Fats and oils
Dietary fat is a macronutrient that has historically engendered considerable controversy and continues to do so. Contentious areas include optimal amount and type for cardiovascular disease risk reduction, and role in body weight regulation.
Dietary Fats and Oils – The Good, Bad, and Ugly
Dietary fats and oils are unique in modern times in that they have good, bad, and ugly connotations. The aspects of dietary fat that are classified as good include serving as a carrier of soluble vitamins (vitamins A, D, E, and K), enhancing the bioavailability of fat-soluble bioactive substances (e.g., absorption of fat-soluble micronutrients), providing essential substrate for the synthesis of metabolically active compounds (e.g., essential fatty acids for eicosanoid synthesis), providing critical structural components (e.g., cell membranes and lipoprotein particles), preventing carbohydrate-induced hypertriglyceridemia, and serving an energy-dense form of reserve metabolic fuel (triglyceride). The aspects of dietary fat that are classified as bad include serving as a reservoir for fat-soluble toxic compounds. The aspects of dietary fat that are classified as ugly include providing a concentrated form of metabolic fuel in times of excess and contributing saturated and trans fatty acids that promote atherosclerotic plaque formation, the underlying cause of heart disease, stroke, and phlebitis.
Lipids – In Food and in the Body
Fatty acids are the basic components of larger lipid compounds or serve as substrates for bioactive molecules. They are composed of an acyl (hydrocarbon) chain with a methyl and a carboxyl group at either end. The majority of fatty acids have an even number of carbons. The range of chain lengths for common fatty acids is broad, 12–22 carbons, although shorter- and longer-chain fatty acids occur naturally. The predominant fatty acids, in the human body and food, are depicted in Table 1. In addition to chain length, fatty acids differ from each other with regard to the number, type, and location of double bonds. Fatty acids with no double bonds are referred to as saturated, with one double bond as monounsaturated, and with two or more double bonds as polyunsaturated (Figure 1).
The double bonds within unsaturated fatty acids can be in either the cis (hydrogen atoms on same side of the acyl chain) or trans (hydrogen atoms on opposite sides of the acyl chain) conformation. The presence of a cis relative to a trans double bond results in a greater bend or kink in the hydrocarbon chain. This kink impedes the fatty acids from aligning (packing together). In a cell membrane, this results in increased fluidity; in food, it results in oils that are liquid or fats that are soft at room temperature. The vast majority of fatty acids occur in the cis conformation. Two fatty acids with the same number of carbons and double bonds, and position of double bonds, but with at least one double bond differing in conformation, are referred to as geometric isomers (e.g., oleic acid (18:1cis) and elaidic acid (18:1trans)) (Figure 2).
Fatty acids also vary with regard to the location of the double bonds within the acyl chain. Fatty acids with the same number of carbons and double bonds, and conformation of the double bonds, but having different double bond locations within the acyl chain, are referred to as positional isomers (e.g., oleic acid (18:1cis) and elaidic acid (18:1trans)) (Figure 2). The most common positional isomers differ in the location of the double bonds from the methyl end of the acyl chain. Fatty acids in which the first double bond occurs at the third or the sixth carbon are termed o-3 (n-3) or o-6 (n-6) fatty acids, respectively (e.g., a-linolenic acid (18:3n-3) and g-linolenic acid (18:3n-6)).
Most double bonds occur in a nonconjugated sequence, that is, a single carbon atom with single carbon–carbon bonds separates the carbons making up the double bonds. Some double bonds occur in the conjugated form, without an intervening carbon separating the double bonds. Conjugated double bonds tend to be more reactive chemically (e.g., more likely to become oxidized). Enzymes that metabolize fatty acids distinguish among both geometric and positional isomers. The metabolic products of different fatty acid isomers, especially positional isomers, have different and at times opposing biological effects.
Some fatty acids are classified as essential. An essential nutrient is a nutrient that the body cannot synthesize or cannot synthesize in amounts adequate to meet requirements. Linoleic acid (18:2) and fatty acids that can be derived from linoleic acid (Figure 3), such as arachidonic acid (20:4), are classified as essential fatty acids. Their essentiality is due to an inability of humans, in contrast to plants, to introduce a double bond after the ninth carbon in a fatty acyl chain (from the carboxyl end).
