Insulin, one of the body’s most important hormones, is intertwined with many bodily processes that are sure to impact personal training clients and athletes. Our bodies release this hormone primarily in response to the blood sugar jolts experienced following the consumption of carbohydrates (complex or simple), commonly done post-workout as a means of refueling the exercised muscles and promoting glycogen replenishment. Ensuring sufficient glycogen stores helps support the muscle-building process by aiding in protein synthesis. Exploring the physiology surrounding insulin and important factors to bear in mind is relevant information for any fitness professional working with clients.
What is Insulin?
Insulin primarily functions to allow nutrients in the bloodstream to enter body tissues and enables the storing of the body’s fuel sources. It promotes glycogen formation, muscle tissue development, and storage of fat. Even in healthy individuals, any abnormality in insulin secretions stemming from poor eating habits and/or lack of exercise can lead to serious metabolic disorders. Years of inappropriate food choices/portion sizes can result in the development of type 2 diabetes, which medical professionals often see accompanied by obesity or, at the other end of the spectrum, hypoglycemia.
For weight training purposes (and in the interest of general health), undergoing a 6-hour-fasted blood sugar test performed can reveal whether one’s insulin secretions fall within a safe category. As a point of reference, the normal range for fasted blood sugar lies between 70 and 110 mg/dl.
The Role of Excess Sugar Intake
The bloodstream’s storage capacity for glucose caps out at approximately 80 calories (the equivalent of 20 grams of sugar). When recently ingested glucose raises the blood sugar level in excess of this quantity, the pancreas releases insulin into the bloodstream to transport the excess glucose to bodily tissues. We refer to this excess amount of glucose as “insulin-carried”; it must be present for the uptake of glucose by most of the body’s tissues, with the exception of the brain and the liver. The cells in these organs obtain their glucose from another non-insulin-dependent transporter.
If too much insulin-carried blood glucose remains present after the liver and muscle tissue have taken their maximum capacity, the excess gets stored in extra-muscular fat cells. Interestingly, muscle tissues take up insulin-carried glucose very gradually, in contrast to fat cells, which absorb and store this form of glucose very quickly.
Exercise and Insulin Resistance
Type 2 diabetes manifests itself as resistance to the insulin already produced/released by the pancreas. Insulin resistance, or IR, creates additional challenges for trainers and exercise professionals, as it often coexists with other serious health conditions such as obesity, cardiovascular disease, and polycystic ovary syndrome. According to the American Diabetes Association, exercise – both aerobic and resistance — remains the most optimal way to combat IR. Exercise renders the body more insulin-sensitive, fostering the building of muscle tissue that can absorb blood glucose.
Exercise Prescription for IR Clients
When working with insulin-resistance clients, personal trainers should emphasize daily- and weekly-accumulated volume. Experts suggest at least 2.5 hours of exercise per week to help stave off the development of type 2 diabetes, and gradually work up to 4–7 hours per week to achieve and maintain overall well-being. Following a workout, glucose uptake by skeletal muscles can remain elevated for several hours. Many trainers and clients find that breaking daily aerobic sessions into multiple shorter bursts of 10-20 minutes per day may foster insulin sensitivity throughout the day. Since the common comorbidity issues associated with IR can make prolonged exercise challenging to maintain, short bouts of aerobic movement can feel more successful for the client.
Resistance training, too, can dramatically improve both insulin sensitivity and glucose uptake. If the client shows dedication, a trainer could design a program incorporating weightlifting three times per week, focusing on major muscle groups. Since the body’s skeletal muscle accounts for 75–95% of its glucose uptake, we can easily understand why increasing muscle mass can have a significant effect on maintaining blood glucose at a healthy level. Once again, starting out slowly works well; consider designing the initial program around 1 set of 10–15 repetitions, at approximately 40–60% of the client’s 1RM.
Understanding The Science
One large-scale study among 6,561 randomly selected men and women sought to determine a relationship between insulin and weightlifting. Men who never engaged in strength training had 2.50 times the odds of developing IR when compared to those actively involved in heavy bodybuilding. While a strong relationship between strength training and insulin resistance existed for men, the results in women did not reflect this at all.
Why does this occur? One model predicted that during exercise, insulin concentration increases by 30% in the blood plasma and by twice that in the tissues comprising skeletal muscle. Experimental observations corroborated this supposition. Perhaps the significant increase in the surface area associated with exercise-induced capillary recruitment explains why insulin concentrations in these areas increase during exercise, ultimately enhancing insulin-dependent glucose uptake.
Insulin & Amino Acids
Experts have found that exercise promotes an “insulin-like” effect on the muscles, of which natural bodybuilding athletes strive to take full advantage. Intense training opens muscle tissue insulin receptor sites, enabling amino acids to move into the tissue fibers and serve as catalysts/building blocks for repair and growth. Eating only protein, however, does not stimulate insulin release into the blood. Ensuring that insulin will be present for amino acid uptake into muscles (essential for protein synthesis) requires ingesting sufficient amounts of carbohydrates together with complete proteins in order to stimulate the pancreas to make it readily available.
Anabolism occurs most effectively in the presence of a steady insulin supply. As such, serious weight-training athletes typically consume both a moderate-sized pre-workout meal and a reasonably sized post-workout meal. Amino acids remain available for protein synthesis for only about 3-4 hours postprandial.
Coaches and personal trainers therefore advise clients to aim for the ingestion of complete proteins and complex carbohydrates every 3-4 hours. This nutrient combination stimulates the pancreas to make insulin available. Careful timing of meals enables its release and allows anabolism to occur more efficiently. (When adhered to closely, such an exercise/nutrient intake protocol truly negates the use of supplemental insulin, especially when we consider the risks.)
What are Optimal Insulin Secretions?
Ingesting simple sugars causes serum blood sugar levels to rise quickly, resulting in an over-release of insulin. This leads to a drop in serum glucose levels, followed by a similar drop in insulin levels. Understanding and interpreting a glycemic index chart paves the best path to avoid this effect and plan for optimal insulation secretions. The index classifies carbohydrates according to their absorption rates into the bloodstream, on an empty stomach. The glycemic index rates foods on a scale of 0-100. The higher the number, the quicker the rate of absorption.
To avoid an over-release of insulin, bodybuilders select foods with as low an absorption rate as possible. Even though certain “natural healthy foods” such as potatoes and carrots contain simple sugars and rate a bit high on this index, they remain a good food choice when combined with other low-glycemic-index carbohydrates. This will act to moderate the overall rate of absorption. Soluble fiber, such as the kind contained in most natural complex carbohydrates, helps to slow the rate of absorption.
- Mead, J.R., Irvine, S.A., Ramii, D.P. (2002). Lipoprotein lipase: structure, function, regulation and role in disease. Journal of Molecular Medicine, Dec; 80(12):753-69. Epub 2002 Oct 24.
- Jeukendrup, A.E. and Gleeson M. (2010). Sports nutrition: An introduction to energy production and performance, 2nd ed. Human Kinetics, Champaign, IL