Lipids

Chapter

5

Photo © PhotoDisc

Lipids

• Lipids are organic molecules that dissolve easily in organic solvents such as alcohol, ether, or acetone.

• Lipids are much less soluble in water.

Lipids

• Lipids are generally hydrophobic and lipophillic.

• Lipid vary greatly in their level of solubility.

Triglycerides

• Triglycerides are the largest category of lipids.

• Fat cells store triglycerides in adipose tissue.

Triglycerides

• In foods, we call triglycerides fats and oils.

• Fats are usually solid and oils liquid at room temperature.

Fatty Acids

• Fatty acids are composed of both triglycerides and phospholipids.

• They are often attached to cholesterol.

Fatty Acids

• Fatty acids determine essential characteristics of a fat such as whether it is liquid or solid at room temperature.

• Fatty acids that are not attached to other compounds are called “free” fatty acids.

• Some free fatty acids have distinct flavors.

Fatty Acids Are Key Building Blocks

• Chain Length• 4–24

carbons

Chain Length

• Foods contain fatty acids with chain length between 4 and 24 carbons.

• The shorter the chain, the more liquid the fatty acid.

• Shorter fatty acids are more water soluble.

Fatty Acids Are Key Building Blocks

• Degree of saturation

• Saturated

• All single bonds between carbons

• Monounsaturated

• One carbon-carbon double bond

• Polyunsaturated

• More than one carbon-carbon double bond

Degree of Saturation

• Foods with more unsaturated fatty acids typically have lower melting points are tend to be liquid at room temperature.

• Foods with more saturated fatty acids typically have higher melting points and tend to be solid at room temperature.

Liquid or Solid?

Fatty Acids Are Key Building Blocks

• Types of fatty acids• Cis and trans

• Hydrogenation produces trans fatty acids• Trans fatty acids raise LDLs

• Essential fatty acids• Linoleic acid and alpha-linolenic acid• Can’t be made in the body• Used to make eicosanoids

Phospholipids

• About 2 percent of dietary lipids are phospholipids.

• The human body can make those it needs.

Phospholipids

• Phospholipids are unique in that they are soluble in both fat and water.

• They are found in cell membranes.

• They are found in blood and other body fluids where they keep fats suspended in the watery environment.

Phospholipids

• Structure• Glycerol + 2 fatty acids +

phosphate group

• Functions• Component of cell membranes• Lipid transport as part of

lipoproteins• Emulsifiers

• Food sources• Egg yolks, liver, soybeans,

peanuts

Sterols

• A small percentage of dietary lipids are sterols.

• Cholesterol is a primary example.

• Cholesterol is an important component of cell membranes and a precursor to sex hormones, adrenal hormones, vitamin D, and bile salts.

Sterols: Cholesterol

• Functions• Component of cell membranes• Precursor to other substances

• Sterol hormones• Vitamin D• Bile acids

• Synthesis• Made in the liver

• Food sources• Found only in animal foods

Nonessential and Essential Fatty Acids

• The body can make saturated and Omega-9 Fatty Acids, so it is not essential to get them from the diet.

• The body cannot manufacture omega-6 linoleic and omega-3 alpha-linolenic acids. They must come from food and are called essential.

Triglycerides

• Triglycerides are the major lipids in both the diet and in the body.

• They add flavor and texture to foods.

• They are an important source of energy for the body.

Triglycerides

• Structure• Glycerol + 3 fatty acids

• Functions• Energy source and reserve• Insulation and protection• Carrier of fat-soluble

vitamins• Sensory qualities in food

Triglycerides in Food

• Sources of omega-3 fatty acids• Soybean, canola, walnut, flaxseed oils• Salmon, tuna, mackerel

• Sources of omega-6 fatty acids• Vegetable oils• Nuts and seeds

Photos © Photodisc

Energy Reserve

• We store excess dietary fat as body fat.

• Fat is stored inside fat cells called adipocytes.

• Adipocytes form body fat tissue called adipose tissue.

