Dietary fat and fatty acids Most of the fat in the diet consists of TAG that are composed of three fatty-acid molecules bonded to glycerol. The contribution of TAG to total energy intake varies among individuals and populations, ranging from 15% to 40% of total nutrient energy. The fatty acids of TAGs are of several types: saturated, cis-monounsa-turated, trans-monounsaturated, and polyunsatu-rated fatty acids. All fatty acids affect lipoprotein levels in one way or another. Table 2 lists the major fatty acids of the diet and denotes their effects on serum lipoproteins. Also shown are the effects of carbohydrates, which also influence serum lipopro-tein metabolism. It should be noted that all lipopro-tein responses are compared with and related to those of cis-monounsaturated fatty acids, which are widely accepted to be neutral, or baseline.
Saturated fatty acids The saturated fatty acids are derived from both animal fats and plant oils. Rich sources of dietary saturated fatty acids include butter fat, meat fat, and tropical oils (palm oil, coconut oil, and palm kernel oil). Saturated fatty acids are straight-chain organic acids with an even number of carbon atoms (Table 2). All saturated fatty acids that have from eight to 16 carbon atoms raise the serum LDL cholesterol concentration when they are consumed in the diet. In the USA and much of Europe, saturated fatty acids make up 12-15% of total nutrient energy intake.
Table 2 Macronutrient effects on serum lipoprotein cholesterol
Saturated Palmitic Myristic Lauric Caproic Caprilic Stearic trans-Monounsaturated cis-Monounsaturated Polyunsaturated n-6d n-3d Carbohydrate
aFirst number denotes number of carbon atoms; second number denotes number of double bonds.
bThe dash (-) indicates that there is no change in level compared with that produced by cis-monosaturated fatty acids (oleic acid) (C18:1 n-9). All the lipoprotein responses to oleic acid are considered 'neutral', i.e., no effect.
cThe letter 'n' and number indicates at which carbon atom, numbered from the terminal methyl group, the first double bond appears. Abbreviations: VLDL, very low-density lipoproteins; LDL, low-density lipoproteins; HDL, high-density lipoproteins; DHA, docosahexanoic acid (C22:6 n-3); EPA, eicosapentanoic acid (C20:5 n-3).
The mechanisms whereby saturated fatty acids raise LDL cholesterol levels are not known, although available data suggest that they suppress the expression of LDL receptors. The predominant saturated fatty acid in most diets is palmitic acid (C16:0); it is cholesterol-raising when compared with cis-monounsaturated fatty acids, specifically oleic acid (C18:cis1 n-9), which is considered to be 'neutral' with respect to serum cholesterol concentrations. In other words, oleic acid is considered by most investigators to have no effect on serum cholesterol or lipoproteins. Another saturated fatty acid, myristic acid (C14:0), apparently raises LDL cholesterol concentrations somewhat more than does palmitic acid, whereas other saturates - lauric (C12:0), caproic (C10:0), and caprylic (C8:0) acids - have a somewhat lesser cholesterol-raising effect. On average, for every 1% of total energy consumed as cholesterol-raising saturated fatty acids, compared with oleic acid, the serum LDL cholesterol level is raised about 2 mg dl"1 (0.025 mmoll"1).
One saturated fatty acid, stearic acid (C18:0), does not raise serum LDL cholesterol concentrations. The main sources of this fatty acid are beef tallow and cocoa butter. The reason for its failure to raise LDL cholesterol concentrations is uncertain, but may be the result of its rapid conversion into oleic acid in the body.
Traws-monounsaturated fatty acids These fatty acids are produced by hydrogenation of vegetable oils. Intakes of trans-monounsaturates vary from one country to another depending on consumption of hydrogenated oils. In many countries they contribute between 2% and 4% of total nutrient energy intake. A series of trans acids are produced by hydrogenation: most are monounsaturated. For many years, it was accepted that trans-monounsatu-rated fatty acids were neutral with respect to LDL cholesterol concentrations. However, recent studies have shown that they raise LDL cholesterol concentrations to a level similar to that of palmitic acid when substituted for dietary oleic acid. In addition, they cause a small reduction in serum HDL cholesterol concentrations. Thus, trans-monounsaturates must be placed in the category of cholesterol-raising fatty acids.
