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From http://en.wikipedia.org/wiki/Glycemic_index :

The glycemic index, or glycaemic index, (GI) provides a measure of how quickly blood sugar levels (i.e., levels of glucose in the blood) rise after eating a particular type of food. The effects that different foods have on blood sugar levels vary considerably. The glycemic index estimates how much each gram of available carbohydrate total carbohydrate minus fiber) in a food raises a person's blood glucose level following consumption of the food, relative to consumption of pure glucose. Glucose has a glycemic index of 100.

A practical limitation of the glycemic index is that it does not take into account the amount of carbohydrate actually consumed. A related measure, the glycemic load, factors this in by multiplying the glycemic index of the food in question by the carbohydrate content of the actual serving.

From http://www.ncbi.nlm.nih.gov/pubmed/23803881 :

Compared with an isocaloric low-GI meal, a high-GI meal decreased plasma glucose, increased hunger, and selectively stimulated brain regions associated with reward and craving in the late postprandial period, which is a time with special significance to eating behavior at the next meal.

From http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2584181/table/t1/

Table 1

In this blog post, I share on carbohydrate and diet and nutrition for the health and fitness of people in India and worldwide including related to weight loss and gain. Information is from reputed sources and the references are at the end. 

There are four major classes of biomolecules - carbohydrates, proteins, nucleotides, and lipids. Carbohydrates, or saccharides, are the most abundant of the four. 

Carbohydrates have several roles in living organisms, including energy transportation, as well as being structural components of plants and arthropods.

Carbohydrate derivates are actively involved in fertilization, immune systems, the development of disease, blood clotting and development.

They are called carbohydrates because the carbon, oxygen and hydrogen they contain are generally in proportion to form water with the general formula Cn (H2O)n.

Most organic matter on earth is made up of carbohydrates because they are involved in so many aspects of life, including:

Energy stores, fuels, and metabolic intermediaries.

Ribose and deoxyribose sugars are part of the structural framework of RNA and DNA.

The cell walls of bacteria are mainly made up of polysaccharides (types of carbohydrate).

Cellulose (a type of carbohydrate) makes up most of plant cell walls.

Carbohydrates are linked to many proteins and lipids (fats), where they are vitally involved in cell interactions.

Carbohydrates and nutrition

People think of bread, pasta and rice when they think of carbs, but all these foods are rich in carbs too.

Bread, pasta, beans, potatoes, bran, rice and cereals are carbohydrate-rich foods. Most carbohydrate rich foods have a high starch content. Proteins and fats require more water for digestion than carbohydrates. Carbohydrates are the most common source of energy for most organisms and animals, including humans.

Carbohydrates are not classed as essential nutrients for humans. We could get all our energy from fats and proteins if we had to. However, our brain requires carbohydrates, specifically glucose. Neurons cannot burn fat.

1 gram of carbohydrate contains approximately 4 kilocalories (kcal)
1 gram of protein contains approximately 4 kcal
1 gram of fat contains approximately 9 kcal

However, proteins are used in both forms of metabolism - anabolism (building and maintaining tissue and cells) and catabolism (breaking molecules down and releasing/producing energy). So, the consumption of protein cannot be calculated in the same way as fats or carbohydrates when measuring our body's energy needs.

Not all carbohydrates are used as fuel (energy). A lot of dietary fiber is made of polysaccharides that our bodies do not digest.

Most health authorities around the world say that humans should obtain 40 to 65% of their energy needs from carbohydrates - and only 10% from simple carbohydrates (glucose and simple sugars).

Should I go for a high-carb or low-carb nutritional approach?

Every couple of decades some 'breakthrough' appears which tells people either to 'avoid all fats', 'avoid carbs', 'you can't go wrong with carbohydrates', etc. Carbohydrates have been and will continue to be an essential part of any human dietary requirement for hundreds of years, unless a fundamental mutation occurs.

