Nutrition is the most important key related to the health of an individual. Obtaining adequate nutrition is the fundamental requirement of every individual for survival. Recent decade’s people started talking about nutrition and awareness is increasing because overnutrition and undernutrition both causes diseases. Overnutrition gives rise to obesity, cardiovascular diseases, diabetes, cancer and so many diseases.
So here I want to tell you some important facts about nutrition. Quality and quantity both are very important to know about a healthy diet.

Nutrients in the diet can be divided in to two parts:
A. Macronutrients (carbohydrate, protein, and fat)
B. Micronutrients (Vitamins and minerals)

The three macronutrients all have their own specific roles and functions in the body and supply us with calories or energy. For this reason, the body requires these nutrients in relatively large amounts to grow, develop, repair and feel good.
Micronutrients do not contribute to energy balance but need in small amounts because they are not synthesized in our body and help to maintain energy balance, metabolism, cellular, functions, physical and mental health.

Energy balance
With rule of thermodynamics energy intake should be equal to energy output or energy expenditure. Energy intake is regulated by macronutrient content of food. Carbohydrate, fat and protein provide fuel for oxidation in mitochondria to generate energy (ATPs).
In general energy provided by each component differs.
1 g carbohydrates = 4 kcal
1 g of protein = 4 kcal
1 g fat = 9 kcal
Regulation of energy balance is very important for healthy life. Energy intake in the form of food and energy expenditure at various forms first in the form of food metabolism which is called dietary induced thermogenesis (energy required for digestion of food), BMR basal metabolic rate energy expenditure required to maintain metabolic functions in tissues and hence sustain life. Metabolic energy is also required for thermal regulation, and expenditure is higher in cold or hot environments. Another and most important factor is muscular activity which is usually different in all individuals depending upon their occupation and lifestyle.
Regulation of energy balance is coordinated in the hypothalamus, which receives afferent signals that indicate nutritional status in the short term and the long term.

Macronutrients
Carbohydrates
Carbohydrates are basically sugar molecule and based on structure they are divided in to two forms:
Simple carbohydrates and complex carbohydrates.
Simple carbohydrates are monosaccharides and disaccharides, whereas complex carbohydrates are oligosaccharides and polysaccharides.

Carbohydrates broken down to monosaccharides before absorption from the gut and supply energy. Actually carbohydrate should not be considered as an essential nutrient because they can be synthesized in the body from glycerol and protein. However if the available carbohydrate is less than 100 gram per day then increased lipolysis leads to ketosis.
Therefore sugar in original form (intrinsic sugar) as in fruits, vegetables and milk are never restricted but sugar is an extrinsic form should always be restricted.
Starches in cereal foods, root foods and legumes provide the largest proportion of energy in most diets around the world. Starches digested by amylase enzyme which is produced by pancreas and saliva, some starches digested by salivary amylase only and produce rapid delivery of glucose to the blood.
Carbohydrates source
Simple carbohydrates      Intrinsic: fruits, milks, vegetables
Extrinsic (extracted, refined): beet or cane sucrose,high fructose                                                                                                      corn syrup
Complex carbohydrates starch polysaccharides :(Rapidly digestible); cereals,whole                                                                                                     wheat, rice, root vegetables like potato
(Slowly digestible); lentils, legumes(peas,beans)

So bottom lines for carbohydrates are
1. Immediate source of energy or main fuel of body.
2. In case of starvation or going outside without carbohydrates or very less carbs, your body will use Glycogen as a fuel, glycogenolysis starts by using your protein from muscle mass and organs , gluconeogenesis starts (conversion of amino acids to glucose) to maintain normal glucose level in the body and the brain. So In starvation you are losing your muscle mass.
3. Carbs prevent ketosis – Even when fat is used for fuel, the cells need a bit of carbohydrate to completely break it down. Otherwise, the liver produces ketone bodies, which can eventually build up to unsafe levels in the blood causing a condition called ketosis. Ketosis can also cause the blood to become too acidic and the body to become dehydrated.

Fats
Fats are the highest energy density molecule; they provide most calories to the body
Dietary fat has a critical role in the body, one gram of fat provide 9 kcals, excessive consumption of fat may give rise to obesity and several diseases. But wisely use of fat is important to keep our body healthy.
Fats are different types like saturated fatty acid, unsaturated fatty acids (monounsaturated and Polyunsaturated Fatty Acids which is also called as PUFA).
Sources of Saturated fatty acids are animal fats butter, ghee or lard. Because your body can make all the saturated fatty acids it needs, you do not need any in the diet. High intakes of most saturated fatty acids are linked to high levels of LDL (low-density lipoprotein), or bad, cholesterol and reduced insulin sensitivity. Saturated fat and Trans Fat are associated with cardiovascular diseases so intake of these should be less than 2%. And replacing them with PUFA will reduce the risk of cardiovascular disorders.
Monounsaturated fat sources include avocados, nuts, seeds and olives. Peanut, canola and olive oils are additional sources.
PUFA are linoleic acid and Alpha linoleic acid which are essential fatty acids and sources are plant seeds oil. Other PUFAs are omega 3fatty acids and omega 6 fatty acids. Sources of omega 3 fatty acids are bluefish, herring, lake trout, mackerel, salmon, sardines, and tuna. Sources of omega 6 fatty acids are sunflower seeds, Brazil nuts, pecans and pine nuts. Some cooking oils are good sources too, such as corn, sunflower, safflower and sesame oils.
Hydrogenated oils, trans fats and saturated fats are not good for health, so try to reduce the intake of those and replace them with PUFA.

Proteins
Proteins are made up of 20 different amino acids, in which 9 are essential amino acids means they can not be synthesized in the body and we must get them through diet, we require them for our body growth, others are synthesized in the body by transamination. Proteins of animal origin, particularly from eggs, milk and meat, are generally of higher biological value than proteins of vegetable origin. However, when two different vegetable proteins are eaten together (e.g. a cereal and a legume), their amino acid contents are complementary and produce an adequate mix, an important principle in vegan diets.
Proteins in the body are constantly broken down and re-synthesized. Our bodies reuse most of the released amino acids, but a small portion is lost and must be replaced in the diet. The Recommended Dietary Allowance (RDA) for protein for adults is 0.8 g/kg of body weight. Because of their rapid growth, infants have the highest RDA for protein at 1.5 g/kg of body weight. The RDA gradually decreases until adulthood. It increases again during pregnancy and lactation to a level of 1.1 g/kg. The RDA for an adult weighing 140 pounds (63.6 kg) is a mere 51 grams of protein, an amount many of us consume before mid-afternoon. Usually people think that protein rich food is only animal derived, yeah definitely animal origin like meat,salmon, chicken, egg etc are rich in protein but vegetarians also get enough protein through legumes, cereals, beans, lentils, sprouts, nuts, peanut butter, broccoli, multigrain breads, cheese etc. If you eat a variety of foods, you will meet your protein needs.

Please note: Any medical information published on this website is not intended as a substitute for informed medical advice and you should not take any action before consulting with a health care professional

Content source – Davidson’sDavidson’s Principles and Practice of Medicine(22ndedition) http://www.naturalbalancefoods.com/community/dietary-needs/what-are-macronutrients-micronutrients/
http://www.innerbody.com/nutrition/macronutrients