Triacylglycerol is the major form of dietary lipid in fats and oils, whether derived from plants or animals. Triacylglycerol is composed of three fatty acids esterified to a glycerol molecule (Figure 4). The physical properties of the triacylglycerol are determined by the specific fatty acids esterified to the glycerol moiety and the actual position the fatty acids occupy. Each of the three carbons comprising the glycerol molecule allows for a stereochemically distinct fatty acid bond position: sn-1, sn-2, and sn-3. A triacylglycerol with three identical fatty acids is termed a simple triacylglycerol. These are exceedingly rare in nature. A triacylglycerol with two or three different fatty acids is termed a mixed triacylglycerol and makes up the bulk of the fat. The melting point of a triacylglycerol is determined by the physical characteristics and position of the fatty acids esterified to glycerol – their chain length; number, position, and conformation of the double bonds; and the stereochemical position.
Approximately 90% of the molecular weight of triacylglycerol is accounted for by the fatty acids. The fatty acid profile of the diet is reflected, in part, in the fatty acid profile of the adipose tissue triacylglycerol, particularly for essential fatty acids. Such data have been used to approximate long-term food intake patterns of humans.
Mono- and diglycerides have one and two fatty acids, respectively, esterified to glycerol. In nature, they occur only in trace amounts. They are primarily intermediate products of triacylglycerol digestion, clearance from the bloodstream, or intracellular metabolism. They are used as emulsifiers in processed food.
Once consumed, triacylglycerol is hydrolyzed into free fatty acids and monoglycerides in the small intestine prior to absorption. Once these compounds enter the intestinal cell, they are used to resynthesize triacylglycerol. This lipid is then incorporated into nascent triglyceride-rich lipoprotein particles, termed chylomicrons, for subsequent introduction into peripheral circulation. Chylomicrons are secreted directly into the lymph before entering the blood stream. Once in circulation, triacylglycerol is hydrolyzed before crossing the plasma membrane of peripheral cells. The primary enzyme that hydrolyzes triacylglycerol in plasma is lipoprotein lipase. Lipoprotein lipase hydrolyzes triacylglycerol into free fatty acids and 2-monoacylglycerol. The enzyme is attached to the luminal surface of capillary endothelial cells via a highly charged membrane-bound chain of heparin sulfate-proteoglycans. The ability of lipoprotein lipase to bind both the chylomicron particle and the cell surface ensures the cellular uptake of free fatty acids that are generated from the hydrolysis. Once inside the cell, free fatty acids can be oxidized to provide energy, metabolized to biologically active compounds, or resynthesized into triacylglycerol for storage as a potential reservoir for fatty acids for subsequent use.
There are only trace amounts of phospholipid in dietary fats and oils. However, because the fatty acids in fats and oils provide substrate for the synthesis of phospholipid in the body, this subtype of fat is important to discuss. Phospholipid is a critical component of all cells, both plant and animal. It is composed of two fatty acids esterified to the sn-1 and sn-2 positions and a moiety frequently referred to as a polar head group to the sn-3 position of glycerol, the latter group via a phosphate bond (Figure 5). Phospholipid molecules are amphipathic, that is, there are both hydrophobic and hydrophilic domains in the molecule. The two fatty acids confer hydrophobic properties and the polar head group confers hydrophilic properties. The specific fatty acids esterified to the glycerol backbone tend to be unsaturated fatty acids. The different polar head groups, most commonly phosphorylcholine, phosphorylserine, phosphorylinositol, and phosphorylethanolamine, result in phospholipids that vary in size and charge. Because of their amphipathic nature, phospholipids serve as the major structural component of cellular membranes and, in so doing, also serve as a reservoir for metabolically active unsaturated fatty acids. In the small intestine, because of their amphipathic properties, they play an important role in facilitating the emulsification and absorption of fat and fat-soluble vitamins. On the surface of lipoprotein particles, they provide a critical component in the packaging and transport of lipid in circulation. In cells, they form biolayers that serve as the plasma membrane and intracellular membranes.
