What Happens When Pregnant Women Eat More Animal Protein

Are high-protein diets during pregnancy healthful or harmful? That question was answered about 40 years ago in the infamous Harlem Trial of 1976: a “randomized controlled trial of nutritional supplementation pregnancy, in a poor black urban population in the United States.” The study, which I discuss in my video The Effect of Animal Protein on Stress Hormones, Testosterone, and Pregnancy, “was begun when protein was commonly assumed to be deficient in the diet of the poor.” Had researchers actually analyzed their diets before they started, they would have realized that this wasn’t true, but why let facts get in the way of assumptions? So, the researchers split poor black pregnant women into three groups, each receiving one of the following treatments: (1) an extra 40 grams of animal protein a day, which is essentially a couple cans of Ensure, (2) an extra 6 grams of animal protein, or (3) no extra protein. Then they sat back and watched what happened. The high-protein group suffered “an excess of very early premature births and associated neonatal [infant] deaths, and there was significant growth retardation” in the babies who survived. More protein meant more prematurity, more deaths, and more growth retardation, which you can see reflected in the chart at 1:00 in my video.

What’s more, animal protein intake during pregnancy has been associated with children becoming overweight later in life and getting high blood pressure. The “offspring of mothers who reported eating more meat and fish had higher systolic blood pressure” in adulthood. This was part of another failed dietary intervention trial in which mothers were advised to eat a pound of meat a day. The increased weight gain and high blood pressure may be due to the obesity-causing chemical pollutants in the meat supply, as I’ve discussed in my video Animal Protein, Pregnancy, and Childhood Obesity, or the animal protein-induced rise in the growth hormone IGF-1. Or, it could be due to a steroid stress hormone called cortisol.

As you can see in the chart at 2:01 in my video, a single meal high in animal protein can nearly double the level of the stress hormone in the blood within a half hour of consumption, much more than a meal closer to the recommended level of protein. When subjects are given a meal of crab, tuna fish, and cottage cheese, the stress hormone level shoots up. If they’re instead given some barley soup and a vegetable stir-fry on rice, the stress hormone level goes down after the meal, as you can see at 2:27 in my video. Imagine eating meat-fish-dairy meals day after day. Doing so “may chronically stimulate” our stress response axis “and increase the release of vasoactive hormones” that could increase our blood pressure. And, all that extra cortisol release has been linked to increased risk for elevated blood levels of insulin, triglycerides, and cholesterol.

When men on a high-protein diet, “such as meat, fish, poultry, egg white,” were switched to a high-carb diet of bread, vegetables, fruit, and sugary junk, their cortisol levels dropped about a quarter within 10 days. At the same time, their testosterone levels shot up by about the same amount, as you can see at 3:09 in my video. High-protein diets suppress testosterone. That is why, if men eating plant-based diets begin to eat meat every day, their testosterone levels go down and some estrogens actually go up, and that’s why bodybuilders can get such low testosterone levels. It’s not the steroids they’re taking. If you look at natural bodybuilders who don’t use steroids, there is a 75 percent drop in testosterone levels in the months leading up to a competition. Testosterone levels were cut by more than half, which is enough to drop a guy into an abnormally low range, as you can see at 3:47 in my video. It’s ironic that they’re eating protein to look manly on the outside, but it can make them less and less manly on the inside. And, from an obesity standpoint, in general, a drop in testosterone levels may increase the risk of gaining weight and body fat. What does cortisol have to do with weight?

There’s actually a disease caused by having too much cortisol, called Cushing’s syndrome, which can increase abdominal obesity. Even in normal women, though, chronic stress and chronic high cortisol levels can contribute to obesity. What’s more, if they’re pregnant, high-meat and low-carb diets may increase cortisol levels in the moms, which can lead to inappropriate fetal exposure to cortisol, which, in turn, can affect the developing fetus, resetting her or his whole stress response thermostat and leading to higher cortisol levels in later adult life. This can have serious, life-long health consequences. Every maternal daily portion of meat and fish was associated with 5 percent higher cortisol levels in their children as much as 30 years later, though green vegetable consumption was found to be protective. Higher meat consumption, such as three servings a day compared to one or two, was associated with significantly higher cortisol levels, but eating greens every day appeared to blunt some of that excess stress response, as you can see at 5:12 in my video.

As well, the adult children of mothers who ate a lot of meat during pregnancy don’t only have higher stress hormone levels, they also appear to react more negatively to whatever life throws at them. Researchers put them through the Trier Test, which involves public speaking in front of a panel of judges, following by a live math exercise. You can see in my video at 5:36 a chart comparing the stress hormone responses in those whose moms ate less than two servings of meat per day, about two servings a day, or about two to three servings a day. Note that before the test started, the cortisol levels of the two groups eating less meat started out about the same, but their exaggerated cortisol response was laid bare when exposed to a stressful situation. The real-world effects of this are that after that sort of test, when people are given their own private snack buffet with fruits and veggies versus fatty, sugary, comfort foods like chocolate cake, guess who may eat less of the fruits and veggies? Those who have high chronic stress levels. “Cortisol has been implicated as a factor in motivating food intake” even when we aren’t really hungry.

