decline was also observed in a meta-analysis of more than 150 studies dating back to the early 1920s, translating into a decline in total energy expenditure of about 6%. biochemistry energy expenditure breakdown liver 27% brain 19% skeletal muscle 18% kidneys 10% heart 7% other organs 19% postprandial thermogenesis increases in basal metabolic rate occur at different degrees depending on consumed food composition. about 70% of a human's total energy expenditure is due to the basal life processes taking place in the organs of the body (see table). about 20% of one's energy expenditure comes from physical activity and another 10% from thermogenesis, or digestion of food (postprandial thermogenesis). all of these processes require an intake of oxygen along with coenzymes to provide energy for survival (usually from macronutrients like carbohydrates, fats, and proteins) and expel carbon dioxide, due to processing by the krebs cycle. for the bmr, most of the energy is consumed in maintaining fluid levels in tissues through osmoregulation, and only about one-tenth is consumed for mechanical work, such as digestion, heartbeat, and breathing. what enables the krebs cycle to perform metabolic changes to fats, carbohydrates, and proteins is energy, which can be defined as the ability or capacity to do work. the breakdown of large molecules into smaller molecules—associated with release of energy—is catabolism. the building up process is termed anabolism. the breakdown of proteins into amino acids is an example of catabolism, while the formation of proteins from amino acids is an anabolic process. exergonic reactions are energy-releasing reactions and are generally catabolic. endergonic reactions require energy and include anabolic reactions and the contraction of muscle. metabolism is the total of all catabolic, exergonic, anabolic, endergonic reactions. adenosine triphosphate (atp) is the intermediate molecule that drives the exergonic transfer of energy to switch to endergonic anabolic reactions used in muscle contraction. this is what causes muscles to work which can require a breakdown, and also to build in the rest period, which occurs during the strengthening phase associated with muscular contraction. atp is composed of adenine, a nitrogen containing base, ribose, a five carbon sugar (collectively called adenosine), and three phosphate groups. atp is a high energy molecule because it stores large amounts of energy in the chemical bonds of the two terminal phosphate groups. the breaking of these chemical bonds in the krebs cycle provides the energy needed for muscular contraction. glucose because the ratio of hydrogen to oxygen atoms in all carbohydrates is always the same as that in water—that is, 2 to 1—all of the oxygen consumed by the cells is used to oxidize the carbon in the carbohydrate molecule to form carbon dioxide. consequently, during the complete oxidation of a glucose molecule, six molecules of carbon dioxide and six molecules of water are produced and six molecules of oxygen are consumed. the overall equation for this reaction is c 6 h 12 o 6 + 6 o 2 ⟶ 6 co 2 + 6 h 2 o (30–32 atp molecules produced depending on type of mitochondrial shuttle, 5–5.33 atp molecules per molecule of oxygen.) because the gas exchange in this reaction is equal, the respiratory quotient (r.q.) for carbohydrate is unity or 1.0: r.q. = 6 co 2 6 o 2 = 1.0. ==1.0.} fats the chemical composition for fats differs from that of carbohydrates in that fats contain considerably fewer oxygen atoms in proportion to atoms of carbon and hydrogen. when listed on nutritional information tables, fats are generally divided into six categories: total fats, saturated fatty acid, polyunsaturated fatty acid, monounsaturated fatty acid, dietary cholesterol, and trans fatty acid. from a basal metabolic or resting metabolic perspective, more energy is needed to burn a saturated fatty acid than an unsaturated fatty acid. the fatty acid molecule is broken down and categorized based on the number of carbon atoms in its molecular structure. the chemical equation for metabolism of the twelve to sixteen carbon atoms in a saturated fatty acid molecule shows the difference between metabolism of carbohydrates and fatty acids. palmitic acid is a commonly studied example of the saturated fatty acid molecule. the overall equation for the substrate utilization of palmitic acid is c 16 h 32 o 2 + 23 o 2 ⟶ 16 co 2 + 16 h 2 o (106 atp molecules produced, 4.61 atp molecules per molecule of oxygen.) thus the r.q. for palmitic acid is 0.696: r.q. = 16 co 2 23 o 2 = 0.696. ==0.696.} proteins proteins are composed of carbon, hydrogen, oxygen, and nitrogen arranged in a variety of ways to form a large combination of amino acids. unlike fat the body has no storage deposits of protein. all of it is contained in the body as important parts of tissues, blood hormones, and enzymes. the structural components of the body that contain these amino acids are continually undergoing a process of breakdown and replacement. the respiratory quotient for protein metabolism can be demonstrated by the chemical equation for oxidation of albumin: c 72 h 112 n 18 o 22 s + 77 o 2 ⟶ 63 co 2 + 38 h 2 o + so 3 + 9 co ( nh 2 ) 2 the r.q. for albumin is 0.818: r.q. = 63 co 2 77 o 2 = 0.818. ==0.818.} the reason this is important in the process of understanding protein metabolism is that the body can blend the three macronutrients and based on the mitochondrial density, a preferred ratio can be established which determines how much fuel is utilized in which packets for work accomplished by the muscles. protein catabolism (breakdown) has been estimated to supply 10% to 15% of the total energy requirement during a two-hour aerobic training session. this process could severely degrade the protein structures needed to maintain survival such as contractile properties of proteins in the heart, cellular mitochondria, myoglobin storage, and metabolic enzymes within muscles. the oxidative system (aerobic) is the primary source of atp supplied to the body at rest and during low intensity activities and uses primarily carbohydrates and fats as substrates. protein is not normally metabolized significantly, except during long term starvation and long bouts of exercise (greater