| Susan Carlson, PhD |
In 2011, I posted a graph on my blog in which I summarized some of the studies that have measured the LA content of fat tissue in US adults over time (1). It showed a remarkably consistent upward trend. Last year, a University of Kansas nutrition researcher named Susan Carlson contacted me and asked if I had published my findings in a scientific journal, because she wanted to cite the trend in one of her papers. I said I hadn’t published them, but that I would love to do so together.
I knew my graph captured an important trend, but to publish the finding in a scientific journal, I’d have to do a much more rigorous analysis of the literature. So I performed a systematic literature search for all studies that have measured the LA content of body fat in US adults. Together, Susan and I molded my findings into a compelling paper and published it in a good journal, Advances in Nutrition (2).
We identified 37 studies that met our criteria, the first published in 1960 and the last in 2010. The key finding is that there was a substantial and consistent upward trend in the LA content of body fat over this period (R2 = 0.83; p = less than 0.001). You can see it, with trend line added, below:
To determine how these changes relate to our diet, we obtained data on LA intake over time in the US, generously provided by National Institutes of Health researcher Joe Hibbeln (3). As expected, there is a strong correlation between the LA we eat and the LA in our fat tissue (R2 = 0.81; p = less than 0.001). This has been shown in controlled fat modification trials and cross-sectional studies, but to our knowledge it has never been confirmed using population-level data of people eating typical diets that change over decades.
This change in the composition of our fat tissue is primarily due to our greatly increased intake of seed oils, especially soybean oil, but also corn, sunflower, safflower, cottonseed, and peanut oil (4). Our total added fat intake has increased over the last half-century, and the types of added fats we use (including in processed food and restaurants) have shifted from animal fats like butter and lard to seed oils. I’ve illustrated that in the graph below*:
This adds up to a much higher total LA intake than the historical norm for the US and most of Western Europe. In fact, the current LA content of fat tissue in Europe is still quite a bit lower than it is in the US—it’s about where the US was in 1970 (5, 6, 7). This may reflect a less processed/industrialized diet.
In our paper, we deliberately remain neutral on whether this trend is harmful or beneficial. We want our paper to be a resource for the scientific community that anyone can cite comfortably, regardless of how they feel about LA.
*USDA Economic Research Service data, roughly corrected for waste at a rate of 28.8%, and an artifact in 2000.
