Why pushing for preconception weight loss is more likely to harm than help

On Nutrition

One thing that both breaks my heart and fires me up is when I’m in one of the online dietitian/therapist groups I belong to and I see a plea for a recommendation of an OB-GYN or fertility-medicine doctor who doesn’t tell fat women (I’m using fat as a neutral descriptor, like short, tall or thin) that they have to lose weight before they get pregnant.

The recommendation to lose weight to improve the odds of a successful, healthy pregnancy is widespread, but without rigorous evidence to back it up. So why do many experts, medical societies, public health programs and treatment centers push for weight loss before starting infertility treatment? The answer is a mix of weight bias and epidemiological research showing that there is an association between “obesity” and infertility. But association is not the same as cause and effect.

“Health care providers know that obesity is associated with greater complications in pregnancy. Fertility providers are also aware of the association between obesity and lower fertility. The time to pregnancy can be twice as long as for women with an elevated BMI [body mass index],” said Angela Thyer, a doctor and founding partner of Seattle Reproductive Medicine. “It is from these historic studies that doctors hypothesized that if women could lose weight, they could improve their time to conception and fertility. They recommended weight loss without knowing if it really helped.”

A recent randomized controlled trial — the type of study that can demonstrate cause and effect, found that weight loss did not help. Results of the FIT-PLESE trial, published in January in PLOS Medicine, found that in 379 women with a BMI at or above 30 and unexplained infertility — unexplained, because the women were ovulating normally — a 16-week intensive weight loss intervention that included a low-calorie diet, physical activity (increasing step counts up to 10,000 per day) and medication that reduced fat absorption was no better at improving fertility or birth outcomes than an intervention that simply increased physical activity. This was despite the women in the weight loss intervention losing about 7% of their starting weight. The authors concluded that there is not strong evidence to recommend weight loss prior to conception in women who have a BMI in the “obese” range and unexplained infertility.

“It takes well-designed studies like this one to change existing dogma,” Thyer said, adding that this study was surprising to many providers because it disproved long-held beliefs about weight loss and fertility. “Now, we can say with confidence that there is not evidence that weight loss improves conception in women who are obese or overweight with ovulatory infertility.”

While the study may have been surprising, it has company. A 2016 randomized controlled trial published in the New England Journal of Medicine found that women with infertility — some who were ovulating regularly and some who were not — undergoing a similar intervention involving a low-calorie diet had fewer healthy births than the women who did not attempt weight loss.

“A reduction in body fat can be beneficial for reducing insulin resistance and inflammation, which may improve conception rates,” said registered dietitian Judy Simon, owner of Mind Body Nutrition. “But if a person decides to go on a severe calorie-restrictive diet, they are more likely to be missing key nutrients that optimize fertility and pregnancy. I am amazed that a woman may have an average or low BMI and be cleared for IVF [in vitro fertilization], yet she may have an eating disorder and be poorly nourished. In fact, women who present to a fertility clinic are more likely to have a past or current eating disorder. Weight does not equal health, especially when nutrition and lifestyle habits are factored in.”

Thyer said that if weight loss might help a specific patient for a specific purpose, it needs to be approached with compassion and ideally, with expert guidance. “When weight loss is recommended with no direction, patients can leave medical treatment feeling dejected and despondent, and disappear for years,” she said. “And we know as women age, their fertility also decreases. I have unfortunately seen women in my practice where this has occurred.”

For more than a decade, Simon and Thyer — who are speaking on “Check Your Weight Bias at the Door: How to be a body-affirming health care provider” at the Seattle Reproductive Symposium in May — have offered Food for Fertility classes that focus on learning eating competence — including meal planning and cooking together — and making lifestyle changes in a supportive group environment. “Many of our participants who were preparing for IVF actually conceived naturally after eating regularly and feeling more confident in their bodies,” Simon said. All with no focus on weight loss.

“We do not withhold fertility treatments for any women because of BMI,” Thyer said. “We encourage all women to optimize their lifestyle prior to conception by following a plant-forward, whole foods, pro-fertility diet, getting regular activity every day, reducing use of substances like alcohol, tobacco and cannabis, aiming for restful, restorative sleep, reducing stress, and maintaining a loving and supportive network of friends and family as they are on their path to pregnancy.”

Some providers may still be recommending weight loss based on older information, Thyer said. “Thankfully, the tide is shifting. Many providers are now aware that weight loss alone may not help fertility for most patients with a higher BMI. Many are also aware that restrictive diets are not healthy, are likely to fail over time, and that weight cycling is worse than maintaining current weight.”

Simon said she has had patients with a BMI over 55 who were turned away by fertility clinics. “Luckily they found other help,” she said, adding that dealing with infertility is already stressful, and when women feel guilt and shame about their size, that can damage their mental health. “I have had great success with working with women in fat bodies supporting themselves during preconception pregnancy and postpartum. I support them advocating for themselves and their size.”


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