•Lipids
Chapter 4

•Lipids

•Low-fat diets do not always mean weight loss

•“Good” (unsaturated)

–Eat in moderation

•“Bad” (saturated fat and trans fats)

–Limited

•3 classes of lipids

–Triglycerides (fats and oils)

–Phospholipids (e.g., lecithin)

–Sterols (e.g., cholesterol)

–Triglycerides

•Approximately 98% of the fat in foods

•Made of the same elements as carbohydrates

–Carbon, hydrogen, and oxygen

–Proportionately more carbon and hydrogen atoms to oxygen atoms, triglycerides yield more calories per gram than carbohydrates

•Triglycerides (cont’d)

•Fatty acids

–Basically chains of carbon atoms with hydrogen atoms attached

–Vary in the length of their carbon chain and in the degree of unsaturation

–Carbon chain length

oAlmost all fatty acids have an even number of carbon atoms in their chain

oLong-chain fatty acids

oMedium- and short-chain fatty acids

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation

oWhen all the carbon atoms in a fatty acid have four single bonds each, the fatty acid is said to be “saturated” with hydrogen atoms

oAn “unsaturated” fatty acid does not have all the hydrogen atoms it can potentially hold; therefore, one or more double bonds form between carbon atoms in the chain

Monounsaturated

Polyunsaturated

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oFatty acids can attach to glycerol molecules

oTypes and proportions of fatty acids present influence the sensory and functional properties of the food fat

oAll food fats contain a mixture of saturated, monounsaturated, and polyunsaturated fatty acids

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oSaturated fats

Tend to be solid at room temperature

Animal fats provide approximately 60% of fat in American diet

The only vegetable fats that are saturated are palm oil, palm kernel oil, and coconut oil

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oSaturated fats (cont’d)

Commonly known as a “bad” fat because it raises blood cholesterol levels

Raises total and LDL cholesterol levels

As levels rise, risk of coronary heart disease rises

May also make the inner lining of arteries more prone to inflammation and thebuildup of fatty plaques by interfering with the normal protective action of HDL-cholesterol

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oUnsaturated fats

Liquid at room temperature

  • Monounsaturated fats are the predominant fat in olives, olive oil, canola oil, peanut oil, avocado, cashews, almonds, and most other nuts
  • Polyunsaturated fats are the predominant fat in corn, soybean, safflower, and cottonseeds oils, and also in fish
  • Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oUnsaturated fats (cont’d)

Commonly known as “good fats” because when they are eaten in place of saturated fats they lower LDL-cholesterol and raise HDL-cholesterol

Position of the double bond

  • Part of an unsaturated fatty acid’s identity is determined by the location of the first or only double bond along the carbon chain
  • Location of first double bond significant
  • Determines essentiality of fatty acid

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oUnsaturated fats (cont’d)

Linoleic acid

  • The essential n-6 fatty acid
  • Especially abundant in plant oils

Alpha-linolenic acid

  • The essential n-3 fatty acid
  • Found in flaxseed, canola, soybean, and walnut oils; and in nuts, especially walnuts

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oUnsaturated fats (cont’d)

Alpha-linolenic acid (cont’d)

  • Can convert alpha-linolenicacid to eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in the body
  • Commonly referred to as “fish oils”

Stability of fats

  • Degree of unsaturation influences the stability of fats
  • Polyunsaturated fats are most susceptible to rancidity

–Food manufacturers may add antioxidants or hydrogenate to prolong shelf life

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oUnsaturated fats (cont’d)

Hydrogenation and trans fats

  • Hydrogenation is a process that adds hydrogen atoms to heart-healthy polyunsaturated oils to saturate some of the double bonds so that the resulting product is less susceptible to rancidity and has improved function

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oUnsaturated fats (cont’d)

Hydrogenation and trans fats (cont’d)

  • 1970s
  • Process of hydrogenation changes the placement of the hydrogen atoms
  • Most trans fat in a typical American diet comes from partially hydrogenated oils and foods made with partially hydrogenated oils

•Triglycerides (cont’d)

•Fatty acids (cont’d)

–Degree of saturation (cont’d)

oUnsaturated fats (cont’d)

Hydrogenation and trans fats (cont’d)

  • Trans fats, like saturated fat, raise LDL-cholesterol and lower HDL-cholesterol
  • Ounce for ounce, trans fats are more unhealthy than saturated fats
  • FDA labeling must include trans fats
  • Trans fats being eliminated from commercial use

•Triglycerides (cont’d)

•Functions of triglycerides in the body

–Primary function of fat is to fuel the body

–Other important functions in the body:

oInsulate and cushion internal organs

oHelp to regulate body temperature

oFacilitate the absorption of the fat-soluble vitamins A, D, E, and K when consumed at the same meal

