nutrition

 dietary reference intakes(dris) a. definition of the dris : estimates of the amounts of nutrients required to prevent deficiencies and maintain optimal health. the dris establish upper limits on the consumption of some nutrients, and incorporate the role of nutrients in lifelong health, going beyond deficiency diseases. the dris consist of four dietary reference standards for the intake of nutrients designated for specific age-groups, physiologic states, and sexes . 1. estimated average requirement (ear): the ear is the average daily nutrient intake level estimated to meet the requirement of one half the healthy individuals in a particular life stage and gender group. in other words ear is the intake at which the risk of inadequacy is 50%. it is useful in estimating the actual requirements in groups and individuals. 2.recommended dietary allowance (rda): the rda is the average daily dietary intake level that is sufficient to meet the nutrient requirements of nearly all (97 to 98 percent) individuals in a life stage and gender group. in other words rda is the intake at which the risk of inadequacy is 2-3% .the rda is not the minimal requirement for healthy individuals rather, it is intentionally set to provide a margin of safety for most individuals. the ear serves as the foundation for setting the rda. 3. adequate intake (ai): the ai is set instead of an rda if sufficient scientific evidence is not available to calculate an ear or rda. the ai is based on estimates of nutrient intake by a group (or groups) of apparently healthy people that are assumed to be adequate. for example, the ai for young infants, for whom human milk is the recommended sole source of food for the first four to six months, is based on the estimated daily mean nutrient intake supplied by human milk for healthy, full-term infants who are exclusively breast-fed. 4. tolerable upper intake level (ul): ul is the highest average daily nutrient intake level that is likely to pose no risk of adverse health effects to almost all individuals in the general population. as intake increases above the ul, the potential risk of adverse effects may increase. the ul is not intended to be a recommended level of intake. uls are useful because of the increased availability of fortified foods and the increased use of dietary supplements. the ul applies to chronic daily use. for some nutrients, there may be insufficient data on which to develop a ul. b. using the dris most nutrients have a set of dris (figure). usually a nutrient has an ear and a corresponding rda. most are set by age and gender, and may be influenced by special factors, such as pregnancy and lactation in women. when the data are not sufficient to estimate an ear (or rda), then an ai is designated. the ai is judged by experts to meet the needs of all individuals in a group, but is based on less data than in establishing an ear and rda. intakes below the ear need to be improved because the probability of adequacy is fifty percent or less . intakes between the ear and rda probably need to be improved because the probability of adequacy is less than 98 percent, and intakes at or above the rda can be considered adequate. intake above the ai can be considered adequate. intakes between the ul and the rda can be considered at no risk for adverse effects. ii.energy requirement in humans the estimated energy requirement is the average dietary energy intake predicted to maintain an energy balance (that is, when calories consumed are equal to the energy expended) in a healthy adult of a defined age, gender, and height whose weight and level of physical activity are consistent with good health. differences in the genetics, metabolism, and behavior of individuals make it difficult to accurately predict a person s caloric requirements. however, some simple approximations can provide useful estimates: for example, sedentary adults require about 30 kcal/kg/day to maintain body weight moderately active adults require 35 kcal/kg/day and very active adults require 40 kcal/kg/day. a. energy content of food the energy content of food is calculated from the heat released by the total combustion of food in a calorimeter. it is expressed in kilocalories(kcal or cal). the standard conversion factors for determining the metabolic caloric value of fat, protein, and carbohydrate are shown in( figure). note that the energy content of fat is more than twice that of carbohydrate or protein, whereas the energy content of ethanol is intermediate between fat and carbohydrate. b. how energy is used in the body the energy generated by metabolism of the macronutrients is used for three energy-requiring processes that occur in the body: resting metabolic rate, thermic effect of food (formerly termed specific dynamic action), and physical activity. 1. resting metabolic rate: the energy expended by an individual in a resting, postabsorptive state is called the resting (formerly, basal) metabolic rate (rmr).it represents the energy required to carry out the normal body functions, such as respiration, blood flow, ion transport, and maintenance of cellular integrity. in an adult, the rmr is about 1800 kcal for men (70 kg) and 1300 kcal for women (50 kg). from fifty to seventy percent of the daily energy expenditure in sedentary individuals is attributable to the rmr (figure ). 2. thermic effect of food: the production of heat by the body increases as much as thirty percent above the resting level during the digestion and absorption of food. this effect is called the thermic effect of food or diet-induced thermogenesis. over a 24-hour period, the thermic response to food intake may amount to five to ten percent of the total energy expenditure. 