Fat Storage

Digestion and Absorption

• Mouth and stomach• Minimal digestion of triglycerides

• Small intestine• Emulsified by phospholipids• Digested by pancreatic lipase• Absorbed into intestinal cells

• Formed into chylomicrons and moved into lymphatic system

Fat Digestion

Lipids in the Body

• Lipoproteins carry lipids around the body• Chylomicrons

• Delivers dietary lipids from intestines to cells and liver

Lipids in the Body• Very low density lipoproteins

(VLDL)• Deliver triglycerides to cells

• Low-density lipoproteins (LDL)• Deliver cholesterol to cells

• High-density lipoproteins (HDL)• Pick up cholesterol for removal or

recycling

Lipids in the Diet• Recommended intake

• Reduce saturated and trans fat intake

• Total fat: 20–35% of calories• Need ~ 2% of calories as

essential fatty acids• Improve balance of

omega-3: omega-6 fatty acids

Photos © Photodisc

Lipids in the Diet• Fat substitutes

• Different types of composition

• Olestra• Sucrose + fatty

acids• Indigestible –

provides zero kcals• Reduces absorption

of fat-soluble vitamins

Lipids and Health

• Obesity• High-fat diets promote weight gain

• Heart disease• High saturated and trans fat intake

raises LDL cholesterol

• Cancer

AHA Position on Omega-3 Fatty Acids

• The American Heart Association recommends eating fish (particularly fatty fish) at least two times a week. Fish is a good source of protein and doesn’t have high saturated fat that fatty meat products do.

• Fatty fish such as mackerel, lake trout, herring, sardines, albacore tuna, and salmon are high in two kinds of of omega-3 fatty acids: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).

Function of Triglycerides

• Fat performs a number of essential functions in the body.

• Fat is also a major energy source. When at rest, muscles prefer to use fat for fuel.

A Healthy Balance

• An appropriate balance of calories from fat and carbohydrates can have multiple health benefits aside from controlling weight.

• These include cholesterol levels, vitamin intake, etc.

Fat = Storage• Evolution has selected fat, rather than

glycogen, as its primary energy storage medium.

• A gram of fat stores more than six times as much energy as a gram of glycogen.

Micronutrient Carrier• Fat is a

micronutrient carrier.

• Fat holds more than just energy.

• It also carries important nutrients, such as fat-soluble vitamins and carotenoids.

Recommended Fat Intake

• The Dietary Guidelines for Americans recommends a fat intake of 20 to 35 percent of total calories.

• Saturated fat should supply no more than 10 percent of our total calories, or about one-third of our fat calories.

ADA Position on Fat Replacers

• It is the position of the American Dietetic Association that the majority of fat replacers, when used in moderation by adults, can be safe and useful adjuncts to lowering the fat content of foods and may play a role in decreasing total dietary energy and fat intake.

• Moderate use of low-calorie, reduced-fat foods, combined with low total energy intake, could potentially promote dietary intake consistent with the objectives of Healthy People 2010 and the 2005 Dietary Guidelines for Americans.

The Women’s Health Initiative

• This study sought to explore whether a dietary intervention that reduces intake of total fat and increases intakes of fruits, vegetables, and grains will reduce the risk of cardiovascular disease.

• The results found that the intervention had no effect on the risk of coronary heart disease, stroke, or overall cardiovascular disease.

• Since this study was initiated, scientific thinking has evolved to focus on modifying intakes of specific types of fat.

NCEP Tips for Healthful Eating Out

• Choose restaurants that have low-fat, low-cholesterol menu items.

• Don’t be afraid to ask for foods that follow your eating pattern.

• Select poultry, fish, or meat that is broiled, grilled, baked, steamed, or poached rather than fried.

Diet and Cancer

• The evidence linking dietary fat to cancer is inconclusive. The case looks strong when we compare cancer rates between countries: Overall cancer rates are generally higher in countries with high fat intake, and lower in countries where people eat less fat. But in population studies within those countries, the evidence linking fat to cancer is weaker.