Nutrition - Lipids (fats and oils) | angelfirenm.info
Overview of lipids, covering fats and oils, saturated and unsaturated fats, triglycerides (triacylglycerols), phospholipids, and steroids. Fatty Acids Soaps and Detergents Fats and Oils Waxes Phospholipids Eicosonoids Terpenes Steroids Lipid Soluble Vitamins Biosynthetic Pathways. Lipids: Fats and Oils. 1. Rasheed Perry, MPhil student; 2. Introduction to lipids. Functions of lipids. Triglycerides in foods. Trans fats.
Image modified from OpenStax Biology. Fat molecules are also called triacylglycerols, or, in bloodwork done by your doctor, triglycerides. While many fatty acids are found in fat molecules, some are also free in the body, and they are considered a type of lipid in their own right.
Saturated and unsaturated fatty acids As shown in the example above, the three fatty acid tails of a triglyceride need not be identical to each other. Fatty acid chains may differ in length, as well as in their degree of unsaturation. If there are only single bonds between neighboring carbons in the hydrocarbon chain, a fatty acid is said to be saturated.
The thing that fatty acids are saturated with is hydrogen; in a saturated fat, as many hydrogen atoms as possible are attached to the carbon skeleton. When the hydrocarbon chain has a double bond, the fatty acid is said to be unsaturated, as it now has fewer hydrogens.
The double bonds in unsaturated fatty acids, like other types of double bonds, can exist in either a cis or a trans configuration. In the cis configuration, the two hydrogens associated with the bond are on the same side, while in a trans configuration, they are on opposite sides see below. A cis double bond generates a kink or bend in the fatty acid, a feature that has important consequences for the behavior of fats.
Saturated fatty acid example: Unsaturated fatty acid examples: Saturated fatty acids tails are straight, so fat molecules with fully saturated tails can pack tightly against one another. This tight packing results in fats that are solid at room temperature have a relatively high melting point. In contrast, cis-unsaturated fatty acid tails are bent due to the cis double bond. This makes it hard for fat molecules with one or more cis-unsaturated fatty acid tails to pack tightly.
So, fats with unsaturated tails tend to be liquid at room temperature have a relatively low melting point — they are what we commonly call oils. For instance, olive oil is mostly made up of unsaturated fats 2 2. Trans fats are rare in nature, but are readily produced in an industrial procedure called partial hydrogenation.
In this process, hydrogen gas is passed through oils made mostly of cis-unsaturated fatsconverting some — but not all — of the double bonds to single bonds. Trans-unsaturated fatty acids can pack more tightly and are more likely to be solid at room temperature.
Partial hydrogenation and trans fats might seem like a good way to get a butter-like substance at oil-like prices. Unfortunately, trans fats have turned out to have very negative effects on human health.
Because of a strong link between trans fats and coronary heart disease, the U. This characteristic permits both the operational classification and the ultracentrifugal separation of the different classes of lipoproteins. Each lipoprotein class is heterogeneous in its protein constituents.
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Nine distinct apolipoproteins have been separated and described. Most investigators group the apolipoproteins into five families designated apo A, apo B, apo C, apo D, and apo E on the basis of their chemical, immunologic, and metabolic characteristics. A third member of the apo A family, apo A-IV, is a minor component of chylomicrons. There are two forms of apo B: Apo D is a minor component of HDL. The apolipoproteins serve both structural and functional roles.
Some apoproteins are ligands for specific cell surface receptors, e. For reviews of lipoprotein structure and metabolism, see Havel and Stanbury et al. Relationship of Plasma Lipid and Lipoprotein Levels to Atherosclerotic Cardiovascular Diseases Epidemiologic Evidence for CHD Most major epidemiologic studies have focused on white men, but a few have provided information about women and nonwhites of both sexes.
Total Cholesterol TC TC is used in this chapter as an abbreviation for the total cholesterol in either serum or plasma.
Although this difference should be considered in comparing the results of studies with one another when numbers of subjects are large and small systematic biases might affect the comparison, it does not affect the major results or conclusions of studies discussed in this report in which serum or plasma is used in analyses of cholesterol. Thus, TC is used interchangeably for both serum and plasma total cholesterol.
Until the past decade, TC, rather than lipoprotein cholesterol, was measured in most epidemiologic studies because reliable methods for measuring lipoprotein cholesterol in large numbers of people were not available. Therefore, most data on disease risk are based on TC level. In the Seven Countries Study, investigators studied 16 populations of middle-aged men residing in seven countries: Examination methods, laboratory procedures, and quality control procedures were standardized.
