Scientists have long known that what we commonly call “fats” are a subclass of lipids, a group of compounds made up of carbon, hydrogen, and a small amount of oxygen that generally can’t mix or dissolve in water (they are water insoluble).

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Lipids play important roles in the body, such as acting as a major component of cell membranes, giving them flexibility and integrity. They also facilitate the transport of nutrients, including fat-soluble vitamins (A, D, E, and K) and phytochemicals, as well as enhancing their absorption. Adipose tissue also cushions, protects, and insulates the kidney, heart, and other organs, shielding them from temperature extremes and acting as a natural shock absorber.

Fats are a concentrated source of energy, providing 9 kcal per gram, typically accounting for one-third or more of our total calorie intake. Fat is significantly more energy dense than carbohydrates and protein, which provide only 4 kcal per gram.

Lipids are diverse in structure and function. We will discuss the four most common lipid classes: fatty acids, triglycerides, sterols (such as cholesterol), and phospholipids (such as lecithin). Although some use the term “lipid” interchangeably with “fat,” the word “fat,” more precisely, refers to triglycerides, which make up 95% of all lipids in our foods and 99% of the stored fat in our bodies. (INFOGRAPHIC 6.1)

Question 6.1

It’s no wonder the Inuit often enjoyed blubbery seal meat and other fatty foods—fats increase the palatability (taste and flavor) of food and contribute to texture and aroma. Eating fat also contributes to the sensation of feeling full (satiety) in part by slowing gastric emptying, keeping food in the stomach a bit longer. Relative to high-carb, low-fat foods, carbohydrate foods with higher amounts of fat are digested and absorbed over longer periods, resulting in a lower glycemic response (see Chapter 5).

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Dietary fat confers some health benefits, but because many tasty foods are also fatty foods, it can be easy to eat too much fat. Any excess fat in the diet is efficiently deposited as fat in the body (adipose tissue), making high-fat, energy-dense diets one reason why people tend to gain weight.

When Dyerberg and Bang made their historic trip to Greenland, they were eager to learn about the fish-oriented dietary pattern of the Inuit, and how it affected their health. Chapter 7 discusses what we know today about the effects of dietary lipids on health and disease.

Fatty acids are a type of lipid, but they are also the primary components of both triglycerides and phospholipids. Fatty acids consist of a chain of carbon atoms with hydrogen atoms attached to each carbon atom (a hydrocarbon chain): a carboxyl group attached to one end of the fatty acid chain, and a methyl group at the other end (three hydrogens bonded to a carbon atom—also called the “omega” end of the fatty acid).

Fatty acids differ in length, and so they are categorized by the length of their hydrocarbon chains, as well as in their degree of saturation (how many hydrogen atoms fill the available bonds with carbon). Short-chain fatty acids have fewer than 6 carbons; medium-chain fatty acids have 6 to 12 carbons, and long-chain fatty acids have more than 12 carbons. Both properties, degree of saturation, and chain length determine their function in the body and role in health and disease.

Some fatty acids are saturated, with hydrogen atoms filling every possible bond with carbon atoms. Saturated fatty acids are relatively solid at room temperature, and most commonly found in animal products (such as meats and dairy), as well as some vegetable products (such as coconut oil, palm kernel oil, palm oil, and cocoa butter). The rest are unsaturated, with less hydrogen and one or more double bond (or point of unsaturation) between carbon atoms, and are generally liquid at room temperature. Unsaturated fats are found most abundantly in plant foods, such as seeds, nuts, grains, and most vegetable oils. Fatty acids with one point of unsaturation are called monounsaturated fatty acids (abundant in olive and canola oils and nuts); those with more than one point of unsaturation are called polyunsaturated fatty acids (abundant in corn, safflower, sunflower, sesame, and soybean oils). The arrangement of the hydrogen atoms on either side of the double bonds can be in either a “cis” or a “trans” orientation, which has important health implications, as will be discussed later in the chapter. (INFOGRAPHIC 6.2)

Question 6.2

Triglycerides are lipids made up of three fatty acid chains bound to one glycerol, a small three-carbon molecule that makes up the glycerol backbone of each triglyceride. The fatty acid chains form the “tail” of the triglyceride. All triglycerides are composed of a mix of short-chain, medium-chain, and long-chain fatty acids. They seldom contain exclusively one type of fatty acid.

