By Christian Jarrett

The phenomenon of mothers gaining weight during and beyond pregnancy is well-researched and understood – much of it has to do with the hormonal changes that assist fetal growth and preparation for lactation. Less researched and recognised, other than through jokes about “dad bods”, is that many expectant fathers also gain weight, and that the pounds tend to stay on (one study found that fathers weigh, on average, 14 pounds more than childless men).

In Health Psychology Review, a team led by Darby Saxbe at the University of Southern California highlight the evidence for perinatal weight gain in fathers, and they review seven potential casual mechanisms for why it happens, which they hope will stimulate further research. The lack of empirical research on this phenomenon before now “is striking”, they write.

Weight gain in fathers-to-be and new fathers is a serious public health issue, the researchers point out, because while this added size may have had adaptive benefits in our evolutionary past (for instance, by helping the new father cope with food shortages and deter aggressors), in the modern, sedentary world it is only likely to be associated with health problems, such as increased risk of cardiovascular disease.

Saxbe and her team first considered three potential behavioral mechanisms behind paternal weight gain, beginning with lack of sleep. New fathers typically suffer sleep disruption and sleep problems are well-known to lead to weight gain, including through affecting the hormones that normally regulate appetite.

Then there is the lack of exercise – one study found that young men who became fathers dropped an average of five hours per week of exercise.

The researchers also looked into whether dietary changes could be a relevant mechanism. However, while there is evidence that some new mothers increase their intake of high-fat and high-sugar food, and generally increase their calorie intake, Saxbe and her colleagues could find no evidence for such dietary changes associated with fatherhood.

Next, the researchers considered direct hormonal and even brain changes associated with fatherhood. Most consistently, there is evidence that fatherhood lowers men’s testosterone levels, possibly reflecting changing priorities away from competition for mates and toward greater commitment and caring for offspring. Saxbe and her team believe lower testosterone is another plausible mechanism for fathers’ weight gain since lower levels of this hormone are known to be associated with greater fat deposits and reduced muscle mass.

The researchers also looked into the possible role of raised cortisol – a hormone associated with stress – but they found the evidence for this as a potential mechanism is mixed at best.

The team also considered two psychological factors – depression and stress. They found both to be plausible mechanisms – they are known to increase around the time of fatherhood, and both have bi-directional links with weight gain (though the researchers added that the idea of depression as a cause, rather than merely a consequence, of weight gain is less supported).

Finally, Saxbe’s team considered the role of expectant fathers’ pregnant partners. As well as the possible part played by phantom pregnancy (or couvade syndrome), there are the normal dynamics of being in a couple to consider, as partners tend to influence each other’s lifestyles and habits – for instance, one study found that having an obese partner increased one’s own obesity risk six-fold.

Of course, the various potential mechanisms that the researchers outlined are closely interlinked. Future longitudinal research that identifies when exactly expectant fathers are most vulnerable to weight gain will help elucidate where each of the proposed mechanisms lies in a causal chain. The researchers also suggest the need for cross-cultural research to see if paternal weight gain is universal, together with investigations into the role of social background and personal psychological factors such as self-control and motivation.

Such research endeavors are important not only for men’s health, Saxbe and her team point out, but also for family health, especially given prior evidence suggesting that fathers’ obesity may be a greater risk factor for children’s obesity than mothers’ obesity (in fact Saxbe and co suggest that fathers are the “forgotten parent” in childhood obesity research given the lack of interventions targeted at fathers).

“New fathers deserve more study and attention,” the researchers conclude. “Paternal perinatal weight gain is a real and intriguing phenomenon that may have lifelong health implications for men and their families. In this new area of inquiry, there are exciting opportunities for researchers to discover why and when fathers to gain weight over the transition to parenthood and how best to intervene to reduce the risk of paternal obesity.”

—The weight of fatherhood: identifying mechanisms to explain paternal perinatal weight gain

Christian Jarrett (@Psych_Writer) is Editor of BPS Research Digest