than 90 minutes.) at rest approximately 70% of the atp produced is derived from fats and 30% from carbohydrates. following the onset of activity, as the intensity of the exercise increases, there is a shift in substrate preference from fats to carbohydrates. during high intensity aerobic exercise, almost 100% of the energy is derived from carbohydrates, if an adequate supply is available. aerobic vs. anaerobic exercise studies published in 1992 and 1997 indicate that the level of aerobic fitness of an individual does not have any correlation with the level of resting metabolism. both studies find that aerobic fitness levels do not improve the predictive power of fat free mass for resting metabolic rate. however, recent research from the journal of applied physiology, published in 2012, compared resistance training and aerobic training on body mass and fat mass in overweight adults (strride at/rt). when you consider time commitments against health benefits, aerobic training is the optimal mode of exercise for reducing fat mass and body mass as a primary consideration, resistance training is good as a secondary factor when aging and lean mass are a concern. resistance training causes injuries at a much higher rate than aerobic training. compared to resistance training, it was found that aerobic training resulted in a significantly more pronounced reduction of body weight by enhancing the cardiovascular system which is what is the principal factor in metabolic utilization of fat substrates. resistance training if time is available is also helpful in post-exercise metabolism, but it is an adjunctive factor because the body needs to heal sufficiently between resistance training episodes, whereas with aerobic training, the body can accept this every day. rmr and bmr are measurements of daily consumption of calories. the majority of studies that are published on this topic look at aerobic exercise because of its efficacy for health and weight management. anaerobic exercise, such as weight lifting, builds additional muscle mass. muscle contributes to the fat-free mass of an individual and therefore effective results from anaerobic exercise will increase bmr. however, the actual effect on bmr is controversial and difficult to enumerate. various studies suggest that the resting metabolic rate of trained muscle is around 55 kj/kg per day. even a substantial increase in muscle mass, say 5 kg, would make only a minor impact on bmr. longevity in 1926, raymond pearl proposed that longevity varies inversely with basal metabolic rate (the "rate of living hypothesis"). support for this hypothesis comes from the fact that mammals with larger body size have longer maximum life spans (large animals do have higher total metabolic rates, but the metabolic rate at the cellular level is much lower, and the breathing rate and heartbeat are slower in larger animals) and the fact that the longevity of fruit flies varies inversely with ambient temperature. additionally, the life span of houseflies can be extended by preventing physical activity. this theory has been bolstered by several new studies linking lower basal metabolic rate to increased life expectancy, across the animal kingdom—including humans. calorie restriction and reduced thyroid hormone levels, both of which decrease the metabolic rate, have been associated with higher longevity in animals. however, the ratio of total daily energy expenditure to resting metabolic rate can vary between 1.6 and 8.0 between species of mammals. animals also vary in the degree of coupling between oxidative phosphorylation and atp production, the amount of saturated fat in mitochondrial membranes, the amount of dna repair, and many other factors that affect maximum life span. one problem with understanding the associations of lifespan and metabolism is that changes in metabolism are often confounded by other factors that may affect lifespan. for example under calorie restriction whole body metabolic rate goes down with increasing levels of restriction, but body temperature also follows the same pattern. by manipulating the ambient temperature and exposure to wind it was shown in mice and hamsters that body temperature is a more important modulator of lifespan than metabolic rate. organism longevity and basal metabolic rate in allometric scaling, maximum potential life span (mpls) is directly related to metabolic rate (mr), where mr is the recharge rate of a biomass made up of covalent bonds. that biomass (w) is subjected to deterioration over time from thermodynamic, entropic pressure. metabolism is essentially understood as redox coupling, and has nothing to do with thermogenesis. metabolic efficiency (me) is then expressed as the efficiency of this coupling, a ratio of amperes captured and used by biomass, to the amperes available for that purpose. mr is measured in watts, w is measured in grams. these factors are combined in a power law, an elaboration on kleiber's law relating mr to w and mpls, that appears as mr = w^ (4me-1)/4me. when me is 100%, mr = w^3/4; this is popularly known as quarter power scaling, a version of allometric scaling that is premised upon unrealistic estimates of biological efficiency. the equation reveals that as me drops below 20%, for w < one gram, mr/mpls increases so dramatically as to endow w with virtual immortality by 16%. the smaller w is to begin with, the more dramatic is the increase in mr as me diminishes. all of the cells of an organism fit into this range, i.e., less than one gram, and so this mr will be referred to as bmr. but the equation reveals that as me increases over 25%, bmr approaches zero. the equation also shows that for all w > one gram, where w is the organization of all of the bmrs of the organism's structure, but also includes the activity of the structure, as me increases over 25%, mr/mpls increases rather than decreases, as it does for bmr. an mr made up of an organization of bmrs will be referred to as an fmr. as me decreases below 25%, fmr diminishes rather than increases as it does for bmr. the antagonism between fmr and bmr is what marks the process of aging of biomass w in energetic terms. the me for the organism is the same as that for the cells, such that the success of the organism's ability