Carbohydrates are always carbon, oxygen and hydrogen containing molecules. They vary in structure and can be easily classified into simple carbohydrates or complex carbohydrates. Simple carbohydrates are easily absorbed and are often referred to as sugars. Technically speaking, all carbohydrates eventually end up as a type of sugar in the blood stream to be later used as energy.
A more technical name for a sugar is a saccharide, the simplest of all being a monosaccharide. A monosaccharide is a single sugar unit and when combined together in different amounts or with other monosaccharides make larger units that are required by the body in different amounts. Examples of monosaccharides are:
A disaccharide refers to two monosaccharides joined together. Examples of these are:
Poly – is a prefix to mean many, which is why polysaccharides refer to many monosaccharides joined together to form substances like starches or fibres. Due to the complex nature of polysaccharides these are often called complex carbohydrates and are also more difficult to digest. Starches are still digested down to their smallest component of a monosaccharide whereas fibre can be either soluble or insoluble (see the section later explaining this in more detail).
The main function of a carbohydrate, irrespective of their structure is to provide energy to the body and to provide a storage form of energy for the liver and muscles. Carbohydrates also regulate digestion and final utilisation of protein and fat.
How are Carbohydrates digested, absorbed and used in the body?
Simple sugars (or monosaccharides) are very easily digested, they are what we call a high glycaemic (GI) food (see GI table detailing what constitutes a simple sugar in the High GI section). Polysaccharides are more complex and require more time to be fully digested; they are called low GI foods. The glycaemic index refers to the rate at which a carbohydrate containing food is absorbed into the blood stream. Mixed meals of a carbohydrate with a protein and/or a fat containing food will also affect the GI of the food.
The GI of a food can be either low GI, intermediate GI or high GI. Foods are all tested and classified as below:
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Fibre in the Diet
Fibre in the diet is a form of complex carbohydrates and can either be soluble (able to dissolve in water and forms a gel like substance) or insoluble (unable to be dissolved in water). Both are not able to be absorbed however they provide essential fuel for bacteria in the gastrointestinal tract. They have many other benefits and a healthy diet should contain between 25 – 35g of fibre per day.
- Provides bulk to the stool to improve the satiety (feel of fullness) value of food.
- Controls blood sugar levels by slowing down absorption of foods.
- Prevents constipation and regulates bowel movements.
- Plays a vital role in binding cholesterol and thereby protecting the heart from heart and artery blockages.
- Delays gastric emptying of foods, also improving the satiety value of foods.
- Regulates blood sugar levels, therefore plays a vital role in diabetic management as well as weight control.
- Prevention of heamorrhoids
- Reduction of colorectal cancer
An important factor of a high fibre diet is that it must be combined with a high intake of fluids, to prevent constipation.
Ideally a diet that is between 45 – 65% of total energy contributing from carbohydrate foods is recommended. The variation depends on a person’s physical activity level and individual requirements. This is approximately 6 – 8g/kg/day. Majority of the carbohydrates in the recommended diet should come from complex carbohydrates.
What is a carbohydrate?
Most baked goods (including bread, roti, cupcakes, rolls, Etc.) made with white flour, sweets, chocolate, cooldrinks, fruit juices, fudge, honey, sugar (white and brown), pasta (made with white flour), certain low fibre cereals.
Vegetables, Brown rice, Buckwheat, beans, lentils, sweet potato, baby potatoes, oats, oat bran, soya beans, whole wheat products, quinoa, and chickpeas.
The use of carbohydrates in exercise and performance
The general public is urged to always follow a low GI rule however during and after exercise, a higher GI is recommended to assist with sports performance. A low GI diet ensures sustained energy levels, sportsmen included should consume low GI e – 2 hours prior to exercise and then continue to eat low GI foods a few hours post an exercise session or competition. Depending on the intensity and duration of an exercise session the need for a high GI food is often there. A session lasting longer than 2 – 3 hours often indicates a higher GI food is necessary. Energy stored in the muscle as glycogen is a stored form of the monosaccharide, glucose. There is a maximal storage amount that can be stored in the muscle, which is why at a certain point in an exercise session lasting longer than 2 – 3 hours this glucose needs to be replaced by something that is easier to absorb.