Dietary sources of cholesterol are limited to foods of animal origin. Cholesterol is an amphipathic molecule that is composed of a steroid nucleus and a branched hydrocarbon tail. Cholesterol occurs naturally in two forms: nonesterified (free cholesterol) and esterified (cholesteryl ester). If esterified, the fatty acid is linked to cholesterol at the number 3 carbon of the sterol ring.
Free cholesterol is a component of cell membranes and, along with membrane phospholipid fatty acid, determines membrane fluidity. Cholesterol intercalates into the phospholipid bilayer restricting motility of the fatty acyl chains and hence decreases fluidity. Free cholesterol is critical for normal nerve transmission. It makes up approximately 10% (dry weight) of total brain lipids. Cholesterol is a precursor of steroid hormones (e.g., estrogen, testosterone), vitamin D, adrenal steroids (e.g., hydrocortisone, aldosterone), and bile acids. This latter property is exploited in certain approaches to decrease plasma cholesterol concentrations by preventing the resorption of bile acids (recycling), hence forcing the liver to use additional cholesterol for bile acid synthesis and in so doing, creating an alternate mechanism for cholesterol net excretion.
The receptor-mediated cellular uptake of cholesterol from plasma lipoprotein particles is critical in maintaining intracellular and whole-body cholesterol homeostasis. Once internalized, intracellular free cholesterol can have three metabolic effects. It inhibits the activity of 3-hydroxy 3- methylglutaryl CoA (HMGCoA) reductase, the rate-limiting enzyme in endogenous cholesterol biosynthesis. This property serves to decrease the intracellular rate of cholesterol biosynthesis commensurate with the uptake of cholesterol from extracellular sources (plasma lipoproteins), thereby minimizing intracellular accumulation. Intracellular free cholesterol inhibits the synthesis of receptors that take up lipoproteins containing apoproteins B100 or E from the plasma, thereby limiting the amount of additional cholesterol taken up by the cell. Intracellular free cholesterol also increases the activity of acyl CoA cholesterol acyltransferase (ACAT), the intracellular enzyme that converts free cholesterol to cholesteryl ester. A high level of intracellular free cholesterol is cytotoxic, whereas cholesteryl ester is a highly nonpolar molecule and coalesces to form lipid droplets within the cell, preventing interaction with intracellular components and subsequent detrimental effects.
Free cholesterol can be esterified intracellularly, as indicated, by ACAT. ACAT uses primarily oleoyl CoA as substrate, resulting primarily in cholesteryl oleate. Free cholesterol can also be esterified in plasma by lecithin cholesterol acyltransferase (LCAT). LCAT uses phosphotidylcholine as a substrate, resulting primarily in the products cholesteryl linoleate and lysolecithin. Cholesteryl ester is less polar than free cholesterol and this difference dictates how the two forms of cholesterol are handled, as mentioned above, intracellularly, and also as noted below, in the blood stream.
Approximately one-third of cholesterol in plasma circulates as free cholesterol and approximately two-thirds as cholesteryl ester. Cholesterol in circulation is carried on all subclasses of lipoprotein particles: both intestinally derived chylomicrons and hepatically derived very low density lipoprotein, intermediate- density lipoprotein, low-density lipoprotein (LDL), and high-density lipoprotein (HDL). Free cholesterol is incorporated into the phospholipid monolayer surface of lipoprotein particles, whereas cholesteryl ester is incorporated into the core of the lipoprotein particle. The majority of the cholesterol in circulation is carried on LDL particles. Cholesteryl ester is the major component of atherosclerotic plaque. In the arterial wall, cholesteryl ester is either derived from the infiltration of lipoprotein-associated cholesteryl ester resulting from LCAT activity or synthesized in situ as a result of ACAT activity, depending on the mode of entry. The fatty acid profile of the cholesteryl ester in arterial plaque can provide some hint as to its source.
Historically, dietary cholesterol has been associated with increased cardiovascular disease risk. However, within the range currently consumed and on the basis of more recent data indicating that dietary fat type has a greater effect on cardiovascular disease risk indictors than dietary cholesterol, the emphasis has shifted.
