Worried about gaining too much weight while pregnant? I explore the dangers of dieting during pregnancy on baby’s risk of obesity and disease in adulthood.
Most of us understand that we need to eat more during pregnancy to support the amazing things happening inside our belly. Despite pregnancy being an exciting time, for a lot of women who struggle with body image, it can be hard to watch the baby weight pack on throughout the trimesters. Being a new mom myself, I am always horrified when I hear some women discuss what their doctors have suggested they do to prevent themselves from gaining too much (or any!) weight during early pregnancy. Keto diets, gluten free diets, cleanses, I’ve heard it all. So today I wanted to chat about the dangers of dieting during pregnancy. Spoiler alert: it’s not just an issue for baby’s development and growth right now. It actually may have some serious consequences long term. Let’s break this down for you so you see the bigger picture.
Dangers of Dieting During Pregnancy on Baby’s Risk of Obesity and Birth Defects
Regardless of the size of mom before she got pregnant, dieting during pregnancy deprives baby of crucial pregnancy nutrients. Not getting enough calories also means you likely won’t be getting enough protein, folic acid, vitamin B12, iron, vitamin D and calcium, among many other vital nutrients. An undernourished mother means her baby is more likely to suffer from a wide range of health complications and disabilities, so let’s talk a little bit about that here.
Disease and Physical Developmental Risks
Research continues to suggest that undernutrition during pregnancy (again, regardless of mom’s size) may be linked to intrauterine growth restriction (IUGR), resulting in issues with the developing renal, pulmonary, vascular, cardiac, and endocrine systems, difficulties breastfeeding, preterm delivery, and low birth weight.
Low birth weight (LBW) specifically has been associated with a greater risk of developing various diseases. Cohort studies from one UK population found that LBW babies had higher rates of coronary heart disease as adults. Follow-ups on these populations revealed that LBW was associated with higher blood pressure, insulin resistance and Type 2 Diabetes in adult men. Similarly, two cohort populations from Finland showed that both stroke and coronary heart disease were associated with low birth weights, and “thinness at birth” was related to metabolic syndrome and Diabetes in adults. The famous Nurses Health Study in the US also showed very similar findings. This study including 70,297 women found that birth weight was linked to risks of high blood pressure, stroke and coronary heart disease. In other words, the long term result is not just having a skinny baby- but also having some serious long term effects.
Dieting behaviours during pregnancy, including restrictive eating, fasting diets, diets for weight loss, and disordered eating, especially the use of diuretics, has also been associated with increased risk of neural tube defects (NTDs).
Mental Developmental and Intelligence Risk
Even “moderate dieting” has been found to stunt brain development, reduce IQ and promote behavioural problems in primate animal models. A human study echoed these findings, suggesting that babies born to moms who had low caloric intakes during pregnancy had lower math scores. In a very common study from the 1940s, the Dutch Hunger Winter, pregnant women were exposed to famine which had a huge influence on their offspring. The study found that the offspring of these women undergoing famine had an increased risk of cardiovascular, kidney, lung and metabolic disorders and reduced cognitive function. A more recent 2011 study involving primates, found that eating less during pregnancy impaired fetal brain development. Yes this is a non-human study, however the evidence is mounting in this area and it does appear that cutting calories may be associated with reduced brain function.
Metabolic Health Risk
Dieting, in general, has also been linked to some long-term metabolic issues, not just during pregnancy, but honestly, all throughout the life cycle. The “thrifty phenotype” hypothesis proposes that when a baby is exposed to a calorie-restricted environment in the womb, they start to learn that they need to slow down our metabolism and hold onto every calorie in order to survive. Pretty smart baby, if you ask me. However it becomes a problem when they get out in the real world and they’re exposed to ALL THE FOOD. The result? An increased risk for excess weight gain and other metabolism disturbances.
It seems that while the world is constantly worrying about mommas gaining too much weight, undernourishment and dieting may be just as likely to increase baby’s tendency to gain excess body fat. This is often cited by researchers often as a “U or J shaped” pattern between a mother’s weight gain and child body fat as they age. In other words, pregnant women with either the lowest or highest weight gains during pregnancy tend to see similar risk in producing large babies with higher risks of metabolic disease.
This seems to be because the changes that are going on in utero do not match the environment once the baby is born. Barker and colleagues came up with a hypothesis where they predicted that the baby born with the lowest birth weight had the greatest risk of metabolic disease later in life. They predicted that adverse events that occur during fetal development (like restrictive dieting) increases a child’s susceptibility to adulthood diseases. A clear example of this hypothesis is in the infamous Dutch Hunger Winter study. More on that coming up.
Weight Gain Throughout Life
The profound impact of both under- and over-nutrition during pregnancy are seen in one of the most well-known events providing crucial insight on the impact of prenatal nutrition on fetal health – the Dutch Hunger Winter. During this major famine lasting for about 6 months, some pregnant women became very undernourished in either early pregnancy or later pregnancy. As a result, babies affected by the famine in the first trimester had heavier than normal birth weights, while those in late gestation had low birth weights. Babies who encountered the famine earlier had higher rates of coronary heart disease, increased circulating lipids, blood clotting factors and obesity. Those impacted mid-gestation were more likely to have poor kidney function as adults, whereas babies impacted late gestation had higher chances of developing glucose intolerance and Type 2 Diabetes. A more recent 2017 study found that the offspring of mothers who drank at least one diet beverage (artificially sweetened) per day were more likely to be overweight or obese by age seven compared to mothers who drank water instead. We’ve discussed the potential issues with artificial sweeteners here, here and here if you want a little more context.
This has been echoed by research on babies in India who are born at the lowest birth weight with the smallest abdominal circumference. Despite being tiny, these babies tend to have the highest distribution of fat which they not only tend to maintain into adulthood, but it also predisposes them to metabolic disorders down the road.
While all of the mechanisms are currently debated, some animal research suggests that the offspring of calorie-restricted mothers tend to display signs of leptin resistance! These offspring tend to consume more calories and gain weight faster once outside the womb. Remember when we dug into the research on leptin resistance here?
Some researchers have found that babies from mommas who were undernourished had underdeveloped pancreatic beta-cells, increasing the risk for baby to suffer from glucose intolerance and Diabetes later in life. A systematic review of the literature concurred, suggesting that low birth weight babies tend to display challenges with insulin metabolism later in life, which may pre-dispose them to Type 2 Diabetes. In a more recent 2015 Taiwanese study, women who did not gain enough weight during their pregnancy (based on the Institute of Medicine weight gain guidelines) were more likely to be diagnosed with Gestational Diabetes Mellitus (GDM). This is really interesting to me as most people associated gestational diabetes with eating (and thereby) gaining too much weight!
Risk of Intermittent Fasting DIETING during Pregnancy
If you’re expecting a little one but are a strict follower of intermittent fasting, it may be time to go back to a normal and more regular eating schedule. Pregnancy is a time when it’s most important to listen to your hunger cues. During periods of fasting, there is a cumulative and significant reduction in blood glucose levels, which is the preferable source of energy the fetus needs for growth. The shortened eating time frame of intermittent fasting, which often involves skipping breakfast and not eating anything after dinner, can result in weight loss, which as you can see, is metabolically damaging to the fetus. It also becomes restrictive during times when pregnant women need additional snacks to get extra calories for their babe or when they feel ill and need to maintain stable blood sugars. Studies on religious fasting of women such as during Ramadan (similar to intermittent fasting) is contradictory, either showing no adverse health effects (also here and here), significant reductions in frequency and occurrence of the fetus’ breathing movements, or major concerns including low birth weights. Another study also compared differences between Muslim adults who experienced fasting in utero during Ramadan and those who did not. Results showed that both adult Muslim men and women who were conceived during this time were thinner and shorter, and those who had been in utero mid- to late-gestation were only a bit thinner than unexposed Muslims. This may be due to the stress that fasting inflicts on the fetus, influencing size and growth of the placenta and programming the fetus for poor long-term health. However, another study adjusted for any confounding factors (e.g. smoking, gestational age) found that infants conceived during Ramadan were no smaller than those who were not. These researchers suggested that fasting around conception time is unlikely to have a profound effect on birth size or premature birth.
While obviously the research is complicated and we can’t know for sure whether fasting or other lifestyle factors cause adverse outcomes to the fetus, the limited and inconclusive nature of research on long-term health impacts of prenatal intermittent fasting provides more of a reason to avoid this kind of practice.
Risk of Keto Dieting during Pregnancy
Joining the list of increasingly popular fad diets is the ketogenic diet. This very low carbohydrate, high fat diet is often adopted by many healthy individuals as a lifestyle choice, including women of child-bearing age and some pregnant women. With such a restrictive diet, many question whether the fetus will normally develop in the womb and be healthy after birth.
Past studies performed on mice found some serious consequences of prenatal exposure to ketogenic diets on offspring, including impaired embryonic growth and brain structure alterations before giving birth and in early life. In a current mice study, two groups of mice were involved: one that was prenatally exposed to a ketogenic diet (4:1 ratio of fat to protein and carbohydrate) and the other was exposed to a standard diet. Researchers found alterations in brain structure and enhanced neurobehaviour, specifically decreased susceptibility to depression and anxiety, and increased hyperactivity, among adult mice with prenatal exposure to a ketogenic diet. Although this was shown in mice, it is still unclear whether this can occur in humans as well.
Both rodent and human adult studies have demonstrated that limited intakes of carbs and available glucose induces ketogenesis, a metabolic process that produces ketones, which the brain and other organs can use for fuel. However, on a prenatal ketogenic diet, the embryo relies on the mother’s ketone supply. If there is an excessive supply of ketones, this can impair growth and development of the fetus’ brain. Research also suggests that ketosis may lead to NTD risks because of studies performed on diabetic women experiencing famine in the first month of pregnancy. It may really just be best to hold off on your diet until babe is safe and sound.
Risk of Pregorexia
For some, gaining weight during pregnancy is a great fear and many become overconsumed with tracking their calories and the number on their scale. This is a condition known as pregorexia. This is not an official medical diagnosis, and the term was coined by the media and the public, but the idea behind it has been around for a long time. Because of this fear, many women will try anything to limit their weight gain as much as possible. As we can see from the research above, this not only puts your babe at risk but also yourself.
The Bottom Line on the Dangers of Dieting During Pregnancy
Dieting or following any sort of irregular diet pattern during pregnancy is not without any risks. As one of the most nutritionally-demanding stages of a woman’s life, pregnancy requires an ample nutrient supply for health and wellbeing of the fetus. We still need the evidence base behind the benefits and safety of intermittent fasting and ketogenic diets before anyone can be sure of its safe practice during pregnancy. What’s most important is the health of your baby, and given the vast amount of research supporting the importance of optimal nutrition in the prenatal diet, nothing should compromise this as top priority.
RD2B Rachel Shim
Updated on October 14th, 2020
Abbey Sharp is a Registered Dietitian (RD), regulated by the Ontario College of Dietitians. She is a mom, YouTuber, Blogger, award winning cookbook author, media coach specializing in food and nutrition influencers, and a frequent contributor to national publications like Healthline and on national broadcast TV shows.