Medical brain notes 30

  DIET AND DISEASE INTRODUCTION Nutrition  is concerned with food and how the body uses it. All nutrients mustbe ingested, and most of them digested, before they can be assimilated and used by the body. Carbohydrates, lipids and proteins are nutrients that are required in comparatively large amounts. Others, such as vitamins, minerals and trace elements, are also necessary but are required in much smaller quantities. Water is also necessary to support life. Nutrients have a variety of metabolic roles and are needed for normal growth, development and the maintenance of health. The body can make some nutrients from others that must be supplied in the diet; the former are  nonessential , the latter  essentialnutrients . Specific daily nutritional requirements are determined by a numberof factors and, indeed, vary throughout the life of any individual. The major factors concerned include the age, sex, physical activity and the general well-being of the person concerned a...

  DIET AND NUTRITION Food is obviously needed to sustain life but a balanced diet is vital for good health. General evidence for good nutrition is a well developed body with an ideal weight, good skin, including hair, and muscle tone, and physical and mental alertness. The food eaten by an individual constitutes the diet and it should supply appropriate amounts and proportions of a variety of substances, usually called nutrients. Some nutrients, the macronutrients, such as carbohydrates, lipids and proteins, are required in amounts that are large compared with the micronutrients, such as vitamins, minerals and trace elements, which are needed in much smaller quantities. Nutrients are needed to sustain a number of activities within the body if good health is to be maintained. They supply the raw materials required for growth and maintenance of tissues, include substances that help regulate metabolism, for example the cofactors and coenzymes necessary for enzyme activities and provid...

  NUTRIENTS AND ENERGY Diets are often described in terms of their energy content, despite the fact that they need to supply materials for needs other than just their energy provision ( Figure 10.1 ). It is possible for a diet to supply an adequate amount of energy but be deficient in some essential raw materials, such as vitamins and minerals needed by the individual. A  balanced diet  is one that supplies adequate energy distributed appropriately between carbohydrates, lipids, and proteins and contains the necessary amounts of vitamins, minerals, trace elements, water and nondigestible fiber. Nutritional energy is usually measured in joules or kilojoules. The number of joules required by any one person needs to be matched to their energy output ( Table 10.1 ). The term  recommended daily amount (RDA)  was originally used by the UK Department of Health in 1979 to define the amounts of certain nutrients needed by different groups within the population. Unfortuna...

  CARBOHYDRATES Carbohydrates have the general formula (CH 2 O) n . The main dietary carbohydrates ( Figure 10.3 ) are mono- and disaccharide sugars, for example fructose, glucose, lactose and sucrose, and polysaccharides, mainly starch, and these are usually the major suppliers of energy. Current recommendations from the WHO suggest that 55% of dietary energy should be in the form of carbohydrates. Most of the carbohydrates should be in the form of starch with no more than 10–15% of energy intake in the form of sugars. However, dietary fiber or roughage largely consists of cellulose, a polysaccharide that cannot be digested and absorbed, although it may be metabolized by bacteria in the large intestine . Foods rich in fiber include wholemeal cereals, flour, root vegetables, nuts and fruits. Dietary fiber stimulates peristalsis and protects against constipation and is known to reduce blood cholesterol and glucose and the incidence of colorectal cancers . Communities that consume hi...

  LIPIDS Dietary lipids are the fats and oils, both of which contain fatty acids that are concentrated sources of energy. Fats are also needed as carriers for the fat-soluble vitamins A, D, E and K. Fatty acids may be saturated or unsaturated, that is, they lack or contain double bonds respectively. The only difference between fats and oils is in their melting points which are related to their degree of unsaturation and chain length: oils are liquid at room temperatures, indicating a higher degree of unsaturation than fats which are solid at these temperatures. Dietary fats ( Figure 10.3 ) consist largely of triacylglycerols together with small amounts of phospholipids and cholesterol. The WHO has recommended that total fats should not supply more than 30% of the energy intake of the diet.   Essential fatty acids Humans lack the enzymes necessary for synthesizing certain unsaturated fatty acids necessary for health and these are  essential fatty acids (EFAs) . A number of...