The obesity explosion in most industrialized countries, and many developing countries, is a result of several contributory factors. One could easily argue for or against higher or lower carbohydrate intake, and give compelling examples, and convince most people either way.

However, some factors have been present throughout the obesity explosion and should not be ignored:

Less physical activity.

Fewer hours sleep each night. A study published in the journal SLEEP4 identified an association with duration of sleep and obesity in both children and adults.

Higher consumption of junk food.

Higher consumption of food additives, coloring, taste enhancers, artificial emulsifiers, etc.

More abstract mental stress due to work, mortgages, and other modern lifestyle factors. A study by scientists from the US and Slovakia, revealed that neuropeptide Y (NPY), a molecule the body releases when stressed, can 'unlock' Y2 receptors in the body's fat cells, stimulating the cells to grow in size and number

In rapidly developing countries, such as China, India, Brazil, Mexico, obesity is rising as people's standards of living are changing. However, for their leaner nationals of a few decades ago carbohydrates made up a much higher proportion of their diets.

Those leaner people also consumed much less junk food, moved around more, tended to consume more natural foods, and slept more hours each night. Saying that a country's body weight problem is due to too much or too little of just one food component is too simplistic - it is a bit like saying that traffic problems in our cities are caused by badly synchronized traffic lights and nothing else.

It is true that many carbohydrates present in processed foods and drinks we consume tend to spike glucose and subsequently insulin production, and leave you hungry sooner than natural foods would.

The Mediterranean diet of the people in Greece or the island of Corfu, with an abundance of carbohydrates from natural sources plus a normal amount of animal/fish protein, have a much lower impact on insulin requirements and subsequent health problems, compared to any other widespread western diet.

Dramatically fluctuating insulin and blood glucose levels can have a long term effect on your eventual risk of developing obesity, diabetes type 2, heart disease, and other conditions. However, for good health we do require carbohydrates.

Carbohydrates that come from natural unprocessed foods, such as fruit, vegetables, legumes, whole grains, and some cereals also contain essential vitamins, minerals, fiber and key phytonutrients.

What happens to sugar levels in the blood?

When we eat food with carbohydrates in them our digestive system breaks some of them down into glucose. This glucose enters the blood, raising blood sugar (glucose) levels. When blood glucose levels rise, beta cells in the pancreas release insulin. Insulin is a hormone that makes our cells absorb blood sugar for energy or storage. As the cells absorb the blood sugar, blood sugar levels start to drop.

When blood sugar levels drop below a certain point alpha cells in the pancreas release glucagon. Glucagon is a hormone that makes the liver release glycogen - a sugar stored in the liver.

In short - insulin and glucagon help maintain regular levels of blood glucose for our cells, especially our brain cells. Insulin brings excess blood glucose levels down, while glucagon brings levels back up when they are too low.

If blood glucose levels are rising too rapidly and too often the cells can eventually become faulty and not respond properly to insulin's "absorb blood energy and store" instruction; over time they require a higher level of insulin to react - we call this insulin resistance.

Eventually, the beta cells in the pancreas wear out - because they have had to produce lots of insulin for many years - insulin production drops and eventually packs in altogether.

Insulin resistance leads to hypertension (high blood pressure), high blood fat levels (triglycerides), low levels of good cholesterol (HDL), weight gain and other diseases. All these illnesses, together with insulin resistance, is called metabolic syndrome. Metabolic syndrome leads to type 2 diabetes.

If over the long-term blood sugar levels can be controlled without large quantities of insulin being released, the chances of developing metabolic syndrome are considerably lower.

Natural carbohydrates, such as those found in fruits and vegetables, legumes, whole grains, etc., tend to enter the bloodstream more slowly compared to the carbohydrates found in processed foods. Good sleep and regular exercise also help regulate blood sugar and the hormone control.

Carbohydrates which quickly raise blood sugar are said to have a high glycemic index, while those that have a gentler effect on blood sugar levels have a lower glycemic index.