3. physical activity: muscular activity provides the greatest variation in energy expenditure. the amount of energy consumed depends on the duration and intensity of the exercise. the daily expenditure of energy can be estimated by carefully recording the type and duration of all activities. in general, a sedentary person requires about thirty to fifty percent more than the resting caloric requirement for energy balance (see figure), whereas a highly active individual may require 100 percent or more calories above the rmr. iii. acceptable macronutrient distribution ranges acceptable macronutrient distribution ranges (amdr) are defined as a range of intakes for a particular macronutrient that is associated with reduced risk of chronic disease while providing adequate amounts of essential nutrients. the amdr for adults is 45 to 65 percent of their total calories from carbohydrates, 20 to 35 percent from fat, and 10 to 35 percent from protein. there is a range of acceptable intakes for the macronutrient, this is, in part, due to the fact that fats and carbohydrates (and, to a limited extent, protein) can substitute for one another to meet the body s energy needs. the amdr thus represents a balance designed to avoid risks associated with excess consumption of any particular macronutrient. for example, very high-fat diets are associated with weight gain and an increased intake of saturated fats that can raise the plasma low-density lipoprotein(ldl) cholesterol concentration and increase the risk of coronary heart disease (chd). conversely, very high-carbohydrate diets are associated with a reduction in plasma high-density lipoprotein (hdl) cholesterol, an increase in plasma triacylglycerol concentration, and an increased risk of chd. the amdr for protein ensures an adequate supply of amino acids for tissue growth, maintenance, and repair. the biologic properties of dietary fat, carbohydrate, and protein are described below. dietary fats the incidence of a number of chronic diseases are significantly influenced by the kinds and amounts of nutrients consumed (figure). the role of dietary fats and the risk for chd have been thoroughly documented. a. plasma cholesterol and coronary heart disease plasma cholesterol may arise from the diet or from endogenous biosynthesis. in either case, cholesterol is transported between the tissues in combination with protein and phospholipids as lipoproteins. 1-ldl and hdl: the level of plasma cholesterol is not precisely regulated, but rather varies in response to the diet. elevated levels result in an increased risk for cardiovascular disease . the risk increases progressively with higher values for serum total cholesterol. a much stronger correlation exists between the levels of blood ldl cholesterol and heart disease. in contrast, high levels of hdl cholesterol have been associated with a decreased risk for heart disease. abnormal levels of plasma lipids(dyslipidaemias) act in combination with smoking, obesity, sedentary lifestyle, and other risk factors to increase the risk of chd. elevated plasma triacylglycerols are also a risk factor for chd, but the association is weaker than that of ldl cholesterol with chd. 2. beneficial effect of lowering plasma cholesterol: clinical trials have demonstrated that dietary or drug treatment of hypercholesterolemia is effective in decreasing ldls, increasing hdls, and reducing the risk for cardiovascular events. the diet-induced changes of plasma lipoprotein concentrations are modest, typically ten to twenty percent, whereas treatment with "statin" drugs decreases plasma cholesterol by thirty to forty percent. b. dietary fats and plasma lipids triacylglycerols are quantitatively the most important class of dietary fats. their biologic properties are determined by the chemical nature of the constituent fatty acids, in particular, the presence or absence of double bonds, the number and location of the double bonds, and the cis-trans configuration of the unsaturated fatty acids. 1. saturated fat: triacylglycerols containing primarily fatty acids whose side chains do not contain any double bonds are referred to as saturated fats. consumption of saturated fats is strongly associated with high levels of total plasma cholesterol and ldl cholesterol, and an increased risk of coronary heart disease. the main sources of saturated fatty acids are dairy and meat products and some vegetable oils, such as coconut and palm oils. most experts strongly advise limiting intake of saturated fats. 2. monounsaturated fats: triacylglycerols containing primarily fatty acids with one double bond are referred to as monounsaturatedfat. unsaturated fatty acids are generally derived from vegetables and fish. when substituted for saturated fatty acids in the diet, monounsaturated fats lower both total plasma cholesterol and ldl cholesterol, but increase hdls. this ability of monounsaturated fats to favorably modify lipoprotein levels may explain, in part, the observation that mediterranean cultures, with diets rich in olive oil (high in monounsaturated oleic acid), show a low incidence of coronary heart disease. the mediterranean diet: the mediterranean diet is an example of a diet rich in monounsaturated fatty acids (from olive oil) and n-3 fatty acids (from fish oils and some nuts), but low in saturated fat. for example, figure shows the composition of the mediterranean diet in comparison with both a "western" diet similar to that consumed in the united states and a typical low-fat diet. the mediterranean diet contains seasonally fresh food, with an abundance of plant material, low amounts of red meat, and olive oil as the principal source of fat. the mediterranean diet is associated with decreased serum total cholesterol and ldl—but little change in hdl when compared with a typical western diet higher in saturated fats. plasma triacylglycerols are unchanged. 