Facts on Fats - Dietary Fats and Health: (EUFIC)
Overview of lipids, covering fats and oils, saturated and unsaturated fats, Because of a strong link between trans fats and coronary heart disease, the U.S. Food in the blood, lower blood pressure, and prevent the formation of blood clots. Animal products and some processed foods, especially fried fast food, are generally high in saturated fats, which have been linked to increased blood. The lipids of the retina also contain very high concentrations of DHA. The blood cholesterol level is the net result of the absorption in the gut .. In the s, a link between total/saturated fat consumption and the risk .. Global, regional, and national consumption levels of dietary fats and oils in and.
Omega-3 fatty acids in particular may reduce the risk of sudden death from heart attacks, decrease triglycerides in the blood, lower blood pressure, and prevent the formation of blood clots. However, fats are essential to the body and have a number of important functions. For instance, many vitamins are fat-soluble, meaning that they must be associated with fat molecules in order to be effectively absorbed by the body.
Fats also provide an efficient way to store energy over long time periods, since they contain over twice as much energy per gram as carbohydrates, and they additionally provide insulation for the body. Like all the other large biological molecules, fats in the right amounts are necessary to keep your body and the bodies of other organisms functioning correctly.
Waxes Waxes are another biologically important category of lipids. Wax covers the feathers of some aquatic birds and the leaf surfaces of some plants, where its hydrophobic water-repelling properties prevent water from sticking to, or soaking into, the surface.
Image of shiny leaf surface covered with wax. Structurally speaking, waxes typically contain long fatty acid chains connected to alcohols by ester linkages, although waxes produced by plants often have plain hydrocarbons mixed in as well 6 6.
Phospholipids What keeps the watery goo cytosol inside of your cells from spilling out? Cells are surrounded by a structure called the plasma membrane, which serves as a barrier between the inside of the cell and its surroundings. Specialized lipids called phospholipids are major components of the plasma membrane. Like fats, they are typically composed of fatty acid chains attached to a backbone of glycerol.
Instead having three fatty acid tails, however, phospholipids generally have just two, and the third carbon of the glycerol backbone is occupied by a modified phosphate group. Different phospholipids have different modifiers on the phosphate group, with choline a nitrogen-containing compound and serine an amino acid being common examples. Different modifiers give phospholipids different properties and roles in a cell. Structure of a phospholipid, showing hydrophobic fatty acid tails and hydrophilic head including ester linkages, glycerol backbone, phosphate group, and attached R group on phosphate group.
A bilayered membrane consisting of phospholipids arranged in two layers, with their heads pointing out and their tails sandwiched in the middle, is also shown. A phospholipid is an amphipathic molecule, meaning it has a hydrophobic part and a hydrophilic part. The fatty acid chains are hydrophobic and do not interact with water, whereas the phosphate-containing group is hydrophilic because of its charge and interacts readily with water.
In a membrane, phospholipids are arranged into a structure called a bilayer, with their phosphate heads facing the water and their tails pointing towards the inside above. This organization prevents the hydrophobic tails from coming into contact with the water, making it a low-energy, stable arrangement.
If a drop of phospholipids is placed in water, it may spontaneously form a sphere-shaped structure known as a micelle, in which the hydrophilic phosphate heads face the outside and the fatty acids face the interior of this structure. Formation of micelle is an energetically favored because it sequesters the hydrophobic fatty acid tails, allowing the hydrophilic phosphate head group to instead interact with the surrounding water 7.
Liquid at room temperature, polyunsaturated fats are missing two or more pairs of hydrogen atoms.
Many common vegetable oils, such as corn, soybean, safflower and sunflower oil as well as fish are high in polyunsaturated fats. See Table 3 for the health effects of polyunsaturated fats in the diet. Fatty acids that are essential to human health but not produced in the body must be obtained through food.
Only two types of fatty acids are considered essential; omega-3 fatty acids and omega-6 fatty acids, both polyunsaturated fats. The Dietary Guidelines for Americans emphasize vegetable oils mono- and polyunsaturated fats as part of healthy eating pattern because they are the major source of essential fatty acids and vitamin E. DHA and EPA are mainly found in higher-fat, cold-water fish, such as salmon, mackerel, sardines and herring. Diets high in EPA and DHA may help reduce the risk of heart disease by helping to thin the blood and prevent blood platelets from clotting and sticking to artery walls, causing blockages.