The glycemic index (GI)

Carbohydrates enter the bloodstream as glucose at different rates - high glycemic index (GI) carbohydrates enter the bloodstream as glucose rapidly, while low GI carbohydrates enter slowly because they take longer to digest and break down.

A meal with lower GI carbohydrates will raise your blood glucose levels more slowly, and over a longer period - this is better for long-term health and body weight control.

People who are relatively physically inactive (sedentary), and don't sleep at least 7 hours every night are especially vulnerable to the long-term detrimental effects of regular consumption of high GI carbohydrates.

Low GI carbohydrates have the following benefits:

You are less likely to put on weight

Low glycemic diets may be better for weight loss. A diet of foods less likely to spike blood sugar levels helps dieters lose more weight, according to a systematic review from Australia.

You will have better diabetes control

Your blood cholesterol levels will most likely remain healthy

Your risk of heart disease is lower

It will take longer for you to become hungry after a meal

Your physical endurance will improve

Reducing the risk of colon cancer recurrence - if you have colon cancer, there may be a lower chance of cancer recurrence. Colon cancer survivors whose diet is heavy in complex sugars and carbohydrate-rich foods are far more likely to have a recurrence of the disease than are patients who eat a better balance of foods, according to a study published in the Journal of the National Cancer Institute.

How can I switch to a low GI lifestyle?

If you eat cereals for breakfast, switch to oats, barley or bran. Make sure the oats are as natural as possible; milling or grinding can ramp up their GI dramatically.

If you eat bread, only consume wholegrain bread.

Eat plenty of fresh fruit and vegetables.

If you have a fruit juice make sure you eat all of the pulp (the meat of the fruit).

Eat rice with the husk still there (brown rice).

If you eat pasta go for whole grain ones.

Eat plenty of salads.

Cut out all junk foods, processed foods, foods with too many additives (flavorings, emulsifiers, etc).

How processing affects the Glycemic Index of carbohydrates?

Milling and grinding of foods always raise their glycemic index. Unfortunately, the processes often eliminate other nutrients, such as minerals, vitamins, dietary fibers as well, leaving what is often no more than starchy endosperm (the inner part of the seed/grain, mainly starch).

What is the difference between the Glycemic Load and Glycemic Index of a food?

The GI refers just to how quickly a food's broken down glucose enters the bloodstream. This may be misleading sometimes. For example, a melon has a high GI, however most of it is water. The glycemic load (GL) takes into account the GI of the carbohydrate as well as how much carbohydrate that food has.

GL = (GI x amount of carbohydrate) divided by 100.

For example:

An apple has a GI of 40 and contains 15 grams of carbohydrate. GL = (40 x 15) divided by 100 = 6g.

A small baked potato has a GI of 80 and contains 15g of carbohydrate. GL (80 x 15) divided by 100 = 12g.

The GL is a better measure for calculating the metabolic effect of foods - but it may not always lead to best nutrition.

What is the disadvantage of using just the glycemic load?

Dietitians, nutritionists and endocrinologists say that GL is useful for scientists, but GI may be better for lay people, especially those with diabetes. Many low GL foods do not have the right nutrients, such as butter or fatty meats which have the wrong types of fats for good long-term health.

Experts from the University of Sydney's (Home of the Glycemic Index) suggest that lay people should use GI as a tool that allows you to "choose one food over another in the same food group - the best bread to choose, etc." and not to get bogged down with the figures.

Slow carbs matter much more than low carbs. A well balanced diet consisting of good quality foods is as important as physical activity and good sleep.

Physical activity is a key factor in weight control and good long-term health. It is as important as a good diet and good sleep.

Sleeping at least 7 hours each night is as important as a well balanced diet and physical activity.

If you are overweight and you want to lose weight, focusing on slow carbs is useful. A well balanced and nutritional diet, such as the Mediterranean diet, plus good sleep and plenty of physical activity, is much more likely to lead to long-term success and good physical and mental health.

From http://www.medicalnewstoday.com/articles/161547.php