In a study Gettler and colleagues conducted in the Philippines, “we found that new dads with newborns have very substantial drops in their testosterone on average, which could reflect changes during pregnancy or immediately in the period after their babies are born.”

The men in the study were in their 20s, and their testosterone went down about 25% on average. “That’s a big biologically meaningful change,” Gettler says. “Far greater than what might be simply due to ageing, which might only drop testosterone levels 10% per decade after the 30s.”

In a similar article published in Evolution, Medicine and Public Health in 2017, Gettler used US population data to show American men who were partnered and living with children had substantially greater body fat, particularly around their waists, than single men not living with children.

The difference in body fat between the two groups — the dad-bod phenomenon — was explained by their lower testosterone levels, Gettler wrote. Age-matched single men not living with babies or young children had higher testosterone on average. Studies have also shown men’s brains change with fatherhood.

As to why those evolutionary ancestors saddled today’s fathers with a steep hormonal drop, Gettler says it’s about priorities.

“The idea is that that transition with this decline in testosterone, whether you’re talking about bird fathers or human fathers or other mammalian fathers, helps to shift priorities away from mating effort, away from competition and more toward partnering with the mother to help raise demanding offspring,” he says.

But humans have also evolved, Gettler explains, such that fathers have the capacity to be involved in a range of ways that are costly in terms of time, energy and resources.

People often have less time to exercise as they start families and dive deeper into work, typically in their 30s.

“Generally, some time around the age of 30, a sedentary person starts to lose muscle at about the rate of half a per cent per year,” says Brad Schoenfeld, an expert on muscle physiology and a professor in exercise science at Lehman College in the Bronx. “That’s usually in that dad-body age range, so certain parts will begin to sag, and any addition of extra body fat in the area only accentuates the effect.”

But it can also be an important time to emphasise men’s health and wellbeing.

“I think it is important to shift the exercise narrative away from a focus on weight loss to a focus on health and wellbeing gain,” says Shelley Keating, an obesity researcher and senior lecturer at the University of Queensland’s School of Human Movement and Nutrition Sciences in Australia.

“For dads, or anyone for that matter, looking to make positive changes, it’s important to understand that most health benefits of exercise occur irrespective of weight change,” Keating says. In other words, it’s not all about what you see when you step on the scale.

Avoiding or unravelling the dad bod takes exercise and healthy diet choices, and before you start, it’s a good idea to consult qualified professionals.

“I always like to indicate that this is best supported by a doctor or dietitian to make sure no essential food groups are being cut and micronutrient requirements are being met,” Keating says.

Men should also be realistic and not too hard on themselves. “Try and make your healthiest choices and your best choices as often as you can, and at the same time, recognise you may not always be able to do that,” says Craig Garfield, a paediatrics professor at Northwestern University’s Feinberg School of Medicine and an attending physician at Lurie Children’s Hospital of Chicago. Garfield also founded the first public health survey of fathers in the perinatal period. The most important thing is for new dads and mums to take care of themselves, he says, because that’s ultimately best for the baby, too.

For most men, an hour or two of strength training a week is enough to push back against the dad bod, Schoenfeld says.

Total body exercise

Schoenfeld recommends working all the major muscles of the body in the same workout session, preferably with exercises that involve multiple joints. This includes push-ups, squats, pull-ups, lunges and overhead presses. Perform a minimum of one leg-pressing exercise (eg, squats), one upper-body-pulling exercise (eg, pull-up) and one upper-body-pushing exercise (eg, bench press) a week. You can use machines or free weights. Aim for a minimum of four weekly sets per muscle group a week.

Cardiovascular training

When the goal is getting healthy and losing fat, exercise at or even below the physical activity guidelines of 150-300 minutes a week of moderate-intensity exercise or 75-150 minutes a week of vigorous-intensity exercise can help shed abdominal fat, Keating says. For weight loss via aerobic exercise alone, more than 60 minutes at least five days a week is recommended.

Time-crunch training

When finding time to exercise is a problem, low-volume, high-intensity interval training (HIIT) programmes allow a shorter exercise session with the same gain or better. But before starting HIIT, older adults and anyone at risk of cardiovascular disease, including those starting exercise after a period of being inactive, should get medical clearance to do so.

Nutrition

Generally, to lose weight you should eat about 500 calories less than normal a day, Keating says. The diet should focus on whole foods and improving diet quality. As with exercise, the sustainability of the diet is a key for long-term weight-loss success, she says. Improving your diet quality can improve your heart health, even in the absence of weight loss.

Do you have a fitness question? Email YourMove@washpost.com and we may answer your question in a future column.