Energy Balance and Regulation of Food Intake8

If the caloric content of the food ingested is less than the energy output that is if the balance is negative-endogenous stores are utilised, glycogen, body proteins and fats are catabolised and the individual loses weight. I

If the caloric value of the food intake exceeds energy output due to heat and work, the energy balance is said to be positive and the energy is stored in the form of adipose tissues and the individual gains weight. Body weight is an index of energy balance. In obese people there are only two ways to loose weight-one by increasing the energy expenditure, two by decreasing the food intake. Maintenance of energy balance is effected by adjustments of four important variables.

Of these food intake compensates for the changes in other variables. The food intake is regulated so as to adjust expenditure of energy. This also brings a long term regulation of reserves closely related to the body weight.

An average normal diet should contain proteins 100G (400 Cal). Fats 100 G (900 Cal) and carbohydrates 400 G (1600 Cal). Total Cal 3000. There are wide variations in individual energy requirements. The average daily energy expenditure in majority of people is 2400–4000 Cal for men, 1700–2900 Cal for women.183

Physical Activity: This is the most important factor. Sedentary workers need less energy. Mental activity involves no expenditure of energy. Manual workers need more energy.

Age: Growing children from 1–19 years need more energy for growth and physical development. With advanced age BMR falls. Energy requirement becomes less.

Body size and weight: BMR is directly proportional to body size and weight.

Climate: During cold climates one has more energy expenditure. During a hot climate, there is less expenditure.

Inadequate caloric intake due to under nutrition and starvation leads to wasting of body with marked loss of adipose tissue and muscle.

Proteins: Proteins are an indispensable constituent of the diet because it is the only source for amino acids including the essential amino acids which cannot be synthesized in the body.

Amino acids are used for tissue growth, for milk proteins formation, to maintain the structure of every tissue cell including its enzymes, for the exocrine and endocrine secretions to maintain the normal plasma concentration. The minimum protein intake of the adult must contain the enough of each essential amino acid to make up their loss.

The proteins of animal origin closely resemble those of human tissue. These are called proteins of high biological value and they are knows as first class proteins.

The proteins of vegetable origin cannot be used so economically for tissue growth and repair. Hence these proteins are called proteins of low biological value. And they are also known as second class proteins.

Calcium: 0.9–1 G/day - source - milk and fortified bread.

Phosphate: 0.88 G/day.

Magnesium 200–400 mg/day: Source - in cereals and vegetables.

Iron not less than 12 mg/day.

Copper + Cobalt, Zinc - trace.

Iodine _ trace - derived from water, milk and as impurity with salt.

Vitamins: Fruits and vegetables provide enough Vit C.

Milk, milk products, fats, meat and eggs provide enough Vit A and D.

Hunger and satiety mechanisms are inborn and dependent on the inherent organisation of the central nervous system, whereas appetite is an acquired and dependent on past pleasurable experience.

Feeding centre is located in the extreme lateral portion of the lateral hypothalamus at the same rostro-caudal plane, as the ventromedial nucleus. Stimulation of this area causes hyperphagia accompanied by effects such as licking, swallowing, chewing and eating automatism. Bilateral lesions of these areas produce complete aphagia and death.

Satiety centre (Brobeck and B.K. Anand) is located in the medial hypothalamus region just lateral to ventro median nucleus. Stimulation of this area produces a decrease in food intake. Bilateral lesion produces hyperphagia and obesity. This produces its effects by inhibitory influences and control the feeding activity and feeding reflexes through brain stem reticular formation, cranial nerve nuclei and spinal cord. With an urge to eat, the animal manifests hyperactivity as restlessness, locomotion, searching, etc. A satiated animal normally does not move about.

Food serves as stimulus for these reflexes and awareness of food brought about through any sensory means, initiate the feeding reflexes. Reticular formation plays a role in exploratory, food-seeking behaviour of the animal. The hypothalamic activity is influenced by limbic system and neocortex.

Limbic structures in the frontal lobe and temporal lobe, modify food intake through a discriminative mechanism. This they termed appetite, while primitive urge of “hunger” they attributed to hypothalamic level. The role of emotions in relation to food is of some interest. Thus Bruch has emphasised that, what we eat, when we eat and how we eat are the functions of the limbic system.