•Triglycerides (cont’d)

•Functions of triglycerides in the body (cont’d)

–Essential fatty acids have specific functions in the body

–Much interest in fish oils

oResults of research inconclusive

•Other Lipids

  • Phospholipids

–Phospholipids have a glycerol backbone with fatty acids attached like triglycerides

oDifference is phosphate group replaces one of the fatty acids

–Both fat-soluble (because of the fatty acids) and water-soluble (because of the phosphate group)

oEnables them to act as emulsifiers

–Structural components of cell membranes that facilitate the transport of fat-soluble substances across cell membranes

•Other Lipids (cont’d)

•Phospholipids (cont’d)

–Precursors to prostaglandins

–Lecithin is the best-known phospholipid

oSupplements unnecessary

oNot an essential nutrient

•Other Lipids (cont’d)

  • Cholesterol

–A sterol

oWaxy substance whose carbon, hydrogen, and oxygen molecules are arranged in a ring

–Occurs in the tissues of all animals

oFound in all cell membranes and in myelin

–Found exclusively in animals

oLiver and egg yolks are the richest sources

•Other Lipids (cont’d)

•Cholesterol (cont’d)

–“Good” and “bad” cholesterol refer to the lipoprotein packages that move cholesterol through the blood

–Not an essential nutrient

–Dietary cholesterol increases total and LDL-cholesterol

•How the Body Handles Fat

•Digestion

–Minimal amount of chemical digestion of fat occurs in the mouth and stomach through the action of lingual lipase and gastric lipases respectively

–Duodenum

oStimulates the release of the hormone cholecystokinin

Stimulates the gallbladder to release bile

  • Prepares fat for digestion

•How the Body Handles Fat (cont’d)

•Digestion (cont’d)

–Most fat digestion occurs in the small intestine

–End products of digestion are absorbed into intestinal cells

–Small amount is excreted in feces

–Digestion of phospholipids is similar

–Cholesterol does not undergo digestion; it is absorbed as is

•How the Body Handles Fat (cont’d)

•Absorption

–About 95% of consumed fat is absorbed

–Small fat particles are absorbed directly through the mucosal cells into capillaries

–Absorption of larger fat particles, namely monoglycerides and long-chain fatty acids, is more complex

oMicelles

oRecombine into triglycerides

oChylomicrons distribute dietary lipids throughout the body

•How the Body Handles Fat (cont’d)

•Fat catabolism

–Catabolism of fatty acids increases when:

oCarbohydrate intake is inadequate (e.g., while on a very low-calorie diet)

oOr catabolism of fatty acids is unavailable (e.g., in the case of uncontrolled diabetes)

–Fatty acids cannot be reassembled to make glucose

–Inefficient choice of fuel for glucose-dependent brain cells, nerve cells, and red blood cells

–How the Body Handles Fat (cont’d)

•Fat anabolism

–Most newly absorbed fatty acids recombine with glycerol to form triglycerides that end up stored in adipose tissue

–Fat stored in adipose tissue represents the body’s largest and most efficient energy reserve

–Adipose cells have a virtually limitless capacity to store fat

–Fat reserves

o1 pound of fat provides 3,500 calories

•Fat in Foods

•Vital functions of fat

–Imparts its own flavor, from the mild taste of canola oil and corn oil to the distinctive tastes of peanut oil and olive oil

–Transfers heat to rapidly cook food, as in the case of frying

–Absorbs flavors and aromas of ingredients to improve overall taste

–Adds juiciness to meats

•Fat in Foods (cont’d)

•Vital functions of fat (cont’d)

–Creates a creamy and smooth “mouth feel” in items such as ice cream, desserts, and cream soups

–Adds texture or body to many foods, such as flakiness, tenderness, elasticity, and viscosity; for example, milk is watery and cheese is rubbery when fat is removed

–Imparts tenderness and moisture in baked goods, such as cookies, pies, and cakes, and delays staling

–Is insoluble in water, thus providing a unique flavor and texture to foods such as salad dressings

•Fat in Foods (cont’d)

•Fat content in MyPyramid varies greatly among groups and between selections within each group

–Generally, three MyPyramid food groups provide little or no fat

oGrains naturally contain very little fat

oUnadulterated vegetables contain little or no fat

oFruits, with the exception of avocado, coconut, and olives, are naturally fat-free

•Fat in Foods (cont’d)

•Milk

–Classified into one of 3 subgroups: full fat, reduced fat, and fat free

oFat in dairy products is predominately saturated and full-fat products have more cholesterol than in the lower-fat options

oFat-free varieties of milk, yogurt, and cheese provide virtually no fat

oThe midrange is reserved for reduced-fat items—they have some of the fat and cholesterol removed yet retain some of the “mouth feel” characteristic of whole milk

