The Masai are a cattle-herding group in Kenya and Tanzania who subsist mostly on meat and milk unusually rich in fat, but who have very low levels of blood cholesterol and do not suffer from heart attacks.
In the 1950s, a group led by Bruce Taylor — the first scientist to induce a heart attack in an ape by cholesterol feeding — studied deceased Masai and showed they had very little atherosclerosis. George Mann later studied five times as many deceased Masai and determined that they had as much atherosclerosis as Americans, but because their blood vessels were more dilated, the atherosclerosis did not cause heart attacks.
Most often, commentators dismiss Taylor's study as smaller and less rigorous and accept Mann's study as definitive. But is that correct? Do the Masai eating their traditional diet really have just as much atherosclerosis as Americans?
This is part speculation, but I believe one possibility is that Taylor was studying more isolated Masai who were more strictly adhering to their traditional diet.
Mann found the size of the lumen to increase rather than decrease with age because the size of the vessels more than compensated for intimal thickening, and found the degree of atherosclerosis to actually reverse itself during the ages of 12-30, when the Masai eat a strictly pure meat, blood and milk diet and are active warriors, which is also when their always-low cholesterol levels are at their lowest.
Without question, Mann's study was more rigorous. It used five times as many autopsies, reported its method of selecting subjects and of determining ages, and gave much more detail about its measurements as well as photographs.
Both studies appear to have been conducted in the same region, among the Loita Masai, who apparently to this day maintain a more traditional lifestyle than other groups of Masai, and currently number some 25,000 (out of 450,000 Masai). The plateau they occupy is apparently relatively isolated and takes an 8-hour drive in a four wheel-drive vehicle to reach (see page 9 here).
However, it also appears that Dr. Mann's study was broader in its geographical area than Dr. Taylor's study. Mann writes, "The tissues were collected in regional hospitals and medical stations of Masailand in Northern Tanzania and Southwestern Kenya from the region of the Loita Masai." He states that the collection of materials "was done by establishing eight collection stations in Masailand."
By contrast, Dr. Taylor's autopsies came only from "the Narok (Kenya) District Hospital." According to the above-linked paper, the isolated Loita live on a plateau elevated 2000 meters in southeastern Narok (the district itself being located in the southwest of Kenya, bordering northern Tanzania). It would therefore appear that Dr. Taylor's study may have been much more specific to the isolated Loita than Dr. Mann's, which included autopsies from a number of hospitals and extended outside of the Narok district.
Indeed, Mann notes the following:
The old and the young Masai do have some access to such processed staples as flour, sugar, confections and shortenings through the Indian dukas scattered about Masailand.
It would seem as if the 25,000 Loita Masai isolated on the plateau by terrain requiring a four-wheel drive vehicle to traverse would have less access to these foods than the 425,000 Masai occupying the surrounding areas.
Of course this cannot be said to be conclusive, but it remains a possibility that Taylor studied Masai who were much more isolated than those whom Mann studied.
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