Heart attack

Dietary fat independently affects heart attack risk. The Nurses’ Health Study found that eating foods high in saturated fats (meat and dairy fat) and trans fatty acids (margarine, hydrogenated vegetable oil, and many processed foods containing hydrogenated vegetable oil) was directly associated with nonfatal heart attacks and deaths from coronary heart disease. Consuming foods high in monounsaturated fat, such as olive oil, and polyunsaturated fat, as found in nuts and most vegetable oil, is linked to a decreased risk. This same study revealed that margarine consumption increased the incidence of heart attack, particularly among women who had eaten margarine consistently for more than a decade. Other studies report a direct association between frequent consumption of meat and butter, and heart attack occurrence.

Depression

The amount and type of dietary fat consumed may influence the incidence of depression. Previous studies have found that diet regimens designed to lower cholesterol levels may reduce death from cardiovascular disease, but may also heighten the incidence of depression. Does low cholesterol cause depression? It appears not, since studies have shown no adverse effect on mood in people taking cholesterol-lowering drugs. The connection more likely has to do with the balance of fats in the diet. Diets to lower blood cholesterol usually focus on restricting total fat intake while increasing the intake of polyunsaturated fats (e.g., corn and soybean oil). These oils are very high in omega-6 fatty acids, but the recommended diets otherwise lack important omega-3 fatty acids (e.g., EPA and DHA). A high intake of omega-6 fatty acids relative to an inadequate intake of omega-3 fatty acids (e.g., from fish and fish oil) have been associated with increased levels of depression. People who eat diets high in omega-3 fatty acids from fish have a lower incidence of depression and suicide.

Diabetes

Fats from meat and dairy cause heart disease, the leading killer of people with diabetes. Diets high in fat, especially saturated fat, worsen glucose tolerance and increase the risk of type 2 diabetes, an effect that is not simply the result of weight gain caused by eating high-fat foods. Saturated fat is found primarily in meat, dairy products, poultry skins and dark meat. In contrast, glucose intolerance has been improved by diets high in monounsaturated oil (e.g., olive oil). There is often difficulty in changing the overall percentage of calories from fat and carbohydrate in the diets of people with type 1 diabetes. However, modifying the quality of the dietary fat is achievable. In adolescents with type 1 diabetes, increasing monounsaturated fats relative to other fats in the diet is associated with better control over blood sugar and cholesterol levels. The easiest way to incorporate monounsaturated fat into the diet is to use olive oil. However, those who are overweight need to be careful—olive oil is high in calories.

Gallstones

A recent study of residents of southern Italy found that a diet rich in animal fats and refined sugars and poor in vegetable fats and fibers were significant risk factors for gallstone formation.

Gastroesophageal reflux disease (GERD)

Conventional treatment includes avoidance of fatty foods.

High triglycerides

People with high triglycerides are typically advised to reduce their weight and limit the consumption of saturated fats (found predominantly in animal products [e.g., meat, eggs, and dairy products], and tropical oil [e.g., palm and coconut oil]). A low-fat diet (55% carbohydrate, 23% fat, 22% fat protein) succeeded in normalizing other blood lipids, including fasting triglyceride levels in one trial. However, the same diet failed to normalize post-meal triglyceride levels in a group of people with high triglycerides. These results suggest that dietary reduction of fasting triglycerides, even if the diet controls other blood lipids, may not be enough to provide optimal protection against coronary heart disease. Many doctors recommend a diet low in saturated fat (meaning avoidance of red meat and all dairy except nonfat dairy) in order to reduce triglycerides and the risk of heart disease.

Immune function

The effect of fats on the immune system is complex and only partially understood. Excessive intake of total dietary fat impairs immune response, but some types of fat may be neutral or even beneficial. For example, at reasonable dietary levels, monounsaturated fat, as found in olive oil, appears to have no detrimental effect on the immune system in humans.

Insulin resistance syndrome (IRS)

The effect of dietary fat on insulin resistance seems to depend on the type of fat eaten. Preliminary studies in animals and humans suggest that insulin resistance is worsened with increased use of saturated fat and improved with increased use of omega-3 fatty acids from fish; the role of other unsaturated fats is less clear. However, diets high in monounsaturated fat have improved insulin sensitivity in both healthy people and in people with diabetes. A diet low in saturated fat, but which allows both fish and monounsaturated fat makes sense for people with IRS because such a diet is associated with heart disease protection. A low-fat diet allowing fish has decreased insulin resistance in people with IRS.

Multiple sclerosis (MS)

A survey of people in 36 different countries suggests that the types of fat people eat might affect MS. In this report, people with MS who ate foods high in polyunsaturated and monounsaturated fatty acids had less chance of dying from MS than those who ate more saturated fats. In another survey, researchers gathered information from nearly 400 people (half with MS) over 3 years. They found that people who ate more fish had less risk of developing MS, while those who ate pork, hot dogs, and other foods high in animal (saturated) fats were at greater risk.

Stroke

The influence of dietary fat on the risk of stroke is not as clear as it is for heart disease risk. Some reports suggest an association between increased fat intake, including saturated fat (primarily found in meat and dairy), and a decreased stroke risk. These unexpected findings may be due to unique dietary conditions in the country studied (Japan) or to flaws in study design. Other evidence suggests the opposite relationship—that people consuming more saturated fat are at higher risk of stroke.

Evidence regarding the role of unsaturated fats (primarily found in vegetable oil, cooked and processed foods made with vegetable oil, nuts, and seeds) is equally unclear, suggesting that unsaturated fats may have varying effects on different types of stroke or that some unsaturated fats differ from others in their influence on stroke risk.

Weight loss

Societies that eat less fat tend to have lower rates of obesity. However, a low-fat diet is no guarantee of normal body weight. Sixty percent of the South African population is overweight, despite a comparatively low fat intake (about 22% of calories from fat). Foods with a high proportion of calories from fat should be eliminated from the diet or limited; these include red meat, poultry skins, dark poultry meat, fried foods, butter, margarine, cheese, milk (except skim milk), junk foods, and most processed foods. Vegetable oil, nuts, seeds, and avocados should be consumed in moderation, although these foods are healthful for people without weight problems.