Did you hear the one about a high fat diet in pregnancy causing neurodevelopmental disorders in offspring? The fearmongering headlines might be enough to make women switch from butter to grape jam on their toast or to revert to cowering in abject terror at the sight of an avocado or handful of macadamia nuts. But is the hype everything it’s cracked up to be?
Since patients who hear these tidbits on the news or read them online are apt to get whiplash from all the contradictory declarations it’s up to healthcare professionals to separate the proverbial wheat from the clickbait chaff. So let’s take a closer look at the study and see if there’s a reason to shun fat during pregnancy.
Before we dive into things let’s ponder how the Mediterranean region has been just notorious through the centuries for babies being born with all kinds of problems due to their mothers eating full-fat foods rather than opting for fat-free feta low-fat prosciutto and flavoring their food with low-sodium nonfat chicken broth instead of good ol’ olive oil.
Okay now that the good-natured sarcasm is out of the way let’s see what the “high fat” diet in the study was comprised of. According to the study (“Exposure to a High-Fat Diet during Early Development Programs Behavior and Impairs the Central Serotonergic System in Juvenile Non-Human Primates”) the control and “high fat” diets differed in several ways besides just the fat content. For starters as a percent of total energy the control diet had protein at 26.8% while the high fat diet had 18.4% protein. As a percent of total energy fat on the control diet was 14.7% compared to 36.6% for the high fat diet. Regarding the composition of fatty acids the high fat diet was significantly higher in saturated and monounsaturated fat and was slightly lower in polyunsaturated fats including omega-3 fatty acids.
Where the diets really went off the rails though was in carbohydrate composition. As a percentage of total energy the control diet was 58.5% carbohydrate while the high fat diet was 50% carbohydrate. At fifty percent carbohydrate it’s unclear how this could be classified as a “high fat” diet at all but journalists clearly weren’t conflicted about this not-so-small point when they crafted headlines like “High-fat diet in pregnancy can cause mental health problems in offspring.” In any case that the high fat diet was half carbohydrate isn’t even the main point. The real kicker is the high fat diet was substantially higher in sucrose and fructose than the control diet. Confounding anyone?
Another problem with this study of course is that it was conducted in macaques not in humans. As our primate relatives macaques are likely a better model for human outcomes than say fruit flies earthworms or yeast cells in petri dishes but they’re still not humans and findings should be interpreted with caution. The physical characteristics of the dams used bring up additional points of contention. First dams put on the high fat diet had a very slightly higher pre-pregnancy body weight compared to dams on the control diet (mean body weight of 10.9kg versus 10.0kg respectively) but a notably higher pre-pregnancy body fat percentage: 25.2% versus 19.3%.
Subject animals’ insulin levels are provided only for the third trimester; there is no information regarding insulin levels at the time of conception or in earlier stages of pregnancy. At the start of the third trimester insulin levels in the high fat diet dams were 40.5µU/ml compared to 26.5µU/ml in the control diet dams. (They also had a higher insulin area under the curve during the third trimester.) Hinted at by their significantly higher pre-pregnancy body fat percentages it’s possible the dams on the high fat diet started out with insulin levels already higher than the animals on the control diet—yet another potential confounder.
But what causes high insulin? What caused the developmental issues in the offspring? Was it a high fat intake per se or was it the combination of increased fat in the context of a diet that was still fifty percent carbohydrate? This is the crucial question none of the smarmy clickbait headlines nor the stories that followed even asked let alone sought to answer. It might be nice to see a follow-up study to this: one where macaques are matched for pre-pregnancy body fat percentage and insulin levels and where a “high fat” diet really is a high fat diet and a not a high fat high carb diet. It’s pretty well recognized at this point that high fat high carb is the worst of both worlds: that’s basically the standard American diet and we know how well that has served us. (In fact the study authors even point this out: “The HFD represents a typical Western diet in respect to percent of calories provided by fat and saturated fat content.” At fifty percent of total energy we could also say this is actually a little lower than many North Americans’ carbohydrate intake!)
A high fat diet can be incredibly therapeutic when it’s low in carbohydrate and a high carb diet can also work for some people when it’s low in fat. It is irresponsible to lay blame for problems on the fat content of a diet when the real culprit could well be the combination of an increased fat intake in a diet that is also high in carbohydrate. This research shouldn’t be dismissed out of hand but it raises more questions than it answers and the spurious conclusions it led to should not have been plastered all over health and nutrition news as if they were definitive.