3. polyunsaturated fatty acids: triacylglycerols containing primarily fatty acids with more than one double-bond are referred to as polyunsaturated fats. the effects of polyunsaturated fatty acids on cardiovascular diseases influenced by the location of the double bonds within the molecule. a. n-6 fatty acids: these are long-chain, polyunsaturated fatty acids, with the first double bond beginning at the sixth carbon atom (when counting from the methy end of the fatty acid molecule, figure . [note: they are also called (omega 6) fatty acid]. consumption of fats containing n-6 polyunsaturated fatty acids, principally linoleic acid obtained from vegetable oils, lowers plasma cholesterol when substituted for saturated fats. plasma ldls are lowered, but hdls, which protect against coronary heart disease, are also lowered. the powerful benefits of lowering ldls are only partially offset because of the decreased hdls. nuts, avocados olives, soybeans, and various oils, including sesame, cottonseed, and corn oil, are common sources of these fatty acids. linoleic acid, along with linolenic acid, are essential fatty acids required for fluidity of membrane structure and synthesis of eicosanoids [note: a deficiency of essential fatty acids is characterized by scaly dermatitis, hair loss, and poor wound healing.] a lower boundary level of five percent of calories meets the ai set for linoleic acid. an upper boundary for linoleic acid is set at ten percent of total calories because of concern that oxidation of these polyunsaturated fatty acids may lead to deletingrious products. b.n-3 fatty acids: these are long-chain, polyunsaturated fattyacids, with the first double bond beginning at the third carbon atom (when counting from the methyl end of the fatty acid molecule, see figure above). dietary n-3 polyunsaturated fats suppress cardiac arrhythmias, reduce serum triacylglycerols, decrease the tendency to thrombosis, and substantially reduce risk of cardiovascular mortality, but they have little effect on ldl or hdl cholesterol levels. the n-3 polyunsaturated fats are found in plants (mainly ?-linolenic acid an essential fatty acid), and in fish oil containing docosahexaenoic acid (dha)and eicosapentaenoic acid (epa). the acceptable range for ?-linolenic acid is 0.6 to 1.2 percent of total calories, although emerging data suggest that higher values may provide protection against chd. c. antithrombotic effects of n-3 fatty acids: the reduced blood platelet reactivity observed with increased consumption of epa and dha n-3 fatty acids results from inhibition of the conversion of arachidonic acid to thromboxane a2(txa2) by platelets. instead, the n-3 fatty acids are converted to txa3which is less thrombogenic than txa2(see figure above). thus, the products of fish oils decrease platelet aggregation and, therefore, are antithrombogenic. in addition to these effects, n-3 fatty acids decrease arrhythmias, and affect a variety of membrane functions. the fatty fish can be remembered as smash: salmon, mackerel, anchovies, sardines, and herring.] [note: generally speaking, western diets contain excess dietary fatty acids that compete with the formation of eicosanoids derived from n-3 fatty acids.] 4. trans fatty acids: trans fatty acids (figure ) are chemically classified as unsaturated fatty acids, but behave more like saturated fatty acids in the body, that is, they elevate serum ldl (but not hdl), and they increase the risk of chd. trans fatty acids do not occur naturally in plants and only occur in small amounts in animals. however, trans fatty acids are formed during the hydrogenation of liquid vegetable oils, for example, in the manufacture of margarine. 5. dietary cholesterol: cholesterol is found only in animal products. the effect of dietary cholesterol on plasma cholesterol is less important than the amount and types of fatty acids consumed. 6. plant sterols: commercially available margarines containing hydrogenated plant sterols and sterol esters (predominantly sitostanol esters), when used in place of regular margarine, can reduce ldl plasma cholesterol concentrations. the mechanism by which these compounds lower ldl cholesterol concentrations is to inhibit intestinal absorption of dietary cholesterol and cholesterol secreted into the bile. c. other dietary factors affecting coronary heart disease 1. soy protein: consumption of 25 to 50 g/day of soy protein causes an approximately ten percent decrease in ldl cholesterol in patients with elevated plasma cholesterol. 2. alcohol consumption: moderate consumption of alcohol (for example, two drinks a day) decreases the risk of coronary heart disease, because there is a positive correlation between moderate alcohol consumption and the plasma concentration of hdls. however, because of the potential dangers of alcohol abuse, health professionals are reluctant to recommend increased alcohol consumption to their patients. red wine may provide cardio-protective benefits in addition to those resulting from its alcohol content, for example, red wine contains phenolic compounds that inhibit lipoprotein oxidation . [note: these antioxidants are also present in raisins and grape juice.] 3. vitamins b6,b12 and folate: an elevated plasma homocysteine level is associated with increased cardiovascular risk. homocysteine, which is thought to be toxic to the vascular endothelium, is converted into harmless amino acids by the action of enzymes that require the b vitamins folate-b6 (pyridoxine),and b12(cobalamine). ingesting foods rich in these vitamins can lower homocysteine levels and possibly decrease the risk of cardiovascular disease. folate and b6 are found in leafy green vegetables, whole grains, some fruits, and fortified breakfast cereals. b12comes from animal food, for example, meat, fish, and eggs.