Carbohydrates

The European Food Safety Authority (EFSA) categorises carbohydrates into two groups. Glycaemic carbohydrates are digested and absorbed in the small intestine, and dietary fibre which cannot be absorbed and pass through to the large intestine (EFSA, 2017).  Glycaemic carbohydrates can be further classified into monosaccharides, disaccharides, oligosaccharides, trisaccharide’s, tetrasaccharides and polysaccharides including starch (Mondal, 2017).  The first part of the word tells you how many glucose molecules are in that carbohydrate.  For example, monosaccharide mean one glucose molecule, di-saccharide mean s two and so on.

Monosaccharides are simple sugars which cannot be further broken down into smaller sugars (Monosaccharides (Simple Sugars) Definition, List, Examples of Foods, 2014).  They include glucose, fructose and galactose. They can be found in sources such as boiled sweets, honey, fruit, vegetables and milk and they are absorbed rapidly by the body (Simmel and Kraft, 2018)

Disaccharides are composed of two monosaccharides linked together to produce a two sugar molecule (Mondal, 2017).  They include sucrose which is composed of a fructose and a glucose molecule.  Lactose which is composed of a glucose and a galactose, and maltose which is two glucose molecules (14.6: Disaccharides – Chemistry LibreTexts, 2019).  These can be found in dairy foods and foods made with milk (lactose), table sugar, any food with added sugar (sucrose).  Maltose is present in starchy foods because starch is broken down to maltose by the enzyme amylase, these foods include cooked potatoes, malt products, and food where starch is fermented by yeast such as sourdough bread and beer (Ipatenco, 2018)

Oligosaccharides are sugar made up of 2-9 monosaccharides linked together (Mondal, 2017).  Trisaccharide’s are composed of three monosaccharides. Tetra saccharides are composed of four monosaccharides (Mondal, 2017).  As the number of monosaccharides increases, the longer it takes for the body to breakdown and absorb the carbohydrate (Simmel and Kraft, 2018). 

Glycogen, Starch and Cellulose are polysaccharides, meaning they are made up of many monosaccharides linked together, and so take a long time for the body to breakdown.  Glycogen is the storage form of carbohydrate in the body.  Starch is the storage form of carbohydrate is plants, and cellulose which is a structural component of plants and acts as dietary fibre (Polysaccharide | chemical compound, 2019) .

Starch can be obtained from cereal grains such as corn, oats, wheat, barley and rice, and their products, legumes, potatoes and other root and tuber vegetables (Polysaccharides Definition, List, Functions, Food Examples, 2014).

Glycogen is a source of fuel for muscular activity.  The body stores glycogen in the liver and muscles.  An average of 80g of glycogen can be stored in the liver, this increases after meals and decreases after exercise or at night (Murray and Rosenbloom, 2018).  Glycogen in the liver is used to help regulate blood glucose levels (Röder et al., 2016).  A hormone produced by the α-cells of the islets of Langerhans in the pancreases called glucagon causes glycogen to be released into the bloodstream as glucose.  This will maintain blood sugar levels to a normal rate of 4-6mmols/l.  Insulin is another hormone released by the β-cells of the pancreases.  It acts as a key to the cells so the glucose in the bloodstream can enter the cells and provide energy.  People suffering from diabetes don’t produce insulin or glucagon.  They must inject insulin or use a continuous insulin pump.  If a diabetics sugar drops (hypoglycaemia), they must take sugar orally, or if they are unconscious, they are given a glucagon injection so glycogen can be released from the liver (Diabetes, 2018).  Muscle can store more glycogen, an average of 500g with a range between 300-700g (Murray and Rosenbloom, 2018). For every 1g of glycogen stored, 3ml of water is stored (Fernández-Elías et al., 2015).  Glycogen can be used as a fuel for exercise in both aerobic and anaerobic exercise.  It is important for athletes to restore muscle glycogen after training or performance.  This requires consuming a sufficient amount of carbohydrates soon after training or performance (Murray and Rosenbloom, 2018). The process of glycogen restoration begins during the first hour of recovery (Herman and Kahn, 2006).  The uptake of glucose into the muscle takes place through glucose transporters called GLUT4 (Herman and Kahn, 2006).  GLUT4 are stored in small vesicles in the muscles.  During physical activity they relocate to the cell membrane where they transport glucose into the muscle cell.  After physical activity they slowly recede back into the vesicle during the first hour post exercise, so the earlier carbohydrate is consumed, the better the glycogen restoration will be (Herman and Kahn, 2006).  

The Glycaemic index is used as an indicator of the speed at which a carbohydrate is absorbed into the bloodstream (Simmel and Kraft, 2018).  The rate at which the glucose is absorbed into the bloodstream is set at a reference value of 100 (i.e. pure glucose would be 100).  A high GI food is absorbed rapidly into the bloodstream, this causes a large spike in blood sugar levels and insulin release.  Blood glucose levels will drop fast causing a “sugar crash” (Simmel and Kraft, 2018).  A low GI food is absorbed slowly into the bloodstream, it causes a small, gradual increase in blood glucose levels.  This will give a longer lasting energy supply (Simmel and Kraft, 2018).  A number of factors affect the GI of foods such as the amount of food eaten, how it is prepared and the composition of the food, if it contains other nutrients such as fat or protein this will slow the absorption (Simmel and Kraft, 2018).  Chocolate is a low GI food because of its fat content.  The glycaemic index of milk chocolate is 42, and for dark chocolate it is 23 (All About Chocolate, 2016).  There is some evidence showing a low GI diet can help weight loss and lower the risk of CVD and type 2 diabetes (Glycemic index diet, 2019).  Below are some examples of low, medium and high GI foods;