  PROTEINS Dietary proteins ( Figure 10.3 ) are necessary to supply the amino acids needed for growth and the general repair and maintenance of tissues. A daily intake of about 65 g and 50 g of dietary protein is required in males and females respectively, which provides about 10–15% of the total energy in a balanced diet, although only about 5% of body energy comes from their catabolism under normal circumstances. Protein must be obtained from a variety of sources to supply all essential amino acids. Humans are unable to synthesize nine of the 20 amino acids found in proteins that have codons in the genetic code and these are therefore essential dietary constituents ( Table 10.2 ). However, the nonessential amino acids can be synthesized if the supply of the essential ones is adequate. The ‘quality’ of dietary protein is important and protein intake needs to be varied, particularly as some plant proteins lack one or more of the essential amino acids.  

  WATER Water is vital for life because it helps stabilize the structures of large molecules, such as proteins and starch, functions as a medium for most biochemical reactions, acts as a solvent for electrolytes, glucose, vitamins, minerals and numerous other small molecules and transports nutrients to, and waste products away from, cells as well as around the whole body. The GIT secretes some 7–9 dm 3 of water that aids the digestion and absorption ofnutrients. To maintain these functions, loss of body water should match intake to ensure water balance . It is recommended that two to three dm 3  of water should be consumed daily, of which about 60% should be liquid water and the rest obtained from seemingly solid foods.

  VITAMINS Vitamins are organic substances that the body cannot synthesize or can make only from chemically closely related compounds. They are needed in the diet but only in relatively small amounts ( Table 10.3 ). Vitamins may be classified as water soluble, such as the B group and vitamin C ( Figure 10.9 ) and fat soluble, such as the vitamins A, D, E and K. Some of the B vitamins and, in general, each fat soluble vitamin comprise a group of closely related compounds called  vitamers . In such cases, the name of the vitamin is used as a collective descriptor. Vitamins have specific biochemical roles and are essential for normal metabolism, growth and good health. Vitamin B 1  or thiamin is an essential component of the coenzyme or prosthetic group, thiamin pyrophosphate (TPP,  Figure 10.10 ). This is necessary for the actions of some enzymes, for example transketolase activity of the pentose phosphate pathway for oxidizing glucose and oxidative decarboxylations ca...

  MINERALS AND TRACE ELEMENTS Minerals and trace elements are inorganic dietary substances required to maintain good health. Minerals include calcium, magnesium, sodium, potassium, phosphate, chloride and sulfate. They are present in the body in amounts larger than 5 g and some are required in dietary quantities of more than 100 mg per day and are provided by a variety of foods ( Table 10.4 ). Both groups have diverse functions. Minerals play roles in promoting growth and are important constituents of body tissues such as bone, teeth, hair, skin and nails, and as cofactors in some enzymes and other proteins. Sodium, potassium and chloride are required to maintain the electrolyte and osmotic composition of intra- and extracellular fluids, generate electrochemical gradients across plasma membranes and for nerve conductance and muscle contraction. Calcium is an essential component of bone and teeth, is required for muscle contraction and is a second messenger for some hormones and neu...

  NUTRITIONAL DISORDERS Nutritional disorders can arise from a deficiency or excess of nutrients and may affect growth or cause specific diseases and even death. They are proving to be major health issues. Nutritional disorders can arise from an inadequate intake of food, such as protein-energy malnutrition, or an excessive intake, for example obesity, or due to an inadequate amount of a specific dietary nutrient, such as scurvy in vitamin C deficiency. PROTEIN-ENERGY MALNUTRITION Protein-energy malnutrition (PEM) is the name for a spectrum of disorders that arise due to lack of food. Despite the name, affected individuals may not be suffering from a lack of protein, however, because of their deficiency of total energy, dietary proteins that would normally be used for tissue repair or growth are used as a fuel. In addition, vitamin and mineral intakes are usually inadequate. In the developed world, PEM is rare and is usually associated with solitary elderly patients who are malnour...

  OBESITY Obesity is  diagnosed when the patient’s BMI is greater than 30 kg m –2  ( MarginNote 10.3 ) . It is characterized by an excess of fat in the body, particularly underthe skin and is generally recognizable when a person is 20% above their IBW. Environmental and genetic factors can determine body weight, but overeating combined with lack of exercise are its usual causes. Very rarely, obesity may be secondary to endocrine disease , for example hypothyroidism that decreases energy requirements, Cushing’s syndrome where the distribution of body fat is altered, and some hypothalamic disorders that are associated with overeating. A number of diseases occur more frequently in obese individuals ( Figure10.25 ), including type 2 diabetes mellitus , coronary heart disease,hypertension , cholelithiasis and osteoarthritis . Not surprisingly, mortality rates are also greater as body weight increases. Some of the complications of obesity are listed in the  Table 10.7 . Th...