A study that found no connection between dietary fat and heart disease was followed by one that differentiated between vegetable and animal fats. The Right Chemistry columnist sorts it all out: Bring on the vegetable fats!

Want to weigh less, be healthier and live longer? Just watch your fat intake, particularly those nasty saturated fats that lurk in meat, dairy products and rich pastries. They're artery-cloggers and will kill you.

That's the notion I grew up with academically and dutifully passed on to students and audiences. The guilt of saturated fats was seemingly so ingrained in the scientific literature that there was no question of questioning it.

So you can appreciate my surprise when a paper appeared in the American Journal of Clinical Nutrition, titled: “Meta-analysis of prospective cohort studies evaluating the association of saturated fat with cardiovascular disease.” It wasn't the title of the paper that was shocking; it was the conclusion: “There is no significant evidence for concluding that dietary saturated fat is associated with an increased risk of coronary heart disease or cardiovascular disease.”

Wow! The American Journal of Clinical Nutrition is probably the prime nutritional journal in the world, and this study was done by researchers at the Harvard School of Public Health, not exactly a shabby institution.

A “meta-analysis” is a study of studies, in which all the information is pooled to achieve greater statistical significance than is afforded by individual studies. In this case, 21 high-quality studies that had monitored the diets of more than a third of a million people for five to 23 years were identified.

During the study period, 11,000 people developed heart disease or had a stroke. Stunningly, the amount of saturated fat in the diet made no difference in determining who would be afflicted. Talk about flying in the face of dogma! I thought I had better have a second look at the origins of the saturated fat-heart disease hypothesis.

The trail is not hard to find and leads directly to the famous Seven Countries Study by University of Minnesota physiologist Dr. Ancel Keys. Prompted by the high rate of heart disease in well-fed Americans when compared with postwar underfed Europeans, Keys investigated the relationship between diet and heart disease. In the seven countries identified in the study, there was a clear association between saturated-fat intake and heart disease.

Finland and the United States, with their massive fat consumption, had the highest incidence of disease, while the Mediterranean countries and Japan, with their reduced intake of saturated fat, had a lower heart-disease rate. Keys hypothesized that saturated fat drove up cholesterol, which in turn caused heart disease.

In 1961, Time magazine featured Keys on its cover, crediting him with tracking down the cause of heart disease. Instantly, saturated fats were transformed into nutritional pariahs. But a little digging into Keys's work unearths an unsettling fact: He actually studied 22 countries and did a little cherry-picking of the data. If one plots all the data, the association between saturated fat and heart disease disappears.

Furthermore, the classic Framingham study that linked blood cholesterol to heart disease showed no association between the disease and saturated-fat intake.

Now, two recent major studies have also found that a diet's fat content doesn't make much difference in terms of weight loss or heart-disease risk. These are no fly-by-night studies. They're featured in such topnotch publications as the New England Journal of Medicine, and Circulation.

A two-year trial, carried out jointly by Israeli, German and U.S. researchers, enlisted 322 moderately obese subjects. They were put on one of three diets:

Low-fat, high-carbohydrate, restricted-calorie.

“Mediterranean” restricted-calorie.

Low-carbohydrate, highfat, non-restricted-calorie.

The biggest weight loss (about 12 pounds) was in the low-carb group, the least (about seven pounds) in the low-fat group! Even more shockingly, the greatest improvement in cholesterol profile was in the low-carb, high-fat group. Furthermore, ultrasound measurement of the thickness of the carotid artery wall, a possible predictor of heart disease, revealed that all three diet groups showed a similar improvement, probably because of a weight-loss-induced drop in blood pressure.

Could saturated fats have been unjustly maligned all these years? Could promoters of low-carb, high-fat diets have been on to something? Had they been unfairly ridiculed by the scientific establishment? Based on the information I had gathered, I was willing to give that thought some credence.

Then, a few days ago, a Harvard research group stirred the pot with yet another meta-analysis, this time focusing on trials in which saturated fats in the diet were replaced by unsaturated fats from vegetable sources. Now there was a significant reduction in heart disease!

How can we then mesh this with the findings of the Israeli study? A careful look at that one reveals that in the high-fat group, the subjects were counselled to rely on vegetable sources of fat.

And how about the previous meta-analysis that found no link to saturated fats? It didn't consider what the fats were replaced with in the low-fat diets. Replacement with refined carbohydrates is of no benefit. But according to the latest Harvard study, replacement with unsaturated fats is!

What do we make of all this? That nutrition is very complicated, and trying to unravel its nuances can drive you crazy. But now we know weight control and heart-disease protection can be achieved by various diets, perhaps, surprisingly, most effectively by low-carb ones, as long as their higher fat content derives mostly from vegetable oils.

But here's something on which low-carb and high-carb proponents agree: Sugar and other refined carbohydrates should be limited.

Mark Twain once quipped that part of the secret of success in life is to eat what you like and let the food fight it out inside. Cute, but I think we're better off by letting the researchers fight it out on the outside. So cut the sugar, limit the saturated fats and bring on the vegetable oils.

At least until the next study comes out.