Beneficial aspects of vegetable oils have been overestimated while the risks of saturated fats may not be as bad as implied. More importantly, blood cholesterol values may not be a reliable indicator of risk of cardiovascular disease, shows a re-analysis of a study done 45 years ago.
This was discovered during a study on linoleic acid and its harmful effects by Christopher Ramsden at the National Institutes of Health. The team re-analysed published and unpublished data from a randomised controlled trial conducted with 9,423 residents of state mental hospitals and nursing home in Minnesota.
Data for the re-analysis was taken from a subset of 2,355 participants who had followed the diet for over a year, had regular cholesterol measurements and had follow-up data for three years.
The group had been randomly assigned to eat a diet low in saturated fat, but high in linoleic acid-rich vegetable oil (corn oil), or a control diet that had the same amount of saturated fat (mostly animal fat) used before the study, but with an increase in linoleic acid.
Both showed a lowering of cholesterol owing to linoleic acid, but the group with vegetable oil had lower cholesterol than the control diet group. Surprisingly participants who had greater reduction in blood cholesterol had higher risk of death.
The low saturated fat diet significantly reduced the level of cholesterol in the blood by 13.8% compared to the control diet, which lowered cholesterol by just 1%. In both groups, for each 0.78mm/l reduction in cholesterol, there was a 22% higher risk of death from any cause. The diets were eaten for an average of 460 days.
Whether this means vegetable oils are bad and saturated fats not so bad will have to be tested further before any emphatic conclusion. It is also not clear if the diet influenced cholesterol as the lowering was seen in both groups and was based on just 149 deaths over the short follow-up period. There was no association between cholesterol reduction and death for the 1,760 people aged under 65.
The new analysis of old data claims that some of the data from the randomized control trials was unpublished and this has "contributed to overestimation of the benefits of replacing saturated fat with vegetable oils rich in linoleic acid".
It is also pointed out that the researchers did not have access to the raw data to be able to confirm these findings which were observed for short lengths of follow-up. Also there was insufficient autopsy information for accurate analysis of the effect of the diets on heart attacks or atherosclerosis.
Experts also stressed there was an established link between high cholesterol and the risk of heart attack or stroke. "More research and longer studies are needed to assesses whether or not eating less saturated fat can reduce your risk of cardiovascular death," Jeremy Pearson of the British Heart Foundation told AFP.
Animal fat in meat, butter, cheese and cream has been blamed for raising cholesterol linked to heart disease and stroke. In 1961, the American Heart Association recommended vegetable oils replace saturated fats, as also the WHO that advises saturated fats should comprise less than 10 percent of total energy intake.
The vegetable oils being consumed in place of saturated animal fats contain polyunsaturated fats called linoleic acid or omega-6 fats. Experts have been pointing to a balanced fat regimen providing a mix of omega-3 and omega-6 fats.
Some of the omega-3 fats are found in organic oils like virgin olive oil, coconut oil, avocados, butter, flax seeds, sesame seeds, almonds, walnuts and pumpkin seeds. Omega-6 rich foods include grape seed oil, sunflower oil, corn oil, margarine, soybean oil, mayonnaise, nuts and seeds. Most experts agree that the omega 6:3 ratio should range from 1:1 to 5:1 but is often as high as 25:1 as in American diets.