Fats and oils derived from plants contain a wide range of phytosterols, compounds structurally similar to cholesterol. The difference between phytosterols and cholesterol is related to their side-chain configuration and steroid ring double bonds. The most common dietary phytosterols are b-sitosterol, campesterol, and stigmasterol (Figure 7). In contrast to cholesterol, phytosterols are absorbed only in trace amounts. For this reason, plant sterols have been used therapeutically to reduce plasma cholesterol concentrations. They compete with cholesterol for absorption, hence effectively reduce cholesterol absorption efficiency.
Dietary Fats and Oils
Fatty Acid Profile of Common Dietary Fats
Dietary fats and oils come from both the animal and plant sources, primarily in the form of triacylglycerol. The fatty acid profile of commonly consumed dietary fats varies considerably. In general, fats of animal origin tend to be relatively high in saturated fatty acids, contain cholesterol, and are solid at room temperature. Oils of plant origin tend to be relatively high in unsaturated fatty acids (monounsaturated and polyunsaturated) and are liquid at room temperature. Notable exceptions include plant oils, termed tropical oils (e.g., palm, palm kernel, coconut oils), and partially hydrogenated fat. Tropical oils are high in saturated fatty acids but remain liquid at room temperature because they contain a high proportion of short-chain fatty acids. Partially hydrogenated plant oils are relatively high in trans fatty acids due to chemical changes induced during processing.
Major Contributors of Dietary Saturated, Monounsaturated, and Polyunsaturated Fatty Acids and Cholesterol
The major types of dietary fats and oils are generally broken down on the basis of animal and plant sources. The relative balance of animal and plant foods is an important determinant of the fatty acid profile of the diet. However, with the increasing prominence of processed, reformulated, and genetically modified foods, it is becoming difficult to predict the fatty acid profile of the diet on the basis of the animal verses plant distinction.
According to the National Health and Nutrition Examination Survey (NHANES) recall data, the 10 major dietary sources of saturated fatty acids in the US diet are regular cheese, whole milk, regular ice cream, 2% low-fat milk, pizza with meat, French fries, Mexican dishes with meat, regular processed meat, chocolate candy, and mixed dishes with beef (Table 2). Hence, regular dairy products contribute the majority (B16%) of saturated fatty acids in the diet, and the top 10 sources contribute B30% of the saturated fatty acids consumed. The increased prevalence of fat-free and low-fat dairy products provides a viable option with which to encourage a population-wide decrease in saturated fat intake. To put the value of decreasing population-wide intakes of saturated fat into perspective, it has been estimated that the isocaloric replacement of 5% of energy from saturated fatty acids with complex carbohydrate, on average, would reduce total cholesterol concentrations by 10 mg dl#1 (0.26 mmol l#1) and LDL cholesterol by 7 mg dl#1 (0.18 mmol l#1). For a person at moderately high risk of developing cardiovascular disease with total cholesterol concentration of 220 mg dl#1 (5.69 mmol l#1) and LDL cholesterol concentration of 140 mg dl#1 (3.62 mmol l#1), such a dietary modification would decrease total and LDL cholesterol concentrations by 4.5% and 5%, respectively. Each 1% decrease in total cholesterol concentrations has been associated with a 2% reduction in the incidence of coronary heart disease. Using this example, such a difference would theoretically translate into a 9% decrease in cardiovascular disease risk. However, it is important to note that decreasing the saturated fatty acid content of the diet should not necessarily be done by displacing fat with carbohydrate. As will be discussed in the next section, the quantity of dietary fat, relative to carbohydrate and protein, also impacts on blood lipid concentrations and lipoprotein profiles. Current data suggest that displacing saturated fatty acids with polyunsaturated fatty acids would result in the greatest decrease in cardiovascular disease risk.
The 10 major dietary sources of monounsaturated fatty acids in the US diet are French fries, regular processed meat, regular cookies, regular miscellaneous snacks, pizza with meat, regular salad dressing, regular cheese, Mexican dishes with meat, sausage, and mixed dishes with beef (Table 2).
The 10 major dietary sources of n-6 polyunsaturated fatty acids in the US diet are regular salad dressing, regular white bread, regular mayonnaise, French fries, regular cake, regular cookies, mixed dishes with chicken and turkey, regular miscellaneous snacks, regular potato chips, and fried fish (Table 2). The distribution of polyunsaturated fatty acids among commonly consumed foods is wide.