Similar differences were found some 15 years later for male and female adolescents and adults in the Jerusalem Lipid Research Clinics Prevalence Study Halfon et al. Other differences among populations have been observed for men in Puerto Rico, Hawaii, and Framingham, Massachusetts Gordon et al. Some of this evidence is reviewed in the report of a Conference on Health Effects of Blood Lipoproteins The results of these various studies, particularly the studies of migrants, indicate that the differences in mean TC levels among populations are due largely to environmental factors, principally diet, rather than to constitutional factors.
Large population differences in mean TC levels have also been observed among children and adolescents; the pattern of variation in these means closely parallels that of the adult values, but at lower absolute values Conference on Blood Lipids in Children, Variation in CHD Rates Among Populations Large differences also exist among populations in the incidence of and mortality from CHD and in the prevalence and severity of atherosclerosis.
For example, in the Seven Countries Study, age-standardized, year incidence of first major CHD events myocardial infarction and coronary death among men free of CHD at entry varied from 3 in 1, on Crete to in 1, in eastern Finland Keys, b. Corresponding figures for year CHD mortality were 0 and 68 in 1, respectively. Variation in Atherosclerosis Among Populations In the International Atherosclerosis Project, the extent of atherosclerosis in the coronary arteries and aortas was measured in 23, autopsied people from 19 populations in 14 countries McGill, b.
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Differences among populations were noticeable at ages 15 to 24 and marked at ages 25 to With few exceptions, ranking the populations according to extent of raised lesions corresponded closely to ranking them by CHD mortality rate. The correlations between median TC and national CHD death rates for these seven countries at 0, 5, 10, and 15 years after TC was measured were. In the International Atherosclerosis Project, there was a correlation of.
These results support the conclusion that variation in CHD rates among populations is determined predominantly by differences in levels of TC. The association may be weak or absent in some populations with low mean levels of TC and low absolute risk of CHD, e. In fact, questions have been raised as to whether the association of serum TC with CHD risk is continuous or whether there is some level of serum TC below which it is not related to risk of CHD e.
In four of the eight studies in the U.
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In the same study, however, the corresponding 7-year CHD mortality rates previously showed a steadily increasing pattern: The data for that trial are shown in Figure From Martin et al. Results from the observations of screenees in the Multiple Risk Factor Intervention Trial also indicated that the association between TC and 5-year risk of CHD death for 23, black men was similar to that forwhite men Neaton et al.
In the pooled results of five U. This observation has sometimes been misinterpreted to mean that the level of TC is relatively unimportant in elderly people. The committee believes that this misinterpretation may arise from failure to distinguish between the concepts of relative risk the ratio between two risks and attributable risk the difference between two risks.
The former is commonly used to evaluate the magnitude of an epidemiologic association; the latter is commonly used to evaluate its public health importance. In fact, in the set of studies cited above, the attributable risk calculated as the difference in risk between the highest and lowest quintiles did not vary consistently with age.
Thus, the corresponding attributable risks were 46, 28, 40, and 42 per 1, in 8 years for men ages 45 to 49, 50 to 54, 55 to 59, and 60 to 64, respectively. Atherosclerosis at Autopsy TC measured by standardized procedures in apparently healthy men was strongly associated with extent of atherosclerosis at autopsy in the Hiroshima Adult Health Study Rickert et al.
There are few data on women. In the Framingham Study, the extent of coronary atherosclerosis in men was positively correlated with TC measured 1, 5, and 9 years before death; in women, only TC measured 9 years before death but not TC measured 5 and 1 years before death correlated significantly Feinleib et al. In a follow-up of the initial report, Freedman et al.
As anticipated, blacks had more extensive fatty streaks than did whites, but there also was a strong positive association between the extent of aortic fatty streaks and the LDL cholesterol concentration within each race group.
In summary, epidemiologic findings among populations and for individuals within populations consistently indicate a strong, continuous, and positive relationship between TC levels and the prevalence and incidence of, as well as mortality from, atherosclerotic CHD.
This relationship has been confirmed in autopsy studies. Few early studies included measurements of lipoprotein cholesterol because of technical difficulties and cost. In a study of schoolboys 6 to 7 years old selected from 26 rural and urban populations in 16 countries Knuiman et al. The basis for these different results is not clear. In one study, a correlation of.
Knuiman and West obtained standardized measurements of HDL-C in a survey of small samples of men in 13 countries and found a correlation of. Thus, the extent to which variation among populations in CHD rates may be affected by associated with variations in mean HDL-C levels is uncertain. In the Oslo Study of men ages 40 to 49 years at entry in andantemortem measurements of plasma lipids and other characteristics were available for men for whom there were also postmortem measurements of the extent of atherosclerosis.
The percentage of the coronary intimal surface involved with raised lesions was positively correlated with TC. Although changes in HDL-C are related to changes in CHD risk in most populations, the benefit that can be expected from raising HDL as a preventive strategy in itself is not entirely clear.