Leptin is a hormone of the adipose tissues. Leptin receptors are located in satiety centre. By its influence it causes satiety and loss of food intake. Deficiency of this hormone leads to hyperphagia and weight gain.

Less food intake and increase in expenditure of energy, exercise will reduce obesity.

When more than 20% of body weight in men and 25% of body weight in women is due to fat, the obesity becomes an obvious feature. The normal values of fat are 12–18% in men and 18–24% in women.

Quetelet Index is the body weight in Kgs divided by the square of the height in meters, e.g. (80/1.8=24). The normal value for this index is 20–25 kg/m².

Adenosine A, receptor hyperactivity inhibits lipolysis and increases insulin sensitivity of adipose tissues and 186cause obesity. Obesity is the predisposing factor for cardio vascular diseases, hypertension, gall bladder diseases, and mammary cancer.

More details regarding ghrelin: (Kojima, Masayasu, and Kenji Kangawa Ghrelin: structure and Function. Physical Rev 85:2005) Small synthetic molecules called growth hormone secretagogues (GHSs) stimulate the release of growth hormone (GH) from the pituitary. They act through the GHS-R, a G protein coupled receptor, whose ligand has only beend discovered recently. Ghrelin is a peptide hormone in which the third amino acid, usually a serine but in some species a theronine, is modified by a fatty acid; this modification is essential for ghrelin's activity. The discovery of ghrelin indicates that the release of GH from the pituitary might be regulated not only by hypothalamic GH-releasing hormone, but also by ghrelin derived from the stomach. In addition, ghrelin stimulates appetite by acting on the hypothalamic arcuate nucleus, a region known to control food intake. Ghrelin is orexigenic; it is secreted from the stomach and circulates in the bloodstream under fasting condition, indicating that it transmits a hunger signal from the periphery to the central nervous system. Taking into account all these activities, ghrelin plays important roles for maintaining GH release and energy homeostasis in vertebrates.

Sites of ghrelin secretion: The main source is stomach. Ghrelin containing cells are abundent in the fundus than in the pylorus. It is also secreted from pancreas.

The existence of ghrelin and its receptor in the hippocampus, a region that is associated with learning and memory, suggests the role of ghrelin in memrory formation.

Ghrelin stimulates GH release primarily from pituitary cells, which indicates that ghrelin can act directly on the pituitary. However, the involvement of the hypothalamus in ghrelin-mediated stimulation of GH release has been strongly suggested.

Ghrelin containing neurons are found in the arcuate nucleus of the hypothalamus, a region involved in appetite regulation. This localization suggests a role of ghrelin in controlling food intake.

Leptin produced in adipose tissues is an appetite suppressor whereas the ghrelin is an potent appetite stimulant.

The hypothalamic arcuate nucleus is the main site of ghrelin's activity in the central nervous system. This also a target site for leptin an appetite suppressor.

Gatrointenstinal function: It increases gastric acid secretion, gastric movement and turnover of gastric and intenstinal mucosa.

It may be useful for the treatment of eating disorder such as anorexia nervosa.

May be useful in the treatment of postoperative gastric ileus and in the treatment of ethanol induced gastric ulcers.

It may be useful in the treatment chronic heart failure.

Ghrelin improves cardiac structure and function

Other potential clinical applications of ghrelin are in osteoporosis, aging and catabolic states including those seen in postoperative patients and in AIDS and cancer associated wasting syndromes.

Mitochondria of brown adiposytes provide a unique machinary that promotes uncoupling of oxidative phosphorylation from ATP synthesis. Thus promotes heat production and expenditure of energy. A protein called uncoupling protein I (UCPI) is present in the mitochondria. This is responsible for uncoupling of oxidative phosphorylation. Brown adiposytes show β3 adrenergic receptors. Sympathetic stimulation increases heat production via these receptors. Binding of these receptors with adrenaline activates the UCP − 1. Exposure to cold, stress, over feeding will stimulate uncoupling of oxidative phosphorylation and thyroid hormones are implicated in this mechanism.188