Ci's-monounsaturated fatty acids The major fatty acid in this category is oleic acid (C18:cis1 n-9). It is found in both animal and vegetable fats, and typically is the major fatty acid in diet. Intakes commonly vary between 10% and 20% of total energy. Oleic acid intake is particularly high in the Mediterranean region where large amounts of olive oil are consumed. Other sources rich in oleic acid are rape-seed oil (canola oil) and high-oleic forms of saf-flower and sunflower oils. Peanuts and pecans also are high in oleic acid. Animal fats likewise contain a relatively high percentage of oleic acid among all their fatty acids; even so, these fats also tend to be rich in saturated fatty acids. When high-carbohydrate diets are consumed, the human body can synthesize fatty acids; among these, oleic acid is the predominant fatty acid produced.
As indicated before, oleic acid generally is considered to be the 'baseline' fatty acid with respect to serum lipoproteins levels, i.e., it does not raise (or lower) LDL cholesterol or VLDL cholesterol concentrations, nor does it lower (or raise) HDL cholesterol concentrations. It is against this 'neutral' fatty acid that responses of other fatty acids are defined (Table 2). For example, when oleic acid is substituted for cholesterol-raising fatty acids, the serum LDL cholesterol concentration will fall. Nonetheless, oleic acid is not designated a cholesterol-lowering fatty acid, but instead, this response defines the cholesterol-raising potential of saturated fatty acids.
Polyunsaturated fatty acids There are two categories of polyunsaturated fatty acids: n-6 and n-3. The major n-6 fatty acid is linoleic acid (C18:2,n-6). It is the predominant fatty acid in many vegetable oils, e.g., corn oil, soya bean oil, and high linoleic forms of safflower and sunflower seed oils. Intakes of linoleic acid typically vary from 4% to 10% of nutrient energy, depending on how much vegetable oil is consumed in the diet. The n-3 fatty acids include linolenic acid (C18:3,n-3), docosahexanoic acid (DHA) (C22:6,n-3), and eicosapentanoic acid (EPA) (C20:5,n-3). Linolenic acid is high in linseed oil and present in smaller amounts in other vegetable oils. DHA and EPA are enriched in fish oils.
For many years, linoleic acid was thought to be a unique LDL cholesterol-lowering fatty acid. Recent investigations suggest that earlier findings overestimated the LDL-lowering potential of linoleic acid. Even though substitution of linoleic acid for oleic acid in the diet may reduce LDL cholesterol levels in some people, a difference in response is not consistent. Only when intakes of linoleic acid become quite high do any differences become apparent. At high intakes, however, linoleic acid also lowers serum HDL cholesterol concentrations. Moreover, compared with oleic acid, it may reduce VLDL cholesterol levels in some people. Earlier enthusiasm for high intakes of linoleic acid to reduce LDL cholesterol levels has been dampened for several reasons: for example, its LDL-lowering ability does not offset potential disadvantages of HDL lowering, and other concerns include possible untoward side effects such as promoting oxidation of LDL and suppressing cellular immunity to cancer.
The n-3 fatty acids in fish oils (DHA and EPA) have a powerful action to reduce serum VLDL levels. This action apparently results from suppression of the secretion of VLDL by the liver. The precise mechanism for this action is not known. However, these fatty acids do not reduce LDL cholesterol concentrations relative to oleic acid. They have been used for treatment of some patients with elevated VLDL concentrations, although drug treatment generally is employed when it is necessary to lower serum VLDL levels.
Carbohydrate When carbohydrates are substituted for oleic acid in the diet, serum LDL cholesterol levels remain unchanged. However, VLDL cholesterol concentrations usually rise and HDL cholesterol concentrations fall on high-carbohydrate diets. Thus, a lack of difference in total serum cholesterol concentrations during the exchange of carbohydrate and oleic acid is misleading. The two categories of nutrients have different actions on lipo-protein metabolism. The differences in response to dietary carbohydrate and oleic acid provide a good example of how measurements of serum total cholesterol fail to reveal all of the changes that are occurring in the lipoprotein fractions.
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