The Quantity or the Quality of Dietary Fats: A Brief Note
Nutritional biochemists such as Burr and Burr had delineated the essential fatty acids in the 1930s. Early hypotheses about dietary fat, blood lipid levels and atherosclerotic disease treated, however, only total fat, in absolute values or percent calories. For example, the early ecologic correlations between FAO/WHO data on fat consumption versus vital statistics on causes of death were used by Ancel Keys to test his early diet-heart hypothesis.
By the early 1950s, Juda Groen in the Netherlands already had clearly in mind the potential differences between vegetable and animal fats, through his observations comparing vegetarian Trappist monks with the high-living Benedictines and from his correspondence with Cornelis de Langen in Java and Isidore Snapper in China, who already were thinking in terms of vegetable oils being inherently different in composition and health effects from mammalian fats.
The burst of new discovery in this area came from the early 1950s in the work at the Rockefeller Institute of Ed Ahrens, and from Larry Kinsell in Berkeley, which demonstrated with controlled dietary substitutions the different effects of animal and vegetable fats on serum lipid composition. This work was soon expanded to experimental pathology in animals and to explorations in human diets.
A classical example of good results from an intellectual failure was Ancel Keys’s recognition that he was late in accepting this evidence of different serum effects of different food fats. Once accepting the difference, however, he was particularly well equipped to do the needed definitive studies of fatty acid composition in careful crossover metabolic ward experiments.
Keys’s work with diet substitutions using numbers of men and natural foods was coincident with that of Mark Hegsted in the early 1960s. With similar experimental design they achieved basically similar predictive equations from linear solutions of regressions of dietary fatty acid change, in substitution for carbohydrates, holding calories constant, against change in serum total cholesterol level. Both investigators reported almost simultaneously that saturated fatty acids were twice as effective in raising total serum cholesterol levels as polyunsaturates were in lowering those levels, and that monounsaturated effects dropped out from the equation, being insignificantly different from carbohydrate effect under this constant conditions.
Thus, the issue of saturated-polyunsaturated fatty acids was satisfactorily resolved. The effectiveness of those equations, independently and equally predicting effects of diet change, have withstood the test of 40-year’s time. The neutral monosaturated fatty acid effect was also resolved until it was raised again by Grundy¹s experiments with olive oil that were differently designed than those of Keys and Hegsted. The latter investigators felt that Grundy’s design could not show, innately or specifically, cholesterol-lowering properties of olive oil but only change in contrast to saturated fatty acids effect.
As to the independent effect of dietary cholesterol on serum cholesterol levels, that issue took many more years to resolve. Hegsted had priority in including diet cholesterol effect into his general experiments and equation, which showed a linear response to diet intake. Keys¹s eventually found, in experiments separate from the fatty acid changes, a curvilinear relationship in which serum cholesterol changed progressively less as diet cholesterol intake was added to the diet. Based on his and others studies, this diminishing effect turned out to be a closer approximation to reality than
the straightline relationship of Hegsted¹s experiments.
At any rate, for several decades the two investigators went back and forth in argument and data on the diet cholesterol effect, one quietly and with moderate language, the other with considerable bombast. In their later years they reconciled differences about the dietary cholesterol effect and subsequently carried out a cordial correspondence. (Henry Blackburn)