It’s no surprise then that a woman’s animal protein intake during pregnancy may lead to larger weight gain for her children later in life—and maybe even for her grandchildren. “Remarkably, recent evidence suggests that the long-term consequences of adverse conditions during early development may not be limited to one generation, but may lead to poor health in the generations to follow, even if these individuals develop in normal conditions themselves.” Indeed, the diet of a pregnant mother may affect the development and disease risk of her children and even her grandchildren. Ultimately, these findings may shed light on our rapidly expanding epidemics of diabetes, obesity, and heart disease.


Whoa, there was a lot to unpack! Rather than break it up, since so much of it was tied together, as you could see, I compiled everything into this one, heftier piece. You may want to read this a second time and watch the video to absorb it all.

For more on how a woman’s diet during pregnancy can affect her children, see Maternal Diet May Affect Stress Responses in Children and Animal Protein, Pregnancy, and Childhood Obesity.

Protein is such a misunderstood nutrient. For more information, check out:

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live presentations:

Morning Sickness Can Be Beneficial

“Since the beginning of time, pregnant women have been reported to suffer from a syndrome variously known as morning sickness, pregnancy sickness, or nausea and vomiting during pregnancy.” The term “morning sickness” is actually misleading, as women can feel sick all day long. Sometimes, it can get so serious that women have to be hospitalized.

Researchers at Harvard’s Brigham and Women’s Hospital found that saturated fat seemed to be a primary dietary risk factor for severe sickness, with five times the odds for every 15 grams intake of saturated fat, “equivalent to one quarter-pound cheeseburger.” The reason saturated fat intake may be such a strong risk factor could be through its effects on estrogen, as “[s]aturated fat has been shown to increase circulating levels of estrogen.”

Why would we evolve to have such a negative reaction to saturated fat? Why would we evolve to get sick at all? As I discuss in my video Morning Sickness May Protect Mother and Child, “[p]regnancy sickness is a universal phenomenon, affecting 70% to 85% of all pregnant women.” If food aversions are included in the criteria, along with nausea and vomiting, the incidence is more like 100 percent. “Because pregnancy sickness is such a common phenomenon, one must question why is this so? Is there a purpose for such a potentially devastating condition?” In the past, pregnancy sickness was dismissed as just being in women’s heads, but recent “studies have reconsidered pregnancy sickness as an embryo-protective mechanism, an evolutionary adaptation to protect the embryo.”

Protect the baby from what? From meat. “Meat is the principal source of pathogens for humans. Meat is also the most common type of food avoided by pregnant women.” So, the development of an aversion to meat during pregnancy could be protective because “meat may have toxins that are mutagenic, carcinogenic, and teratogenic,” meaning causing birth defects, and tainted meat may also be contaminated by pathogens. “Pregnancy is a time of relative immunosuppression.” Normally, we can fend off most meat pathogens. “However, by biological design from evolutionary pressures, pregnant women are immunosuppressed to not reject the developing embryo”––as half the baby (from the father’s side) is foreign. So, morning sickness may have evolved as a way to get us to stay away from meat during this vulnerable time. This would be consistent with a “profound overrepresentation of meat taboos” in sample societies around the world.

If this theory is true, then we should be able to make five predictions. First, if nausea and vomiting in pregnancy are meant to be protective, women who experience them should have better pregnancy outcomes. Indeed, women who suffer from nausea and vomiting are significantly less likely to miscarry or have a stillbirth.

Second, the foods that trigger nausea and vomiting should contain things that can be particularly harmful to the baby, and, in fact, “[o]f all food types, animal protein (including meat, poultry, eggs, and seafood…) is the most dangerous. Meat is the source of a wide range of pathogens that pose a grave threat to pregnant women and developing organisms” that is, their developing embryos.

Third, nausea and vomiting in pregnancy should also coincide with the time when the embryo is most vulnerable, which is approximately weeks 5 through 15, when all the critical organ structures are being formed. And, indeed, that period is right when nausea and vomiting are in fact peaking.

Fourth, pregnant women should find meat and eggs most aversive during this time of heightened embryo sensitivity, and that, too, is the case. And finally, if this theory is true, one should expect a lower frequency of morning sickness among plant-based populations, and, yes, the few societies in which we don’t see such morning sickness problems are the ones that tend to have only plants as dietary staples, rather than meat.


What can you do if you suffer from morning sickness? See Natural Treatments for Morning Sickness.

What other effects can diet have on a healthy pregnancy? See, for example,

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations:

How Can Animal Protein Intake Increase Childhood Obesity Risk?

If pregnant crickets are exposed to a predatory wolf spider, their babies will hatch, exhibiting increased antipredator behavior and, as a consequence, improved survival from wolf spider attack. The mother cricket appears to be able to forewarn her babies about the threat when they are still inside her, so they would be pre-adapted to their external environment. This even happens in plants. If you grow two genetically identical plants—one in the sun, one in the shade—the sun-grown plant will produce seeds that grow better in the sun, and the shaded plant will produce seeds that grow better in the shade—even though they’re genetically identical.