Research also suggests that EPA and DHA may lower levels of triglycerides in the blood, may prevent hardening of the arteries, and may moderately reduce blood pleasure. In addition, consuming DHA during pregnancy 2 servings of oily fish per week is linked with better cognitive development and vision in infants.
For more information on omega-3 fatty acids, see fact sheet Omega-3 Fatty Acids. Omega 6 fatty acids are found in meats and vegetable oils, such as, soybean, corn, and safflower. Linoleic acid LA can be converted to a long chain omega-6 fatty acid called arachidonic acid AAwhich in excess, may be linked to inflammation and other chronic health conditions. Another type of omega 6 fatty acid, called conjugated linoleic acid CLAfound in dairy foods, beef, and lamb, may be associated with a decrease in certain types of cancers and improved body composition.
Research is still exploring these connections. These are unsaturated fats that are processed to become solid at room temperature. Hydrogen atoms are added to unsaturated fat through a process called hydrogenation.
The association of serum cholesterol concentrations with atherosclerosis and myocardial infarction was widely recognized by Dock, The predictive value of serum cholesterol concentration for CHD was firmly established by the Framingham Study in Dawber et al. Nevertheless, appreciation of the relationship of diet to serum cholesterol levels, and thereby to CHD, developed more slowly.
Although there had been an accumulation of epidemiologic evidence mainly ecological correlations supporting the concept that diet, especially dietary fat, was associated with elevated serum cholesterol concentrations and with CHD Keys, ; Keys and Anderson,there also was much skepticism, as illustrated by the comments of YudkinYerushalmy and Hilleboeand Mann Early in the s, the serum-cholesterol-lowering effects of PUFAs were discovered, and epidemiologic and human experimental studies were focused on this issue.
The role of dietary cholesterol remained uncertain until the s, when several careful experiments in humans showed that it had a modest but definite effect. The early development of these concepts, along with the controversies, are found in reviews of the topic by Keysand Ahrens Subsequent sections of this chapter review in detail the epidemiologic and experimental evidence on the relationship between serum cholesterol and CHD, between diet and serum cholesterol, and between diet and CHD.
Plasma Lipids and Lipoproteins Functions and Transport Mechanisms' Lipids are insoluble in water and circulate in plasma in association with certain specific proteins called apolipoproteins. The lipoproteins are large, macromolecular complexes of apolipoproteins and lipids in varying proportions. The four classes of specific lipoproteins that circulate in plasma are called chylomicrons, very-low-density lipoproteins VLDLlow-density lipoproteins LDLand high-density lipoproteins HDL.
The primary function of plasma lipoproteins is lipid transport. The major lipid transported in lipoproteins—triglyceride—is only slightly soluble in water, yet up to several hundred grams must be transported through the blood daily. Hence, transport mechanisms have evolved to permit the packaging of thousands of triglyceride molecules in individual lipoprotein particles, which deliver the transported lipid to specific cells.
Free fatty acids are transported in noncovalent linkage as albumin-fatty acid complexes.
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This latter mode of transport does not permit the high degree of selective targeting of fatty acids to specific sites that is permitted by transport in lipoproteins, but these two modes of fatty acid transport together provide a more versatile system for bulk movement of a major substrate for energy metabolism. Cholesterol is the other major lipid transported in lipoproteins.
It is not used for energy; it is the precursor of steroid hormones and bile acids and is a structural component of cellular membranes. In higher animals, including all mammals, it is transported mainly in the form of cholesteryl esters, which are synthesized in cells or in the plasma compartment itself.
As with triglycerides, the transport of cholesteryl esters in lipoproteins permits specific targeting of cholesterol to tissues that require it for structural purposes or for making its metabolic products. Two of the lipoprotein classes, chylomicrons and VLDL, are composed primarily of triglyceride. Chylomicrons transport exogenous dietary triglyceride, and VLDLs transport endogenous triglyceride.
Chylomicrons are not normally present in postabsorptive plasma after an overnight fast. Lipoproteins are lighter than the other plasma proteins because of their high lipid content. 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. 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.
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.
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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. 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.
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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.
Facts on fats: Dietary fats and health
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.