A balance between energy intake and expenditure is achieved when an adult maintains a stable body weight; effective energy intake (that part of all the food ingested which is actually absorbed from the gut) must equal total energy expenditure. Malnutrition, either under- or over-nutrition, results when one side of the equation “outweighs” the other over a substantial period of time. The amount of energy the body needs in order to function appropriately is determined by the basal metabolic rate; the energy expended due to dietary thermogenesis (heat generated by the body when digesting and absorbing
the food), and for the maintenance of a constant body temperature; the energy needed to deposit or replace cells and tissues; and the energy expended during physical activity.
The different aspects of energy metabolism are tightly regulated by the neuroendocrine system6–8. Electrochemical messages from the nervous system are translated into hormonal messages, delivered to target cells, tissues or organs; in turn, circulating hormones and cytokines act back on neural systems. The hypothalamus, a key component of the neuro-endocrine system, is directly and indirectly involved in energy metabolism.
Through direct innervation of the adrenal medulla, it can stimulate the general release and distribution of noradrenaline, resulting in an increased metabolic rate (mobilisation of glucose and fatty acids from tissues; increased heart rate and force). Cortisol may be released into the circulation from the adrenal cortex through the chain effect of the release of corticotropic-releasing hormone (CRH) from the hypothalamus, acting on the pituitary gland causing the release of adrenocorticotropic hormone (ACTH). Cortisol also acts to mobilise fuels, predominately via the breakdown of muscle protein into amino acids and their sunsequent conversion into glucose. Stress and infection are powerful stimuli for the release of cortisol via this neuroendocrine pathway. An example of a stimulus that originates in a peripheral tissue is the release of leptin from adipose tissue in response to an increase in the amount of triglycerides present in the fat cells; leptin’s action on the hypothalamus results in decreased food intake and lowered metabolic rate.
Factors affecting energy balance
Appetite is controlled by a complex system of sensations (taste, smell, fullness and satiety) as well as neural and humoral factors. Central control of appetite resides primarily in the hypothalamus and the brain stem. Peripheral control of appetite occurs by means of hormones and peptides released by the gut (ghrelin, cholecystokinin, pancreatic polypeptide and others) and other endocrine tissues (insulin and leptin). During episodes of illness and stress, appetite is decreased (see below). Digestion and absorption (see above) affect the actual delivery to the body of ingested food components, and any disruption of the architecture, function and immunity of the intestines (as is the case with HIV infection), is likely to result in intestinal inflammation and/or secondary gastrointestinal infections and consequent malabsorption and diarrhoea, severely reducing the amount of energy available to the body for its functions (see also Chapter 7).
The basal metabolic rate (BMR) accounts for 60–75% of total energy expenditure in the absence of (immediate) past, as well as present, physical activity. Measured upon waking after a 12-hour, overnight fast and prior to the ingestion of any food (i.e.in the post-absorptive state), it is indicative of the energy needed to sustain the metabolic activity of cells and tissues and to maintain blood circulation and respiration in the resting but awake state. Resting metabolic expenditure (REE) is typically measured
only 3–4 hours after a meal at any time of the day and prior physical activity is not controlled for. While sleeping metabolic rate is approximately 5–10% lower than basal metabolic rate, resting metabolic rate is about 10–20% higher than basal metabolic rate.
Basal metabolic rate is dependent on age, gender, body composition, nutritional and health status. An individual’s fat-free mass (bones, muscles etc) is composed of the most metabolically active components of the body and is for this reason the major predictor of basal metabolic rate; the decline in BMR that occurs with aging (approximately 1–2%
every 10 years in weight-stable individuals) is most likely due to the progressive decrease in fat-free mass and the increase in fat mass that occurs over time.
Thermogenesis is the use by the body of increased metabolic oxidations to generate heat. The conversion of energy from food into the high-energy biochemical compounds that can be used as “chemical energy currency” by the body for various metabolic processes is normally an inefficient process, so that roughly 50% of the ingested potential energy is lost to heat production. The dietary thermogenic response to protein consumption (20–30% increase in energy expenditure above BMR) is far greater than the effect caused by the consumption of carbohydrate (5–10% increase in energy expenditure above BMR) and fat (5% increase in energy expenditure above BMR). The ingestion, digestion and absorption of a typical mixed meal elicit an increase in energy expenditure equivalent to approximately 10% of the kilojoules consumed. Ingestion of caffeine, a sympathetic nervous system stimulant, can increase metabolic rate by 10–30%
above baseline for up to 3 hours post ingestion; on a daily basis, a typical caffeine intake can cause up to 3% increase in total energy expenditure.
Growth, pregnancy and lactation are special body states in which energy metabolism adapts upwards to meet the particular extra needs. Infants and children require energy to synthesise and deposit tissue so that their bodies can grow; in the first months of a person’s life, the energy required for growth accounts for approximately 35% of the total energy required by the body. This energy cost of growth decreases to about 3% of the total energy required by the body after 12 months of life and remains low until puberty, when the energy cost of growth again increases to 4% of the total energy required. During pregnancy the basal metabolic rate increases, the energy cost of physical activity due to increased weight-bearing increases by approximately 20%
(lessening in advanced pregnancy), and extra energy is required for the deposition of maternal and foetal tissue. The synthesis of breast milk and the process of lactation accounts for a 4–5% increase in basal metabolic rate.
Typical amounts of physical activity on a daily basis accounts for approximately 20–30% of the total energy expended by an individual, with “fidgeting” making up a surprisingly large fraction of the expenditure. Increasing levels of physical activity, in terms of both intensity and duration, require appropriately increasing amounts of
energy to execute. The body will also continue to require energy once the activity is ceased, proportional to the degree of effort (post-exercise thermogenesis). During prolonged, intense exercise the body heat content will rise, mainly due to inability to lose heat quickly enough. During low intensity exercise, fat stores (muscle and adipose triglycerides) are more likely to be utilised, whereas during more intense exercise carbohydrate stores (muscle glycogen and glucose) are utilised.