What’s happening is called epigenetics, external factors changing gene expression.

Vole pups born in the winter come out growing thicker coats. Vole mothers are able to communicate the season to their babies in utero and tell them to put a coat on even before they’re born. We’re no different. You know how some people have different temperature tolerances, resulting in “battles of the bedroom”? Do you turn the AC on or off? Open the windows? It’s not just genetics. Whether we’re born in the tropics or in a cold environment determines how many active sweat glands we have in our skin.

What does this have to do with diet? As I discuss in my video Animal Protein, Pregnancy, and Childhood Obesity, can what a pregnant woman eats—or doesn’t eat—permanently alter the biology of her children in terms of what genes are turned on or off throughout life?

What happened to the children born during the 1944 – 1945 Dutch famine imposed by the Nazis? They had higher rates of obesity 50 years later. The baby’s DNA gene expression was reprogrammed before birth to expect to be born into a world of famine and conserve calories at all cost. But when the war ended, this propensity to store fat became a disadvantage. What pregnant women eat and don’t eat doesn’t just help determine the birth weight of the child, but the future adult weight of the child.

For example, maternal protein intake during pregnancy may play a role in the obesity epidemic—but not just protein in general. “Protein from animal sources, primarily meat products, consumed during pregnancy may increase risk of overweight in offspring…” Originally, researchers thought it might be the IGF-1, a growth hormone boosted by animal product consumption, that may increase the production of fatty tissue, but weight gain was tied more to meat intake than dairy. Every daily portion of meat intake during the third trimester of pregnancy resulted in about an extra 1 percent of body fat mass in their children by their 16th birthday, potentially increasing their risk of becoming obese later in life, independent of how many calories they ate or how much they exercised.  But no such link was found with cow’s milk intake, which would presumably boost IGF-1 levels just as high.

Given that, perhaps instead of IGF-1, it’s the obesogens in meat, chemicals that stimulate the growth of fatty tissue. “[E]merging evidence demonstrates that environmental factors can predispose exposed individuals to gain weight, irrespective of diet and exercise.” After all, even our infants are fatter, and we can’t blame that on diet and exercise. Animals are fatter, too, and not just our pampered pets—even rats in laboratories and subways are bigger. “The likelihood of 24 animal populations from eight different species all showing a positive trend in weight over the past few decades by chance was estimated at about 1 in 10 million” so it appears something else is going on—something like obesogenic chemicals.

One such candidate is polycyclic aromatic hydrocarbons (PAHs), which are found in cigarette smoke, vehicle exhaust, and grilled meat. A nationwide study of thousands found that the more children were exposed to PAHs, the fatter they tended to be. The researchers could measure the level of these chemicals right out of their urine. Exposure can start in the womb. Indeed, prenatal exposure to these chemicals may cause increased fat mass gained during childhood and a higher risk of childhood obesity.

If these pollutants sound familiar, I’ve covered them before in relation to increasing breast cancer risk in the Long Island Breast Cancer Study Project. So, perhaps they aren’t just obesogens, but carcinogens, as well, which may help explain the 47 percent increase in breast cancer risk among older women in relation to a lifetime average of grilled and smoked foods.

If we look at one of the most common of these toxins, smokers get about half from food and half from cigarettes. For nonsmokers, however, 99 percent comes from diet. The highest levels of PAHs are found in meat, with pork apparently worse than beef. Even dark green leafies like kale can get contaminated by pollutants in the air, though, so don’t forage for dandelion greens next to the highway and make sure to wash your greens under running water.

These are fat-soluble pollutants, so they need lots of fat to be absorbed. It’s possible that even heavily contaminated plant-based sources may be safer, unless you pour lots of oil on your food, in which case the toxins would presumably become as readily absorbed as the toxins in meat.

The good news is they don’t build up in our body. As I show in my video, if we expose people to barbecued chicken, they get a big spike in these chemicals—up to a hundred-fold increase—but our body can get rid of them within about 20 hours. The problem, of course, is that people who eat these kinds of foods every day could be constantly exposing themselves, which may not only affect their health and their children’s health, but maybe even their grandchildren’s health.

Being pregnant during the Dutch famine of the mid-1940s didn’t just lead to an increase in diseases among their kids, but even apparently their grandkids. What a pregnant woman eats now may affect future generations. “The issue of generation-spanning effects of poor conditions during [pregnancy]…may shed light on the epidemic of diabetes, obesity and cardiovascular disease,” which is associated with the transition towards Western lifestyles.


Epigenetics is the science of altering the expression of our genes. No matter our family history, some genes can be effectively turned on and off by the lifestyle choices we make. See, for example:

For more on “obesogenic” chemicals, see:

I previously touched on PAHs in Meat Fumes: Dietary Secondhand Smoke.

In health,
Michael Greger, M.D.

PS: If you haven’t yet, you can subscribe to my free videos here and watch my live, year-in-review presentations: