The Skinny on Dietary Fats

Reading time: 6 – 10 minutes

Fat has a bad reputation, both in food and on the body. It’s certainly true that the U.S. has a problem with body fat; according to the U.S. Centers for Disease Control and Prevention (CDC), about two-thirds of adults in the U.S. are overweight, and fully one-third of adults fall into the more serious “obese” category [1]. Still, appropriate amounts of body fat serve valuable roles. These include helping to maintain the immune system and nervous system, protecting body organs and padding areas where the skeleton would otherwise put pressure directly on the skin (such as the soles of the feet).

The skinny on fat

Too much body fat, however, is associated with a number of negative health effects, including increased risk of heart disease, type 2 diabetes, arthritis, and apnea.

Fat in the diet can contribute to excess fat on the body, as dietary fat is a dense source of calories. This means that even small quantities of fatty food are very calorie-rich. However, too much body fat comes from too much food in general — regardless of whether it’s made up of fat, carbohydrate or protein — since the body converts excess protein and carbohydrate into stored fat. Further, appropriate quantities of fat are important in the diet; dietary fat serves as a satiety signal, increases the absorption of fat-soluble vitamins, and (depending upon the kind of fat) plays other roles as well.

Satiety Signal: a chemical signal that communicates to the brain that the calories consumed in a meal are sufficient to fill energy needs. Receiving and responding appropriately to satiety signals helps prevent overeating.

There are three basic categories of fat in the diet: saturated fats, trans fats and unsaturated fats. For a healthy diet, a growing body of research suggests that you should avoid saturated fats and trans fats, and focus on eating unsaturated fats.

What to avoid: saturated fats

Saturated fats come mainly from animal sources, though there are some plants (avocados and coconut, for instance) that contain saturated fat. Because of their chemical makeup, these fats have high melting points, and are solid at room temperature. Saturated fats contribute to heart disease by increasing low-density lipoprotein, also known as LDL (sometimes called “bad cholesterol”) and should be limited in the diet. According to the American Heart Association, an individual on a 2,000 calorie-per-day diet should limit saturated fat intake to no more than about 16 grams [2].

What to avoid: trans fats

Trans fats are common in the American diet, but they’re not found in nature to any significant extent. Instead, they come about by processing unsaturated oils (including corn and soybean oil). The processes used, called “partial hydrogenation” and “interesterification,” result in the production of plant-based fat that behaves in food like saturated fat. Shortening is an example of trans fat; it’s plant based, but it’s solid at room temperature. Trans fats act strangely in the body: they not only increase LDL, as saturated fat does, they also decrease high-density lipoprotein or HDL. This makes them particularly deleterious components of diet.

In addition to impacting cholesterol, trans fats appear to increase risk of type 2 diabetes. The American Heart Association recommends limiting the amount of trans fat in the diet as much as possible, and sets an upper recommended limit of 2 grams per day [4]. Nutrition labels on foods report how much trans fat is in a serving of the food; whole foods (which don’t have nutrition labels) don’t contain trans fat. One important point to remember is that manufacturers can report that a food contains zero grams of trans fat per serving as long as it contains no more than 0.49 grams per serving. Avoiding trans fat in the diet therefore requires reading ingredient lists as well as nutrition facts; if the words “partially-hydrogenated” or “interesterified” appear in the ingredients, there’s trans fat in the food. The most common sources of trans fat in the diet are processed foods and fast food.

Unsaturated fats are healthy fats

Unsaturated fats, which include both monounsaturated and polyunsaturated fats, come mainly from plant sources, though they’re also found in cold-water fish. Their chemical makeup is such that they have low melting points, and are liquid at room temperature. Some of them are even liquid in the refrigerator or freezer. They confer all the benefits of dietary fat (immune system support, increased fat-soluble vitamin absorption, and so forth) without increasing LDL concentration in the blood. The American Heart Association recommends that 25-35% of daily calories come from fat, the vast majority of which should be unsaturated [3].

Omega-3 fats and omega-6 fats are subcategories of unsaturated fat. They’re both essential in the human diet, but the typical Western diet is much too high in omega-6 fat relative to the amount of omega-3 fat. Generally speaking, omega-6 fat is pro-inflammatory, and increases risk of arthritis, asthma and heart disease when consumed out-of-proportion with omega-3 fat, which is generally anti-inflammatory. Omega-6 fat comes from plant sources, most notably from grains and grain-based oils like corn oil. Omega-3 fat is found in some plant sources (flax is a commonly-cited example), and in fish. Unfortunately, the majority of omega-3 fat in plant sources isn’t a type that humans can use, and has to be converted in the cells into usable omega-3 fats such as docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). Human cells are quite inefficient at this conversion, meaning most of the omega-3 fat from the majority of plant sources isn’t bioavailable. Fatty, cold-water fish are the best source of bioavailable omega-3 fat.

Bioavailability: the extent to which a nutrient is accessible to the body or a medication is available to a target tissue after administration.

At the campaign website of MyPlate, which replaced the Food Pyramid to help consumers eat better, there are a number of nutrition education tips in an easy-to-follow, convenient format. One of the ten tips to making food choices for a healthy lifestyle in Choose MyPlate: 10 tips to a great plate focuses on “foods to eat less often”: these include foods high in solid fats, such as ice cream, pizza, ribs, sausages, bacon and hot dogs. Instead, choose lean, protein-rich foods such as soy, fish, skinless chicken and fat-free or 1% dairy products. Eat foods that are naturally low in fat such as whole grains, fruits and vegetables.


  1. Obesity and Overweight for Professionals: Data and Statistics: U.S. Obesity Trends. Centers for Disease Control and Prevention. Accessed 2011 Nov 15.
  2. Saturated Fats. American Heart Association. Accessed 2011 Nov 15.
  3. Know Your Fats. American Heart Association. Accessed 2011 Nov 15.
  4. Trans Fats American Heart Association. Accessed 2011 Nov 15.
About the Author

Kirstin Hendrickson, Ph.D., is a science journalist and faculty in the Department of Chemistry and Biochemistry at Arizona State University. She has a PhD in Chemistry, and studied mechanisms of damage to DNA during her graduate career. Kirstin also holds degrees in Zoology and Psychology. Currently, both in her teaching and in her writing, she’s interested in methods of communicating about science, and in the reciprocal relationship between science and society. She has written a textbook called Chemistry In The World, which focuses on the ways in which chemistry affects everyday life, and the ways in which humans affect each other and the environment through chemistry.