Table 1: Examples of Low, medium and high GI foods (low is ≤55, medium is 56-69, high is ≥60 (About Glycemic Index, 2017)) (Publishing, 2018; Simmel and Kraft, 2018)

Low GI	pasta, corn tortilla, barley, Wholegrain rye bread, sweet potato (raw), sweetcorn, carrots, salad veg, green peas, cherries, plums, peaches, apples, pears, bananas, kiwi, dried apricots, oranges, milk (full fat and skimmed), ice-cream, yoghurt, soy milk, chocolate (milk and dark), peanuts, soy beans, lentils, kidney beans, chickpeas, Millet and oat porridge
Medium GI	Muesli, raw pineapple, new potatoes (fresh and canned), beetroot, mango, sultanas, sweet potato (boiled), pumpkin (boiled), popcorn, potato crisps, chips (made from potato), wholemeal bread Weetabix, instant oats porridge
High GI	Cornflakes, white bread, white rice (boiled), potatoes (boiled), rice milk, rice crackers, boiled sweets

The EFSA recommends that the total  Carbohydrate intake should be 45-60 E% (EFSA, 2017). 

Dietary fibre is another important form of carbohydrate.   It is not absorbed by the gut, but has many beneficial properties essential for good health (EFSA, 2017).  Dietary fibre includes non-starch polysaccharides, cellulose, hemicellulose, pectin and hydrocolloids (EFSA, 2017).  It is recommended that a minimum of 25g of fibre per day should be consumed (EFSA, 2017).  There are three types of dietary fibre;

Soluble fibre is found in fruit, vegetables, oats, barley and legumes.  It slows the emptying of the stomach so you stay fuller for longer, this may assist it weight loss.  It also lowers LDL cholesterol (Nutrition Australia Victorian Division, 2014). 

Insoluble fibre absorbs water in the colon, this helps prevent constipation.  Insoluble fibre can be found in Wholegrain bread and cereals, seed, nuts, wheat bran and fruit & vegetable skins.  It is important to drink enough water when increasing your intake of insoluble fibre as it could actually cause constipation and intestinal discomfort (Nutrition Australia Victorian Division, 2014).  To find out more about fibre and its many health benefits check out my blog Facts on Fibre.

Resistant starch  also has many health benefits (Gunnars, 2018).  Studies have shown it improves insulin sensitivity (Johnston et al., 2010).  It helps to lower blood sugar levels after meals (Maziarz et al., 2017).  It can help aid weight loss as it gives a feeling of fullness (Bodinham et al., 2010), and it provides ½ the calories of normal starch at 2Kcals per gram (Gunnars, 2018).  One of the main benefits Resistant starch provides is that is acts as a food source for friendly gut flora in the colon, and is referred to as a ‘prebiotic’ (Englyst et al., 1996).  Most food only feed 10% of gut flora, whereas fermentable fibres and resistant starches feed 90% (Macfarlane et al., 2006; Brown et al., 1997).  Resistant starch has many health benefits with regards to colon health.  It reduces the pH of the colon, decreases inflammation and greatly decreases the risk of developing colon cancer, which is the 4^th most common cause of cancer deaths in the world (Hylla et al., 1998; Zimmerman et al., 2012).   Resistant starch can be found in undercooked pasta, under ripe bananas, cooked and cooled potatoes and rice (Nutrition Australia Victorian Division, 2014).

The RDA for carbohydrates based on the food pyramid depends on factors such as age, gender and activity levels (Gov.ie – The Food Pyramid, 2019).  Below is a table showing the RDA for carbohydrates based on these factors:

Table 2: Recommended number of servings of carbohydrate foods based on age, gender and activity level (Gov.ie – The Food Pyramid, 2019)

Sedentary	Active
	Teenagers 13-18	Adults 19-50	Adults 50+		Children 5-12	Teenagers 13-18	Adults 19-50	Adults 50+
Male	4-5	4-6	4	Male	3-5	5-7	4-5	3-4
Female	3	3-4	3	Female	3-4	4	4-5	3-4

However, these recommendations do not cover serious athletes who are completing high intensity training regularly.  There are different RDA’S for carbohydrate which an athlete should consume.  They mainly depend on the type of physical activity, its intensity and how long it lasts.  One journal recommends professional dancers consume 4-5g/kg of body weight in order to maintain weight (Sousa et al., 2013).  The International Association for Dance Medicine and Science recommends dancers consume 55-60 E% of Carbohydrates (Nutrition Resource Paper, 2016), this is more than the 40-60 E% recommended by the EFSA for non-athletes.  The Academy of Nutrition and Dietetic Canada gives a clearer recommendation on how much carbohydrate should be consumed depending on the intensity of the activity.  These recommendations are shown in table 3 below;

Table 3: RDA’s for athletes for carbohydrates depending on exercise intensity (Academy of Nutrition and Dietetics et al., 2016)

Intensity of exercise	Description of activity	RDA
Low	Low intensity or skill-based activities	3-5g/kg/d
Moderate	Moderate exercise programmes (1 hour a day)	5-7g/kg/d
High	Endurance programme (1-3hr/d moderate-high intensity exercise)	8-10g/kg/d
Very High	Extreme commitment (>4-5hr/d moderate-high intensity exercise)	8-12g/kg/d

In conclusion, like fats carbohydrates are a bit complicated.  You need them but you need to be sure you eat the right types.  Avoid eating too much refined carbohydrates such as white bread, white pasta and rice, but don’t go overboard with the high fibre foods either.  If you do you may experience digestive discomfort as well as not absorbing enough calories.  Listen to your own body and slowly increase your fibre intake.  Stay away from sugary pastries, biscuits cakes as they are usually full of saturated fat as well as sugar.  Sweets, soft drinks and energy drinks are full of what’s called empty calories, meaning they contain a load of sugar and no other nutrients, so stay away from them as much as you can.

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