Triglycerides in foods supply energy and may also carry certain fat-soluble vitamins, such as vitamins A, D, E, and K (covered in more detail in Chapter 10). Since triglycerides consist of three fatty acids, they can provide essential fatty acids, the fatty acids our bodies need but cannot synthesize in sufficient amounts, and so must be obtained through our diets. (INFOGRAPHIC 6.3)

Question 6.3

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Chemically, sterols are complex lipids with four interconnected carbon rings with a hydrocarbon side chain.

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The most discussed sterol is cholesterol. A molecule with varied functions, cholesterol is a critical component of our cell membranes and is also needed as a precursor for the synthesis of bile acids, vitamin D, and steroid hormones such as estrogen and testosterone, but it does not provide any energy.

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Cholesterol is synthesized in nearly every tissue in the body, but in particularly large quantities by the liver. Approximately 75% of the cholesterol in blood is made in our body, which provides all the cholesterol needed for body functions. Although a dietary source of cholesterol is not required, we consume cholesterol in animal foods, such as meats and dairy products. Indeed, the presence of cholesterol in cell membranes is one distinguishing characteristic between plant and animal cells, thus dietary cholesterol is found only in foods of animal origin. Plants synthesize other types of sterols (and the closely related stanols), but these types are poorly absorbed by the body and can actually interfere with and lower cholesterol absorption. For this reason, some spreads and other food products are fortified with plant sterols or stanols, to help lower cholesterol levels in the body. Chapter 7 further explores the relationship of cholesterol and other dietary fats on blood cholesterol.

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Phospholipids, the fourth most common category of lipids (after fatty acids, triglycerides, and sterols), are also the primary component of cell membranes and the structures that transport lipids in the blood. Because the body can produce phospholipids, they are not considered an essential nutrient. Phospholipids are similar to triglycerides in structure: they have a glycerol backbone, but with two rather than three attached fatty acids. Attached at the third position is a phosphate group and one of several water-soluble “head-groups.” Together the glycerol backbone, the phosphate, and the head group create a water-soluble head on the phospholipid, while the fatty acid “tails” at the other end are soluble in lipids but not in water. (INFOGRAPHIC 6.4)

Question 6.4

This two-sided structural arrangement of phospholipids—one end water-soluble and the other end fat-soluble—allows phospholipids to suspend fat in water. The fat-soluble tails of phospholipids will surround small fat droplets, leaving the water-soluble head of the phospholipid facing outward toward the water. Placing this water-friendly coating around the lipid droplets allows them to remain stably dispersed (mixed) in the watery environment. An example of a phospholipid is lecithin. In addition to being the most abundant phospholipid in our body, lecithin is also frequently added to food products like salad dressings as an emulsifier because of its ability to keep water and lipids from separating.

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Dyerberg and Bang suspected that there might be something about the mix of fatty acids in the high-fat Inuit diet that protected them from heart disease. Back in their lab in Denmark, the scientists examined blood samples more closely, using an instrument that separated fatty acids by their boiling point. Short, saturated fatty acids appeared through the machine first, followed by longer fatty acids, and, finally, long, unsaturated fatty acids. Immediately, the researchers noticed remarkable differences between the pattern of fats in samples from the Inuit and from people living in Denmark. In particular, the readout from Inuit adults contained a huge spike toward the end, showing their blood contained large amounts of an unknown fatty acid that was relatively long and unsaturated.

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The researchers scratched their heads. What was this mysterious fatty acid, and how was it protecting the Inuit from the number one cause of death in other parts of the world?

Even before Dyerberg and Bang began collecting blood samples from native Greenlanders, they suspected the then-popular belief that all fats were equally bad for health could not be entirely true. They were beginning to understand the important roles that different fats play in the body.