The 10 major dietary sources of cholesterol in the US diet are fried eggs, regular eggs including scrambled eggs, mixed dishes with eggs, mixed dishes with beef, whole milk, regular cheese, fried fish, mixed dishes with chicken and turkey, lean cut meat, and regular processed meat (Table 2). Eggs or foods high in eggs contribute B30% of the cholesterol intake.
Dietary Fat and Cardiovascular Disease Prevention
Quantity of Dietary Fat
When considering the percentage of energy contributed by dietary fats and oils (amount of fat) and cardiovascular disease prevention and management, there are two major factors to consider: impact on body weight and plasma lipoprotein profiles. The potential relationship with body weight is important because overweight and obesity are strongly associated with elevated lipid and lipoprotein concentrations, blood pressure, dyslipidemia, and type 2 diabetes. These factors are all associated with increased cardiovascular disease risk. With respect to plasma lipoprotein profiles, the focus is usually on triglyceride and HDL cholesterol concentrations or total cholesterol/HDL cholesterol ratios.
When body weight is maintained at a constant level, decreasing the total fat content of the diet, expressed as a percentage of total energy, and replacing it with carbohydrate frequently results in an increase in triglyceride concentrations, decrease in HDL cholesterol concentrations, and less favorable (higher) total cholesterol/HDL cholesterol ratios. Low HDL cholesterol concentrations are an independent risk factor for cardiovascular disease. Very low fat diets are of particular concern to individuals with glucose intolerance and excess body weight who have a predisposition to low HDL cholesterol and high triglyceride concentrations or those individuals classified as having metabolic syndrome (having three or more of the following: abdominal obesity, elevated triacylglycerol concentrations, low HDL concentrations, hypertension, elevated fasting glucose concentrations). Because of these findings, the Adult Treatment Panel of the National Cholesterol Education Program (NCEP) revised their guidelines in 2001 from recommending a diet with less than 30% of energy as fat to a diet with 25–35% of energy as fat. About that same time, the American Heart Association and the USDA/HHS 2000 Dietary Guidelines for Americans changed their recommendations to shift the emphasis from a general recommendation to limit intakes of total and saturated fat to limit saturated and trans fat. These modifications have been echoed in more recent updates of these guidelines.
With respect to the quantity of dietary fats and oils and body weight, comprehensive reviews of the long-term data have concluded that even a relatively large downward shift in dietary fat intake, approximately 10% of energy, results in only modest weight loss, 1 kg, over a 12-month period in normalweight individuals and 3 kg in overweight or obese individuals. A recent 2-year intervention study has concluded that there is no advantage, with respect to weight loss or CVD risk indicators, of diets with different proportions of fat, carbohydrate, and protein. The major determinant of successful weight loss was adherence to the protocol, including attendance to group meetings.
Quality of Dietary Fat
Early evidence demonstrated that diets relatively high in saturated fatty acids increase plasma total cholesterol concentrations. Subsequent work demonstrated that this elevation in total cholesterol concentrations is contributed to by increases in both LDL and HDL cholesterol concentrations, the former more so than the latter. More recent work has indicated that the effect of saturated fatty acids on plasma lipoprotein concentrations and cardiovascular disease risk is modified by the macronutrient balance of energy intake. Displacing saturated fatty acids with unsaturated fatty acids, monounsaturated or polyunsaturated fatty acids, lowers both LDL and HDL cholesterol concentrations, polyunsaturated to a greater extent than monounsaturated. Displacing saturated fatty acids with carbohydrate elevates triglyceride and lowers HDL cholesterol concentrations. Observational data indicate that dietary patterns low in saturated fatty acids and high in polyunsaturated fatty acids are associated with the lowest cardiovascular disease risk.
