A 50 YEAR HINDSIGHT ABOUT DIETARY FAT
What we know that ain’t so can cause more trouble than what we don’t know. This statement explains some of the conflicting advice we have received about diet in recent decades.
An eminent nutritionist from Minnesota pointed out in the early 1950's that the mortality rate from heart disease in several countries was proportional to average daily fat intake as determined from national statistics. It was already known that obesity increased diabetes and blood cholesterol and that Americans were on average gaining weight. With a few prominent nutritionists campaigning to reduce fat in the American diet and the presumed “fact” that a high fat diet promotes obesity, the politicians and the media began clamoring for simple dietary advice for the public. The few experts who pointed out what we didn’t know for sure were shouted down. One of the ignored expert’s points was that perhaps the high fat diets seemed harmful because they were lacking in some beneficial nutrients rather than harm resulting from fat itself. After all, when we eat more of one thing, we eat less of something else. Greek, Italian and Spanish diets are quite healthy in spite of being rather high in fat, but they are also high in fruits and vegetables. Suspected beneficial components of the Mediterranean diet are Omega-3 fatty acids, antioxidants, and flavonoids present in fish and vegetables.
It has been shown that overweight research subjects maintained on palatable diets of varying fat content do not change much in weight. Maybe this is like smokers maintaining a constant nicotine intake when given cigarettes of varying drug content. So fat in the diet was demonized without evidence. There is some evidence that a high carbohydrate diet is the most fattening of all especially if sweets are eaten between meals. This may be because simple sugars are the most habit forming food especially when taken on an empty stomach. In this case a small amount, since it requires no time for digestion, is promptly absorbed resulting in a sharp rise in blood sugar. The insulin mechanism over reacts and the blood sugar may fall slightly below normal in 30 minutes or so. A new craving for sugar ensues. Thus carbohydrate is the most habit forming food. Even the taste of sugar stimulates the insulin mechanism in anticipation of the sugar’s appearance in the blood. Rather obviously, this was best studied by using sugar substitutes.
This is similar to the various classes of drugs where the fastest acting are the most habit forming--for example, heroin vs. methadon, Seconal vs. phenobarbital. This is because the rush and the withdrawal are both more sudden. Furthermore, starch from most starchy foods is very quickly digested – new information from long after 50 years ago. Complex carbohydrates like those from sweet potatoes and beans are more slowly digested and absorbed. So the old wisdom about sugar is bad for diabetes and starch is less so is not as true as it seemed to be. In defense of doctors and dieticians, we have always specified fruit in diabetic diets for balance even though virtually all fruit calories are in the form of simple sugars. Thus the concept that less fat in the American diet would help reduce the prevalence of obesity was just dead wrong. Actually the incidence of obesity in the U.S. increased during the 1980s even as the per capita fat intake was declining. The public, the media, and the politicians should have heeded the cautious experts who were singing a softer, more complicated song.
An even more complex and unanticipated error is that of trans-fatty acids. These are not present in vegetable fat, but are produced in vegetable fat solidified by adding hydrogen to unsaturated fats (double bonds between carbon atoms) with heat in the presence of a catalyst. As carried out in a margarine factory, more double bonds are activated than there is hydrogen available to react. Those unrequited, activated bonds switch from cis- to trans- (fatty acids) because the trans- form is more thermodynamically stable. Recent evidence shows that trans-fatty acids elevate blood cholesterol even more than saturated fats (no carbon to carbon double bonds). More bad new news: some of the normal cis-fatty acids in cattle feed are converted to the trans- form in the rumen of cows and presumably other ruminant stomachs (sheep, goats, deer). Read the labels in the grocery. “Partially hydrogenated vegetable oil” on the list means trans-fatty acids are also present. Margarine containing no trans-fatty acids is now available.
In summary: If you are a member of the general population without risk factors (see below), concern yourself with avoiding large amounts of trans-fatty acids created by food manufacturing processes, and accept the small amounts in butter, beef and lamb. Additional efforts to limit saturated fats (mostly from animal sources) are for those people with special risk factors such as elevated blood cholesterol, family history of coronary artery disease, and high blood pressure. Many of these people benefit from medication. All of us should eat vegetables, stay fit with regular exercise and smoke only at Indian peace rituals.
Don’t be too hard on yourselves for failing to pick up on all this in the last 50 years. We are all victims of the cultural milieu into which we are born. Inferior races turned out not to be inferior, women turned out to be educable, etc. When we hear voices crying in the wilderness, it is very difficult to tell the prescient (wisest) message from the spurious (false).
John A. Frantz, M.D.