To take advantage of the GI effect during exercise a sports person should aim to do the following:
Eat/drink low GI before the event:
Low GI foods and drinks assist in sustained release and are slow release foods. They are digested slowly and will still supply energy 1-2 hours after consumption. If a low GI product is consumed 1-2 hours prior to competing, blood glucose will be maintained at a healthy level for the duration of the activity or sporting event.
During the event/exercise session:
For any exercise or competition that lasts longer than 90 minutes, a higher GI (Intermediate for Diabetics) food and drinks would need to be taken. If the duration of the exercise is less than 90 minutes, then a low GI food/drink that was taken before should suffice to maintain blood glucose levels at a healthy level.
It is now very important to eat at least 1 – 1.5 g carbohydrate/kg bodyweight of a high GI carbohydrate food or drink within the first 30 to 60 minutes of completing exercise. (Intermediate for Diabetics, unless competing for 2-3 hours). Why this is important is because the muscles used are able to still absorb glucose from the blood and the window of opportunity for this to happen optimally is between 30 – 60 minutes post exercise. The rate of replenishment of glycogen (stored glucose) occurs the fastest if a high GI product is consumed as soon after exercise as possible.
After about 1 – 2 hours post the event, the meal can still contain higher GI carbohydrates although at the following meal there should only be low GI carbohydrate sources and combined with a protein and fat source.
A technique often used by certain sports men and women is called carbo-loading. Whereby a strategic plan of consuming a high carbohydrate diet to force as much stored glycogen into the muscles as possible for use during the future exercise session or competition. This typically needs to happen at least 3 days prior to competition. This technique is not for everyone to use and it is also important someone tries it out prior to an event in case there are any adverse effects experience (such as diarrhea) on event day. All sports men and women should experiment continually with their diet to determine what works best for them. Athletes should also drink sufficient water at this time and to note that carbo-loading is working, a gain in weight should be observed. If someone is resting during a period of carbo-loading, only low GI foods should be used.
Carbohydrates and Weight Loss
Carbohydrates are often unnecessarily eliminated from the diet due to the belief that carbohydrates make you fat. In reality the total energy of diets has changed and have increased, along with the increased intake of convenience meals and meals higher in processed and refined carbohydrates. By using the best quality of carbohydrates, evenly distributed throughout the day, and when combined by a balanced intake of protein and fats it is possible to alter weight and body composition. There is no need to reduce or eliminate carbohydrates to experience similar results.
Weight loss is caused when there is a calorie deficit and any nutrient when eaten in larger than required amounts will result in weight gain. When carbohydrates are eliminated from the diet, glycogen from muscles are bound to water thereby decreasing stored water in the body. This loss of glycogen as well as water can result in high weight loss initially. Also, with a higher protein and fat diet, the kilojoule intake is also reduced due to the high satiety effect that proteins and fats have. These two factors contribute to the initial large weight loss experienced on a low or no carbohydrate diet however results after a year are the same as that of someone who has followed a balanced diet with moderate intakes all macronutrients.
Carbohydrate Properties & Review
- One gram carbohydrate provides 17kJ (4kcal).
- Carbohydrates can control blood sugar levels when low GI carbohydrates (or complex) carbohydrates make up the bulk of the carbohydrate intake.
- They spare protein in the diet to be used for muscle building
- They are the preferred source of energy in the body
- The diet should be made up of 45 – 65% of energy coming from carbohydrates, depending on physical activity levels.
- Fibre is an important carbohydrate and is needed in amounts ranging from 25 – 35g per day.
- Part of the central nervous system (the brain) relies only on glucose obtained from the diet to assist in functionality.
- Calculation of carbohydrate needs should be calculated after fat and protein needs are calculated and met.