Are vegetable oils healthy?

In small amounts, vegetable oils are healthy, as they contain fats that are essential in our diets.

But our consumption of vegetable oil has increased considerably since the 1960s, and the health effects of high intakes are a source of scientific debate.

What researchers agree on is that vegetable oils such as safflower, sunflower, corn, cottonseed and soybean oils, are rich in a type of fat known as linoleic acid. That acid can lower LDL or "bad" cholesterol when it replaces saturated fats in the diet (which primarily come from animal sources). 

"An overwhelming amount of data supports using vegetable oil in place of animal fat," said Alice H. Lichtenstein, Gershoff Professor of Nutrition Science and Policy and Director and senior scientist of the Cardiovascular Nutrition Laboratory at Tufts University.

But whether consuming vegetable oils ultimately improves health outcomes seems to be less clear. In observational studies, which don't prove cause and effect but look at relationships between variables such as food consumption and disease, linoleic acid intake is inversely associated with heart disease risk in a dose-response manner.

Those findings don't seem to be the final word on the subject. "The gaps are whether the cholesterol-lowering effects of replacing saturated fats with liquid vegetable oils rich in linoleic acid actually translates to improved heart health and whether unintended consequences of relatively high intakes exist," said Daisy Zamora, epidemiologist at the University of North Carolina.

Her meta-analysis of randomized clinical trials on the topic, published in the BMJ, concluded that although replacing saturated fat with vegetable oils does lower LDL cholesterol, it doesn't necessarily translate into decreased deaths from heart disease.

So where does that leave us in regard to vegetable oils? 

First, it's important to note that vegetable oils are rich in essential fatty acids -- specifically linoleic acid and alpha-linolenic acid -- that our bodies need but can't make on their own, so we must consume them.

But the amount that is considered adequate is very small. It is just 12 grams of linoleic acid per day for women between the ages of 19 and 50, and 17 grams for men of the same age. The amount drops slightly over the age of 51.

One tablespoon of safflower oil contains 10 grams of linoleic acid -- well over half of our daily needs. The same amount of soybean oil has 7 grams. And because oils are calorie-dense (a tablespoon of oil has 120 calories), too much in the diet can contribute to weight gain and obesity. 

"You don't want to just pour soybean oil on your salad," Lichtenstein said. "To keep the same number of calories, you need to swap out something else, preferably the croutons or a dinner roll, which are usually made with refined white flour."

According to researchers, there is also some evidence that vegetable oils can promote atherosclerosis when the oils are chemically modified in a process known as oxidation. People who have a high oxidative load from, say, smoking or heavy drinking may be more susceptible to this than those with healthier habits, according to Zamora. Repeatedly heating oils for cooking, especially when deep-frying, can cause oxidation of oils. 

Finally, it's important to keep in mind that not all vegetable oils are created equal. For example, olive oil is low in linoleic acid and is a staple of the Mediterranean diet, which has been associated with beneficial effects on health, including a reduced risk of cardiovascular mortality.

Bottom line? There's no need to eliminate vegetable oils from your diet, but be sure to include other heart-healthy foods on your plate such as fruits, vegetables, whole grains and fatty fish.