Editorial NexoScience
Boys are the more fragile sex in early childhood, at least when there is not enough food to go around. The common assumption runs the other way. In many parts of the world girls are believed to eat last and least, which should leave them thinner and shorter than their brothers. A review of the evidence on childhood undernutrition suggests the opposite is closer to the truth, and that the disadvantage starts before a child is born.
The review, published Feb. 23, 2022, in the journal Nutrients, was written to explain a result the same team had already found. Pooling surveys of children under the age of five, they had calculated that boys carry about a quarter higher odds of wasting, being dangerously thin for their height, and close to a third higher odds of stunting, being too short for their age, than girls living in the same conditions. The gaps are modest and they are not uniform. They were less pronounced in South Asia than elsewhere, which was one of the first clues that biology alone does not explain them.
The evidence came from a sweep of published research, not from a new experiment. This is a narrative review, a structured reading of what the literature already contains rather than a fresh trial or a new pooled estimate. The authors searched the Medline, Embase, Global Health, Popline and Cochrane databases, worked through the reference lists of what they found, and deliberately took in studies that had been too varied in design to enter their earlier meta-analysis. They also plotted national stunting rates from household surveys against country food security scores from the Food and Agriculture Organization, to see how the gap between boys and girls tracked with poverty.
The gap opens in the womb. Male fetuses are already larger than female ones between the 8th and 12th weeks of pregnancy, and they grow faster from there. That head start comes with a hidden cost. A boy’s placenta appears to be the more efficient organ, delivering more growth per gram of placenta, but to hold less reserve capacity, meaning less spare ability to keep pushing oxygen and nutrients across when conditions tighten. It works something like two engines of different sizes fed by one fuel line. The bigger engine runs perfectly well while fuel flows freely, and it is the first to sputter when the flow drops. By one estimate cited in the review, a newborn girl is physiologically about as mature as a boy four to six weeks older, which leaves her better equipped to ride out a bad start. Longitudinal data from Nepal points the same way in reverse. When mothers in one arm of a trial received food supplements, the shortfall in their sons narrowed, as though boys had been the ones held back by the constraint all along.
Low birth weight tells the story in miniature. The standard threshold is a single absolute number, 5.5 pounds (2,500 grams), applied to boys and girls alike, and more girls fall below it for the simple reason that they are born smaller. Yet among babies born under that weight, it is the boys who die more often. A smaller body carrying proportionally more fat looks like an advantage when food is short, not a handicap.
Body fat is not just a reserve, it is part of the immune system. Girls are born with more of it, and with more leptin, the satiety hormone that fat tissue produces and that also helps run immune responses. In a sample of 82 newborns cited in the review, boys averaged around 15 nanograms of leptin per milliliter of blood against around 25 in girls. That matters well beyond appetite. In a Ugandan study of children admitted for treatment of acute malnutrition, those who arrived with the lowest leptin levels faced the highest risk of dying during treatment. Boys are also more susceptible than girls to most infectious diseases, with a few known exceptions including measles, whooping cough and tuberculosis, and infection and undernutrition feed each other. A sick child eats less and absorbs less while needing more energy, and a poorly nourished child catches more infections.
Care practices can widen the gap or flip it. In Senegal and in Guatemala, researchers found that boys were started on solid foods earlier than girls, often because mothers read them as hungrier or less satisfied by breastfeeding alone, which can raise the risk of infection at a vulnerable age. The authors of the Senegal work cautioned that the arrow may point both ways, since the boys were already growing less well when the food was introduced. Pull in the other direction and the bias against girls is easy to find. In northern India, girls were less likely than boys to have eaten solid or semi-solid food in the previous day, and daughters were breastfed for shorter periods than sons. Across an analysis spanning close to a hundred countries, greater societal gender inequality was associated with more child undernutrition in girls and boys alike. The review does not claim that discrimination against girls is imaginary. It claims that it runs alongside a biological tilt pointing the other way.
The authors are blunt about how thin the evidence still is. They wrote that much of the work they reviewed treated sex differences as a secondary finding rather than as the question being investigated, so explanatory and confounding factors were often not properly accounted for, and that the evidence is at times conflicting. None of it is causal. The link between national food insecurity and stunting, for example, is a correlation measured across countries, and a correlation of that kind cannot show that one thing produces the other. A narrative review also involves judgment about which studies to bring in, which is a different exercise from the arithmetic of a meta-analysis.
The measuring tape may be part of the problem. One of the most widely used field tools for spotting a wasted child is a tape wrapped around the upper arm, and it uses a single cutoff for both sexes. Because boys are bigger in absolute terms and need more energy to stay on the same growth curve, that one number may overestimate wasting in girls and miss it in boys. The authors call for mortality outcomes to be analyzed by sex using arm circumference, and for research into whether these differences change treatment outcomes, cognitive development and long-term risk. They also note something uncomfortable about how the field receives this kind of result. When male disadvantage shows up in the data, practitioners they spoke with at an international nutrition conference in 2019 tended to question the quality of the data rather than the assumption behind it. Until that reflex shifts, the useful step available now is an unglamorous one, which is to keep collecting growth data broken down by age and sex and then check whether the children walking into treatment programs match the children the surveys say are at risk.
The study, “Understanding Sex Differences in Childhood Undernutrition: A Narrative Review,” was conducted by Susan Thurstans, Charles Opondo, Andrew Seal, Jonathan C. Wells, Tanya Khara, Carmel Dolan, André Briend, Mark Myatt, Michel Garenne, Andrew Mertens, Rebecca Sear and Marko Kerac, and published in the journal Nutrients (DOI: 10.3390/nu14050948).
