Let's Talk Carbohydrates
An Important Fuel Source
Carbohydrates are one of three main macronutrient groups found in foods. Despite often being falsely portrayed as the 'bad guy' to those looking to lose body weight, carbohydrates play a pivotal role in supplying energy to the working muscles!
Let's strip it down to the basics. To maintain the contraction of skeletal muscles during exercise, active muscle cells require a constant supply of adenosine triphosphate (ATP). Note that ATP is used to fuel all metabolic reactions, often referred to as the energy currency within cells.
When ingested, carbohydrates (CHO) are broken down into glucose molecules. During exercise at intensities greater than approximately 60% maximal oxygen consumption, blood glucose and muscle glycogen (essentially glucose stored as glycogen) are the primary fuels oxidized to produce the ATP (Murray and Rosenbloom, 2018).
As a result carbohydrates are still regarded as the most important macronutrient for endurance exercise performance!
When glucose levels provided by carbohydrates are in excess of what the body requires for immediate energy, it’s converted to glycogen and stored in the muscle and liver.
The following amounts of carbohydrate are stored within the muscles and liver in the form of glycogen.
Liver – 80-110g
Muscle – 12-16g per kg of body weight
Effect of Insulin On Blood Glucose levels
Insulin is a hormone produced by beta cells in the pancreas, the secretion of which increases as blood glucose levels rise. Insulin is largely responsible for stimulating glucose uptake and the conversion of glucose to glycogen for storage in the muscles and liver, thus reducing blood glucose levels (Cahill, 1971).
Effect of Glucagon On Blood Glucose Levels
Glucagon is produced by alpha cells in the pancreas. The secretion of which is stimulated by low blood glucose levels. When blood glucose levels are low, glucagon is secreted which results in the breakdown of glycogen back into glucose to be released into the blood stream. (Jiang and Zhang, 2003).
Why do we need glycogen stores?
There is only 15-25g of free glucose present in the body, 5g of which circulates in the blood (Flatt, 1995). As free glucose supplies quickly become depleted during exercise, the muscles then take glucose from the blood whilst the liver simultaneously breaks down glycogen to deposit more glucose back into the blood in order to keep levels high.
The Glycaemic Index (GI)
The glycaemic index (GI) classifies types of carbohydrates based on how fast they cause an increase in blood glucose concentrations once digested on their own (Jenkins et al., 1981). Often foods are classified on a scale of 0-100.
Low GI – 55 or less
Medium GI – 56 - 69
High GI – 70 - 100
Low GI Carbohydrates
Low GI carbohydrates take the body longer to break down than high GI carbohydrates. Upon the digestion of low GI carbohydrates we observe a slower, more gradual rise in a person's blood sugar levels compared to that of moderate or high GI foods (Little et al., 2009).
As blood sugar levels rise gradually, a huge insulin surge followed by a subsequent fall in blood glucose levels to a below ‘normal’ level is avoided (Little et al., 2009).
Examples Of Low GI Foods
- Basmati rice
- Sweet potatoes
- Wholegrain bread
- Rolled Oats (NOT INSTANT)
- Whole milk
- All bran
High GI carbohydrates
High GI carbohydrates are very easy for the body to quickly break down. Upon the digestion of high GI carbohydrates we therefore observe a rapid increase in blood glucose levels, alongside an accompanying insulin surge.
This huge insulin surge then results in the rapid uptake of glucose to the muscles, which can then cause blood glucose levels to drop below what would be considered as a ‘normal’. This is often referred to as reactive hypoglycaemia and can be detrimental to exercise performance (Little et al., 2009).
Examples Of High GI Foods
- White bread
- Ice cream
- Instant Oatmeal
- Jasmine rice
- Sugary drinks
For more information on carbohydrate fuelling and the appropriate consumption of low / high GI carbohydrates to optimise performance stay tuned for our next article!
Cahill, G.F., 1971. Physiology of insulin in man: the Banting Memorial Lecture 1971. Diabetes, 20(12), pp.785-799.
Flatt, J.P., 1995. Use and storage of carbohydrate and fat. The American journal of clinical nutrition, 61(4), pp.952S-959S.
Jenkins, D.J., Wolever, T.M., Taylor, R.H., Barker, H., Fielden, H., Baldwin, J.M., Bowling, A.C., Newman, H.C., Jenkins, A.L. and Goff, D.V., 1981. Glycemic index of foods: a physiological basis for carbohydrate exchange. The American journal of clinical nutrition, 34(3), pp.362-366.
Jiang, G. and Zhang, B.B., 2003. Glucagon and regulation of glucose metabolism. American Journal of Physiology-Endocrinology And Metabolism, 284(4), pp.E671-E678.
Little, J.P., Chilibeck, P.D., Bennett, C. and Zello, G.A., 2009. Food for endurance-the evidence, with a focus on glycaemic index. CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 4(058), pp.1-13.
Murray, B. and Rosenbloom, C., 2018. Fundamentals of glycogen metabolism for coaches and athletes. Nutrition reviews, 76(4), pp.243-259.