breast Milk Is Positively Associated with Infant Body Composition at 6 Months of Age.”
Authors: Michael Goran, Ashley Martin, Tanya Alderete, Hideji Fujiwara and David Fields