Added benefit, over that derived from lowering LDL, is suggested, however, by the long-term results of the Coronary Drug Project Canner et al. The strength of the association was not significantly improved by adding TC or HDL-C to the equation containing this ratio Castelli et al. The authors concluded that ratios can be useful predictors of risk, but warned that they may not always be as informative as the joint use of the two individual figures used to calculate the ratio.
It seems reasonable to expect that increasing knowledge about the various classes and subclasses of lipoproteins will lead to improved ability to predict risk of atherosclerotic diseases. Whether these prediction formulas will take the form of ratios or more detailed specifications of lipoprotein levels is uncertain. The recent report of the National Cholesterol Education Program identified the absolute level of LDL-C as the key index for clinical decision-making about cholesterol-lowering therapy and as the specific target for therapy.
The authors of the report stated, ''Reliance on a ratio of either total or LDL-cholesterol to HDL-cholesterol as a key factor in decisions regarding treatment is not a practice recommended in this report. Blood pressure and smoking are not combined into a single number because the clinician needs to know both facts separately in order to recommend an intervention.
Similarly, HDL-cholesterol and LDL-cholesterol are independent risk factors with different determinants, and combining them into a single number conceals information that may be useful to the clinician" National Cholesterol Education Program, In summary, of the lipoprotein fractions, LDL has the strongest and most consistent relationship to individual and population risk of CHD.
HDL has generally been found to be inversely associated with risk of CHD in individuals within a population, but in at least three long-term population studies, this inverse association was not seen Keys et al.
These findings together with the results of animal experiments and clinical research reviewed later in this chapter strongly support the conclusions that LDL-C is centrally and causally important in the pathogenic chain leading to atherosclerosis and CHD.
Variation in LDL-C levels explains a large part of individual risk within high-risk cultures and explains almost all the differences in CHD rates among populations. Data are inadequate to characterize ratios of LDL to HDL as a major determinant of the atherosclerotic disease burden among populations; however, the ratio provides improved individual prediction, again within high-risk, high-LDL cultures. Apolipoproteins Apolipoproteins play key roles in both the structure and function of plasma lipoproteins.
Research on the molecular structure, genetic variability, and metabolism of plasma apolipoproteins has progressed rapidly in recent years, particularly with the application of the new techniques of molecular biology. Knowledge about apolipoproteins has added greatly to our understanding of lipoprotein metabolism and how it is related to atherosclerosis. In a number of instances, genetically controlled variations in apolipoproteins affect lipoprotein structure, composition, and metabolism.
For example, polymorphic forms of apolipoprotein E apo E interact with dietary fats to influence plasma lipoprotein concentrations, and assessment of apo E phenotypes is an essential procedure in the diagnosis of familial dysbetalipoproteinemia.
However, the evidence currently available does not clearly show that plasma apolipoprotein levels are better predictors of CHD than are the plasma levels of cholesterol in the major lipoprotein classes. The apo E phenotypes were shown to be due to segregation of three alleles at a single locus Zannis and Breslow, and the major isoforms to be determined by substitution of the amino acid cysteine for arginine Weisgraber et al.
The three major isoforms are called apo E2, E3, and E4. Most people with this phenotype do not have familial dysbetalipoproteinemia, however, and in fact have lower plasma cholesterol levels than the general population see Davignon et al. Thus, a single amino acid substitution in one apolipoprotein can have a substantial effect on plasma cholesterol concentrations and on the plasma lipoprotein profile. This topic has been reviewed thoroughly by Davignon et al. The Apolipoproteins and Atherosclerosis In the s, the availability of better methods for fractionating lipoproteins, for measuring serum apolipoproteins, and for detecting apolipoprotein variants made possible a new series of studies that sought relationships among lipoproteins, apolipoproteins, and atherosclerosis.
This topic was reviewed by Brunzell et al. However, these investigators were not in agreement about whether apolipoprotein levels or lipoprotein cholesterol levels were better indicators of CHD.
Wallace and Anderson concluded their review by stating that apolipoprotein levels appeared promising as predictors of CHD, but that large cohort studies would be required to determine whether they are better predictors of atherosclerotic disease than plasma lipoprotein lipid levels and whether they are useful clinically.
Lp a and Human Atherosclerosis The role of lipoprotein a [Lp a ] in atherosclerosis and atherosclerotic heart disease has been controversial ever since Lp a was discovered by Berg The apoprotein of Lp a has been shown to consist of two peptides—apo a and apo B—linked by one or more disulfide bonds Gaubatz et al. Overall, the evidence concerning the association of Lp a with atherosclerosis is rather scanty, but highly suggestive.
Lp a levels in plasma are generally believed not to be affected by diet. Hoff and associates have demonstrated the presence of apo B, apo A-I, and apo C in the arteries of humans and several animal species Hoff and Gaubatz, ; Hoff et al. The same investigators quantified apo B in lesions Hoff et al.