•Fat in Foods (cont’d)

•Meat and beans

–4 subgroups based on fat content

–Details worth noting:

oThe 1-oz. size cited in MyPyramid is simply a reference, not a serving size or a portion size; typically, a serving size (amount recommended for a meal) is 3 to 4 oz., and a portion size (amount actually eaten at one time) may be much larger

oFat added during cooking, such as frying or basting with fat, increases the overall fat content and counts as choices from the Oils group; it is recommended that meats be prepared by methods that do not add fat, such as baking, roasting, broiling, grilling, poaching, or boiling

•Fat in Foods (cont’d)

•Meats and fat content

–Untrimmed meats are higher in fat than lean-only portions

–“Red meats,” namely beef, pork, and lamb, are higher in saturated fat than the “white meats” of poultry and seafood

–White poultry meat is lower in fat than dark meat; removing poultry skin removes significant fat

–Fat content varies among different cuts of meat. The leanest cuts are beef loin and round; veal and lamb from the loin or leg; and pork tenderloin or center loin chop.

•Fat in Foods (cont’d)

•Meats and fat content (cont’d):

–Beef grades can be used as a guide to fat content because grades are based largely on the amount of marbling

–Shellfish are very low in fat but have considerable cholesterol

–Most wild game is very lean; the fat content in bison, venison, elk, ostrich, pheasant (without skin), rabbit, and squirrel ranges from 2 to 5 g per 3-oz. serving

–Processed meats, such as sausage and hot dogs, may provide more fat calories than protein calories

•Fat in Foods (cont’d)

•Nuts and fat content:

–Nuts have many healthy attributes; they contain plant protein, fiber, vitamin E, selenium, magnesium, zinc, phosphorus, and potassium, in a low–saturated-fat, cholesterol-free package

oNuts’ high-fat content of 13 to 20 g/oz. comes mostly from monounsaturated fats and polyunsaturated fats; walnuts are a rich source of alpha-linolenic acid

•Fat in Foods (cont’d)

•Cholesterol in foods

–Egg yolks have approximately 213 mg of cholesterol. The cholesterol content of typical cuts of beef, pork, lamb, and poultry is generally around 70 mg/3 oz. Veal averages slightly more at about 90 mg/3 oz. The exceptions are organ meats, which are very high in cholesterol. Egg whites, dried peas and beans, are nuts are cholesterol-free.

•Fat in Foods (cont’d)

•Oils

–Allowances are small

–Some items on the oils list actually are included in other groups also

•Dietary Reference Intakes

•The issue of how much of each particular type of fat is needed, how much is optimal, and how much is too much is complex and in some cases controversial

•Total fat

–Neither an AI or RDA is set for total fat due to insufficient data

–An acceptable macronutrient distribution range (AMDR) is estimated to be 20% to 35% of total calories for adults

•Dietary Reference Intakes (cont’d)

•Saturated and trans fat

–Additional macronutrient recommendation issued for both of these fats is that intake should be as low as possible within the context of a nutritionally adequate diet

–Neither of these fats needs to be consumed in the diet

•Dietary Reference Intakes (cont’d)

•Essential fatty acids

–Both an acceptable macronutrient distribution range and adequate intakes have been set for the essential fatty acids linoleic acid and alpha-linolenic acid

–AI should not be confused with recommended dietary allowance

–Essential fatty acid deficiency

oBody cannot make essential fatty acids

oSymptoms of essential fatty acid deficiency include growth failure, reproductive failure, scaly dermatitis, and kidney and liver disorders

•Dietary Reference Intakes (cont’d)

•Cholesterol

–The additional macronutrient recommendation issued for cholesterol is that intake be as low as possible

–Fat in Health Promotion

•Message to eat less fat was too simplistic

•Type of fat in the diet can affect serum lipids more than the total amount of fat

•Eat less saturated fat and keep trans fat consumption as low as possible

–To cut saturated fat intake, experts recommend:

oLimiting meat intake, especially red meat, and choosing lean varieties

oReduced- or no-fat dairy products

oHydrogenated fats should be avoided

•Fat in Health Promotion (cont’d)

•Limit total fat and go for unsaturated fats

–Most effective way to limit total and saturated fat and increase unsaturated fats may be with a plant-based diet

•Limit cholesterol

–Cholesterol intake becomes less important when saturated fat intake is low

•What about “fish oils?”

–American Heart Association suggests healthy people eat at least 2 servings of fish per week

–Fish oil pills are an alternative source of omega-3 fats