Food is any substance humans, animals or plants eat/take in for energy, growth and life maintenance. Examples of food include yam, garri, semolina, banana, apples, meat and fish.

 
On the other hand, nutrients are components of food; that means, food contains nutrients. There are six nutrients (carbohydrates, protein, fats, water, minerals and vitamins) but a particular food may not contain all the nutrients hence we are advised to take a mixture of foods so as to get all the nutrients.
 
The six nutrients referred to above are split into two groups: Macronutrients (carbohydrates, protein, fat and water) and Micronutrients (vitamins and minerals)
 
Once food is eaten by man, the nutrients contained in it undergo different processes of digestion and absorption in the body.
We shall now discuss the six nutrients in some detail.
 
1. Carbohydrates
Carbohydrate, the chief source of energy in man is an organic compound consisting of carbon, hydrogen and water. It is manufactured by plants. 
 
Plants bring together carbon dioxide from the air, and water from the soil in their green leaves and the energy of sunlight to form the simplest form of carbohydrate called glucose. The chlorophyll in the green leaves serves as a catalyst for this reaction. 
 
By this reaction, the energy of sunlight is stored in the carbohydrate which the plant uses and is obtained by animals when the plant is consumed by them. It is this same energy of sunlight in carbohydrates that we get when we eat plants or animals.
 
In other words, the energy you and I use to move around everyday originates from the sun.  
 
Carbohydrates exist in various natural forms such as Glucose, Fructose, Sucrose, Lactose (milk sugar) produced by lactating animals, Starch, Cellulose, Hemicellulose, Pectin, Gums and Mucilages.
 
In a more complex way carbohydrates can be split into Monosaccharides, Disaccharides, Oligosaccharides and Polysaccharides; Glucose and Fructose are monosaccharides.
Finally carbohydrates can also be divided into Sugars (Glucose, Fructose, Sucrose, Lactose); Starches and Fibres (Cellulose, Hemi-cellulose, Pectin, Gums and Mucilages).
Foods rich in carbohydrates are table sugar, honey, jam, jelly, potatoes, cornflakes, rice, bread, yam, cassava products, fruits, vegetables, pasta, plantain, millet, wheat products, peas, fat-free milk, oats, beans and many others.  Foods which do not contain carbohydrates include beef, eggs, chicken, fish, vegetable oils, butter and margarine.
The main use of carbohydrates is provision of energy.  
 
2. Protein
Protein is an organic compound containing carbon, hydrogen, oxygen and nitrogen. The smallest unit of protein is AMINO ACID; thus when we eat protein it is broken down by our body during digestion to amino acids.
The origin of amino acid is plant, plants synthesise amino acids from carbon dioxide, water and nitrogen obtained from soil or in some cases from the air; this is followed by combining several amino acids to form protein. Animals get protein when they eat plants while man gets protein when he eats plants and /or animals.
There are 20 amino Acids, out of which 9 are essential (cannot be manufactured by the body) and thus must be gotten from foods eaten while the other 11 are non-essential (can be manufactured by the body) using nitrogen obtained from other amino acids.
The body combines the various amino acids to form proteins; two types of proteins are formed:- fibrous protein and globular proteins. The fibrous protein is strong mechanically and makes up muscles, hair, nails and connective tissue while the globular protein which is water soluble is found within the cells and constitute the hormones, antibodies, enzymes and body fluid
Sources of protein include meat, poultry products, dairy products, nuts, legumes, cereals, fish and potato. 
Functions of protein include body building (growth), provision of energy, formation of hormones, antibodies, enzymes and body fluids. Proteins also help in the transportation of certain substances in the body. 
 
3. Lipids (Fat and oils)
Lipids are organic compounds that do not dissolve in water but do so in organic solvents such as chloroform and ether. At room temperature; when in solid form, lipids are called fats but in liquid form they are called oils.
There are three nutritionally important classes of lipids:- Triglyceride, Phospholipids and Sterols. 
Cholesterol is a sterol.
The simplest form of lipids is fatty acid; also lipids can be saturated or unsaturated
Functions of lipids include provision of energy, holds body organs and nerves in position, maintenance of body temperature, palatability of food, absorption and transportation of some vitamins and formation of hormones among others.
Sources of lipids include vegetable oils, poultry products, dairy products, fish and meat.  
 