Quantitatively, a-linolenic acid (ALA, 18:3n-3) is the most abundant n-3 fatty acid in the diet. Two other n-3 polyunsaturated fatty acids, sometimes referred to as the very long chain n-3 fatty acids, eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3), are present in smaller amounts. EPA and DHA intakes are associated with decreased cardiovascular disease risk. The relationship of ALA and cardiovascular disease risk is tenuous. Although humans have the ability to elongate and desaturate ALA to form EPA and subsequently DHA, the capacity is low and current recommendations are to consume preformed EPA and DHA. Predominant dietary sources of ALA include soybean and canola oils (Figure 8). The major source of EPA and DHA is fish, specifically dark flesh fish such as salmon and mackerel. Trans fatty acids occur naturally in meat and dairy products as a result of anaerobic bacterial fermentation in ruminant animals.
Trans fatty acids are formed as a result of partial hydrogenation of vegetable oils. Partial hydrogenation, in addition to converting some cis double bonds into trans double bonds, also saturates some double bonds and causes migration of double bonds along the acyl chain; in sum, this results in multiple geometric and positional isomers. Oils are partially hydrogenated to increase viscosity (change a liquid oil into a semiliquid or solid) and extend shelf life (decrease susceptibility to oxidation). Trans fatty acid intake, regardless of the source, is associated with elevated LDL cholesterol concentrations and cardiovascular disease risk. Major contributors of dietary trans fatty acids are commercially baked products, animal products, traditional margarines and shortenings, and commercially fried foods. Mandatory inclusion of trans fatty acid content on Nutrient Facts Panels and bans on the use of partially hydrogenated fats in cities and towns resulted in secular decreased intake.
Composition of Dietary Fats
Types of fat relatively high in saturated fatty acids include butterfat (62%), beef tallow (50%), tropical oils (coconut 87%, palm kernel 81%, palm oil 49%), and lard (39%) (Figure 8). The content of cholesterol in these fats is 33, 14, 0, and 12 mg tablespoon#1, respectively. Types of fat relatively high in monounsaturated fatty acids include canola oil (56%), olive oil (73%), and peanut oil (46%). Types of fat relatively high in polyunsaturated fatty acids include soybean oil (51%), corn oil (58%), safflower oil (74%), and sunflower oil (66%). Vegetable oils do not naturally contain cholesterol.
There are multiple sources of dietary guidance with respect to fats and oils. In general, current recommendations are to consume a diet moderate in total fat (25–35% of energy) and rich in fruits and vegetables, whole-grain products, low-fat and nonfat dairy products, legumes, fish, and lean meats. Liquid vegetable oils are recommended in place of other types of fats (animal fat, partially hydrogenated fat). Important with any type of dietary guidance, especially when it is intended to shift dietary intakes, is to take into consideration availability, price, and personal preference, including regional, cultural, and religious dietary patterns.
Dietary fats and oils have both positive and negative attributes with respect to health outcomes. This makes determining optimal dietary recommendations difficult. Fats and oils are made up primarily of triacylglycerol. The fatty acid profile of the triacylglycerol dictates the physical properties of the fat. During fatty acid biosynthesis, humans are unable to insert a double bond above the ninth carbon of the acyl chain. For this reason, linoleic acid and fatty acids derived from linoleic acid are classified as essential; hence, these must be consumed preformed. Animal fats are the major contributors of dietary saturated fatty acids. Vegetable oils, such as canola and olive, and animal fats, are the major contributors of dietary monounsaturated fatty acids. Vegetable oils, such as safflower, sunflower, and corn oils, are the major contributors of dietary polyunsaturated fatty acids. Foods of marine origin are major contributors of n-3 fatty acids. Partially hydrogenated fat and, to a less extent, animal fats are major contributors of trans fatty acids. Dietary patterns high in polyunsaturated and low in saturated fatty acids have been associated with optimal health outcomes. Very long chain n-3 fatty acids have been independently associated with reduced risk of developing cardiovascular disease. Trans fatty acids have been associated with elevated cardiovascular disease risk. Dietary fatty acid intakes are determined by the sum of individual food choices. Current general dietary recommendations from major health advocacy organization recommend moderate-fat diets rich in fruits and vegetables, whole-grain products, low-fat and nonfat dairy products, legumes, fish, and lean meats. Such a dietary pattern, while accommodating personal preferences, is consistent with a dietary pattern predicted to minimize chronic disease risk.
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