Healthnotes Newswire (March 27, 2008)—We all know that eating too much fat can make us fat, but new evidence adds to the growing awareness that some kinds of fat are worse than others. A new animal study found that a diet high in trans fats increases risk of insulin resistance and obesity, especially abdominal obesity, which is associated with heart disease. Choosing monounsaturated fat instead—as is found in olives, olive oil, avocado, and nuts and seeds—is a healthful alternative.

The new study, published in Obesity, looked at the effect of a high-trans-fat diet on sugar metabolism and weight gain in male African green monkeys. Forty-two monkeys were assigned to eat either a high-trans-fat diet or a low-trans-fat diet for six years. The monkeys on the high-trans-fat diet were given partially hydrogenated soybean oil so that trans fats accounted for approximately 8% of their daily calories—an amount similar to that of people who eat the most trans fats. The other monkeys ate the same diet but their fat was from a blend of oils high in monounsaturated fatty acids.

At the end of the study, the monkeys on the high-trans-fat diet gained four times as much weight and had more body fat than their counterparts, and a greater percentage of their body fat was in the abdomen, a pattern known to be linked to increased cardiac risk. Blood tests revealed that these monkeys showed signs of insulin resistance, a condition that leads to type 2 diabetes, but the other monkeys had normal responses to sugar and insulin.

What are trans fats?

All fats are made up of fatty acids—chains of carbon atoms, some of which have bonds to hydrogen atoms. When a fatty acid has two neighboring carbon atoms that each have a naturally occurring hydrogen bond, these bonds are usually on the same side of the carbon chain, a configuration known as a cis; however, when these bonds are damaged or created artificially through a process called hydrogenation, they tend to form on opposite sides, a configuration known as trans.

Partial hydrogenation is a way of turning a liquid oil into one that is solid at room temperature like margarine or shortening. Natural sources of trans fatty acids include butter, milk, and beef fat, but partial hydrogenation creates a much higher percentage of trans bonds than are found in nature, so partially hydrogenated oils are often referred to as trans fats. Partially hydrogenated trans fats are popular in the food industry because they are less expensive than naturally solid fats like butter, they have a long shelf life, and they can be used for deep-frying.

The trouble with trans fats

The problem comes about when these fats are eaten and incorporated into the body’s cells, especially the cell membranes. Cis fatty acids are bent and flexible, but trans fatty acids are straight and rigid, changing the shape and stiffness of cell membranes, which might make cells in tissues such as blood vessels more susceptible to damage and alter cells’ ability to respond to biochemicals such as hormones and neurotransmitters.

The link between trans fats and heart disease is well established—an estimated 30,000 to 100,000 deaths each year in the United States have been attributed to dietary trans fats. This study gives us reason to wonder if the effects of trans fats on weight gain, blood sugar control, and abdominal fat might be at the root of their negative effects on the heart. If trans fats are found to affect humans in the same way they affected the monkeys in this study, it would give us another clue about how trends in eating habits have led to the current epidemics of overweight, obesity, and type 2 diabetes in the developed world—and give us a better idea about how to reverse them.

“These findings show once again that the composition of the diet is at least as important as the calorie count,” commented Julianne Forbes, a naturopathic doctor practicing in Maine. “In addition to helping people slim down portions and reduce sugar, we can also help them lose weight, avoid diabetes, and prevent heart disease by teaching them to eat less refined foods and read labels on packaged foods to find those made without partially hydrogenated oils.”

(Obesity 2007;15:1675–84)