Lipids and Carbohydrates in Human Nutrition

Role of Lipids and Carbohydrates in Human Nutrition (sources, excess, deficiency)
Lipids are a heterogeneous group of compounds, including fats, oils, steroids, waxes, and related compounds. They are concentrated sources of energy as well as structural components of cell membranes. A certain amount of dietary fat is required for normal body functions.

Fatty Acids (FA) are described according to the number of double bonds present. Saturated fats have a profound hypercholesterolemic (increase blood cholesterol levels) effect. They are found predominantly in animal products (butter, cheese and meat)
 * Saturated fatty acids- no double bonds: palmitic, stearic, arachidic acid
 * Unsaturated fatty acids may contain one or more double bond's e.g. Monoenoic acids like Oleic acid, or Polyenoic acids like Linoleic, Linolenic, Arachidonic acids.

Intake of monounsaturated fats in oils such as olive oil is thought to be preferable to consumption of polyunsaturated fats in oils such as corn oil because the monounsaturated fats apparently do not lower high-density-lipoprotein (HDL) cholesterol levels.

Essential fatty acids have to be provided by the diet, they play vital roles in cell membrane structure. Studies have shown that those following a Mediterranean diet, Arctic-dwelling Inuit - who consume high amounts of n−3 fatty acids from fatty fish - also tend to have higher proportions of n−3, increased HDL cholesterol and decreased triglycerides (fatty material that circulates in the blood), and less heart disease.

Cholesterol
Cholesterol is the principal sterol synthesized by animals, predominantly in the liver. It is an essential structural component of mammalian cell membranes and is required to establish proper membrane permeability and fluidity, also important in the manufacture of bile acids, steroid hormones and vitamin D.

Although cholesterol is important and necessary for biological processes, high levels of cholesterol in the blood have been linked to damage to arteries and cardiovascular disease.

Cholesterol is found in egg yolks and organ meats such as liver and kidneys but can also be synthesised in the body.

Requirement


 * Daily fat intake should be 0.3g animal and 0.4g vegetable fat per kg of bodyweight.


 * Cholesterol should be restricted to 300mg/day or less.

Excess fat intake should be avoided as it is linked to having an increased risk of obesity, coronary heart disease and certain types of cancer.

Carbohydrates
Monosaccharides are the simplest carbohydrates, they cannot be hydrolyzed to smaller carbohydrates.


 * Glucose is made in the body from the digestion of starch and disaccharide’s. The normal blood sugar range is between 3.9-6.1 mmol/litre.
 * Fructose is found in fruits and honey. It is converted to glucose in human metabolism.
 * Galactose is produced via breakdown of lactose (milk) and then changed to glucose for energy.

Disaccharides are two joined monosaccharides and are the simplest polysaccharides. Examples include:


 * Sucrose: table sugar.
 * Lactose: sugar in milk.

Polysaccharides made of many single saccharide units. Most important in human nutrition:
 * Starch (stored as glycogen)

Requirement

Carbohydrates in the human diet provide most of the energy in the human diet. They prevent breakdown of fats and proteins, which would cause excessive production of toxic metabolic by-products.
 * Optimum intake is 4-6g/kg body weight. Sucrose intake should not exceed 10%.

High intakes of sugar occur in many populations, it is linked to glucose intolerance, hyperlipidemia and an increase in the occurrence of dental caries.

Dietary fibre
Dietary fibre is the indigestible portion of plant foods (resistant to human digestive enzymes):

Sources of dietary fibre: whole grains, pulses, vegetables and fruits. 30g/day are required however an excessive intake can reduce bioavailability.
 * Cellulose
 * Non-cellulose polysaccharides
 * Single non-carbohydrate member-lignin.

Exernal Links
http://www.who.int/nutrition/topics/nutrecomm/en/index.html