Father’s Junk Food Habit Linked to Heavier Newborns, Early Study Finds

Fathers who eat a lot of junk food may have newborns that arrive heavier and built a little differently, according to new research. Scientists found that fathers who got more of their daily calories from ultra-processed foods, things like sausages, bacon, sugar-sweetened drinks, and cookies, tended to have newborns with higher birth weight, a higher ponderal index, and thicker skinfolds at two spots on the body: the supra-iliac (hip area) and the thigh. No link showed up with the baby’s overall estimated body fat.

For decades, public health messaging around pregnancy has focused almost entirely on what expectant mothers eat. Fathers have largely been left out of the conversation. But the new work indicates that what a man puts on his plate may matter more than previously thought, adding to a growing body of evidence that a father’s health before and during his partner’s pregnancy can leave a biological imprint on his child.

Researchers studied 43 complete family units, each made up of a father, a pregnant mother, and their newborn. Published in the journal Nutrition Research, the analysis drew these families from the control group of a larger clinical trial run at primary health care units in Ribeirão Preto, Brazil, between 2018 and 2021. All of the mothers in the original trial were overweight before pregnancy. On average, fathers got about 30% of their daily calories from ultra-processed foods, while mothers got about 26%.

How the Father’s Diet Was Studied

Dietary information for both parents was obtained via a 24-hour dietary recall, where trained dietitians asked participants to describe everything they ate and drank the previous day. Each parent completed two of these sessions. Foods were then sorted using a framework called the NOVA classification, which groups foods by how heavily they are processed. Paternal diets leaned heavily on sausages, bacon, sugar-sweetened drinks, and cookies. Maternal diets featured different staples, including hamburgers, ready-to-eat meals, packaged snacks, and industrialized french fries. Researchers found no statistically significant link between how much ultra-processed food fathers ate and how much their partners ate, suggesting these were largely independent habits.

Newborns were measured by trained researchers, ideally between three and five days of life, though some were evaluated up to 28 days after birth. Measurements included weight, length, head and chest size, and a pinch test at specific spots on the body used to estimate fat just beneath the skin. Fat tissue was estimated using a math formula previously validated against air displacement plethysmography, a direct measurement tool, though it is not equivalent to that gold-standard method.

What More Junk Food Meant for Newborns

After accounting for factors like the father’s age, education level, physical activity, smoking status, and alcohol use, the link between a father’s ultra-processed food intake and his newborn’s body measurements held up. Fathers with higher junk food consumption had babies with greater birth weight, a higher ponderal index, a measure of how heavy a baby is relative to their length, and thicker skinfolds at the supra-iliac and thigh sites. One exception: paternal junk food intake showed no link to the newborn’s overall estimated body fat, but did show a link to specific skinfold sites, a distinction the researchers say is worth pursuing.

How a Father’s Diet Might Reach the Womb

So how might a man’s eating influence a baby growing inside the mother’s body? Researchers point to a process called epigenetics, changes in how genes are switched on or off without altering the underlying genetic code. Prior research, separate from this study, has suggested that poor diet in men can alter chemical tags on sperm DNA, which may then influence how certain genes behave after fertilization. Previous research has focused on a gene that plays an important role in fetal growth, and studies have shown that paternal obesity can disrupt its chemical regulation in newborns.

The researchers behind this study propose that a diet high in ultra-processed foods may trigger a similar disruption, possibly by affecting sperm quality through low-grade inflammation and oxidative stress. They are careful to note that none of these mechanisms were tested here; the earlier findings serve strictly as a plausible interpretive frame, not as pathways confirmed by their own data. Whether this entire chain of events plays out in humans, and whether it explains what they saw, remains speculative and is still being studied.

When Mom’s Diet Had the Opposite Effect

One of the more unexpected results was that maternal and paternal junk food intake appeared to push newborn measurements in opposite directions. When mothers ate more ultra-processed foods during early pregnancy (up to 16 weeks), their babies tended to be smaller at birth, with lower birth weight, shorter length, and smaller head size. No such association turned up for maternal diet later in pregnancy.

Researchers offer one possible reading: mothers may affect fetal growth directly through the womb’s environment, while fathers may act earlier, through epigenetic changes set before conception, that may affect sperm. They are explicit that this is a tentative hypothesis, not a settled conclusion, and the split raises questions that future, larger studies will need to sort out.

Caution runs through the whole paper. Only 43 father-mother-newborn triads were included, largely because many fathers declined to take part, citing lack of time or interest, and because data collection paused for seven months during the COVID-19 pandemic. Wide confidence intervals around the main results mean real uncertainty remains about the size, or even the reality, of any true effect. Recruitment was also limited to overweight women, so the results may not generalize to families where the mother has a different body type. Diet was measured during pregnancy rather than before conception, the ideal window.

For now, this stands as one of the first human studies to examine whether fathers’ junk food intake relates to newborn body measurements, and the signals it turned up are worth a closer look. Whether they hold up, and whether they ever translate into advice for men thinking about starting a family, will come down to the larger, more rigorous work the authors say still needs to be done.

Disclaimer: This article summarizes a small observational study nested within a clinical trial. It included 43 families with overweight mothers and found associations, not proof of cause and effect. The results are preliminary and should not be taken as medical or dietary advice. Anyone with questions about diet before or during pregnancy should consult a qualified health care provider.