LDL in lesions stimulated the production of cholesteryl esters by macrophages Clevidence et al. These studies clearly show that apo B-containing lipoproteins accumulated in atherosclerotic lesions. LDL from atherosclerotic lesions contained particles larger than those found in plasma LDL, was more electronegative, and stimulated cholesterol esterification in mouse peritoneal macrophages.
The extent to which these alterations in LDL occurred in the arterial intima, after the LDL had entered the vessel wall, is not clear. This combined evidence implicates apo B-containing lipoproteins in the pathogenesis of atherosclerosis. Apolipoproteins and Dietary Responsiveness In view of the critical roles of apolipoproteins in lipoprotein metabolism, genetic variants in addition to those of apo E probably influence the lipemic responses of individuals to dietary fats and thereby affect the risk of CHD.
This research area is quite active, and new findings are likely to be available in the near future. Summary Apolipoproteins show promise of helping us to understand the mechanisms linking diet to atherosclerosis and cardiovascular disease, but as yet they have not provided predictive power for atherosclerotic diseases beyond that provided by plasma lipid and lipoprotein cholesterol concentrations.
They are not now useful in assessing the relationship of dietary fats to atherogenesis or atherosclerotic cardiovascular diseases on a population-wide basis. Nonfasting triglyceride levels were measured in the Framingham, Honolulu, and Puerto Rico studies. Results indicated that relatively high mean nonfasting triglyceride levels are compatible with low-population CHD rates when the mean TC is also low Gordon et al.
However, in most populations found to have low average TC levels, triglyceride levels are also low Conference on Health Effects of Blood Lipoproteins, Differences Among Individuals A fairly large number of epidemiologic studies have explored relationships between plasma triglyceride levels and the risk of CHD.
The studies conducted before were reviewed in full by Hulley et al. In almost all reported studies, a significant positive association between triglyceride levels and CHD was found upon univariate analysis. In commercial processes, the number of double bonds that are hydrogenated is carefully controlled to produce fats with the desired consistency soft and pliable.
Inexpensive and abundant vegetable oils canola, corn, soybean are thus transformed into margarine and cooking fats. In the preparation of margarine, for example, partially hydrogenated oils are mixed with water, salt, and nonfat dry milk, along with flavoring agents, coloring agents, and vitamins A and D, which are added to approximate the look, taste, and nutrition of butter.
Preservatives and antioxidants are also added. In most commercial peanut butter, the peanut oil has been partially hydrogenated to prevent it from separating out. Consumers could decrease the amount of saturated fat in their diet by using the original unprocessed oils on their foods, but most people would rather spread margarine on their toast than pour oil on it.
Many people have switched from butter to margarine or vegetable shortening because of concerns that saturated animal fats can raise blood cholesterol levels and result in clogged arteries. However, during the hydrogenation of vegetable oils, an isomerization reaction occurs that produces the trans fatty acids mentioned in the opening essay. However, studies have shown that trans fatty acids also raise cholesterol levels and increase the incidence of heart disease.
Trans fatty acids do not have the bend in their structures, which occurs in cis fatty acids and thus pack closely together in the same way that the saturated fatty acids do. Fats and oils that are in contact with moist air at room temperature eventually undergo oxidation and hydrolysis reactions that cause them to turn rancid, acquiring a characteristic disagreeable odor.
One cause of the odor is the release of volatile fatty acids by hydrolysis of the ester bonds. Butter, for example, releases foul-smelling butyric, caprylic, and capric acids.
Microorganisms present in the air furnish lipases that catalyze this process. Hydrolytic rancidity can easily be prevented by covering the fat or oil and keeping it in a refrigerator. Another cause of volatile, odorous compounds is the oxidation of the unsaturated fatty acid components, particularly the readily oxidized structural unit in polyunsaturated fatty acids, such as linoleic and linolenic acids.
One particularly offensive product, formed by the oxidative cleavage of both double bonds in this unit, is a compound called malonaldehyde. Rancidity is a major concern of the food industry, which is why food chemists are always seeking new and better antioxidants, substances added in very small amounts 0.
Antioxidants are compounds whose affinity for oxygen is greater than that of the lipids in the food; thus they function by preferentially depleting the supply of oxygen absorbed into the product.
Because vitamin E has antioxidant properties, it helps reduce damage to lipids in the body, particularly to unsaturated fatty acids found in cell membrane lipids. Summary Fats and oils are composed of molecules known as triglycerides, which are esters composed of three fatty acid units linked to glycerol. The hydrolysis of fats and oils in the presence of a base makes soap and is known as saponification.
Double bonds present in unsaturated triglycerides can be hydrogenated to convert oils liquid into margarine solid.