4. Water
Water makes up about 50 to 70 percent of the total body weight, in the average adult it is about 70 percent but this reduces with age, in the average elderly person it is about 50 percent. The muscular person has more water in his body than a fat person as muscles contain more water.
 
Water is not stored in our body, consequently the average person can survive without food for about two months but can only survive without water for a few days. Loss of about 20 percent of body water may cause death while 10 percent loss could cause severe illness.
 
Water is one of the six nutrients required by our body, the others are carbohydrate, protein, fat, vitamins and minerals.
Water gets into our body through direct intake of water, from foods we eat and from certain activities which take place in the body.
Fruits and vegetables are made up of 85-95 percent water. Foods like potatoes are about 75 percent water.
 
We lose about 10 cups (2500mls) of water daily from our body through sweating, breathing, urine and stool. We must therefore replace this amount through fluid (water, juice, soft drinks, tea, coffee, beer etc) intake and food.
 
We must take between 1000mls and 3000mls of fluids daily to ensure our body is not short of water. On the average, volume of about 2500mls of fluids is adequate.
The functions of water in the body are 1. Regulation of body temperature,2. Removal of waste products, 3. Ensures digestion and absorption of food,  4. Ensures free movement of joints, 5. Transportation of nutrients in the body, 6. Formation of body fluids 
 
5 Minerals
Nutritional minerals are vital to health. There are about twenty one (21) important nutritional minerals required by the body including:
1. Potassium
2. Sodium
3. Chloride
4. Calcium
5. Phosphorus
6. Sulphur
7. Magnesium
Nutritional minerals make up about 5% of body weight or say about 2.8kg in a 70 kg. man. There are about twenty one (21) minerals in the body divided into two groups: macrominerals (7) and microminerals (14).
The macrominerals include calcium, phosphorus, chloride, potassium, sodium, magnesium and sulphur. 
The microminerals include iron, zinc, copper, iodide (iodine), manganese, fluoride, selenium, cobalt, chromium, tin, vanadium, silicon, molybdenium and nickel.
Calcium makes up about 50% of the total weight of minerals in the body, phosphorus makes up 25% while the other nineteen (19) minerals make the balance 25%.
Low minerals in the body can lead to severe health problems. The seven macrominerals are discussed below.
 
6. Vitamins
A vitamin is an organic compound present in small quantities in the body but essential for body processes. Some are manufactured by us while we obtain others from the food we eat 
Some vitamins are stored in some parts of the body such as the liver, lungs, kidneys, and adipose tissue.
There are two groups of vitamins:- Fat-soluble vitamins and Water-soluble vitamins.
The fat soluble vitamins are A, D, E, and K.; they do not dissolve in water and are more available in fatty foods.
The water soluble vitamins dissolve in water and include B and C.; there are about 14 vitamins under Vitamin B.
Vitamins are necessary for the growth and development of man, and in adulthood, for the maintenance of the body.
Lack of vitamins manifests itself in bone deformities, poor vision, skin changes, reduced resistance to diseases, and accelerated aging among others. 
 
7. Phytonutrients (Phytochemicals)
Phytonutrients or Phytochemicals are chemicals produced by plants to stay healthy; they protect plants from insects and ultra violet light. Phytochemicals give plant their colour, aroma and flavour. They are therefore plentiful in colourful fruits, vegetables, spices, whole grains, tea, legumes and nuts. Phytochemicals are not essential for life or for us to be healthy but they can make us healthier. Phytochemicals have anti-oxidant and anti-inflammatory properties and are said to improve our resistance against diseases (immunity). According to the United States Department of Agriculture, consumption of diet rich in phytochemicals can reduce the risk for heart disease and cancer.

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