Diabetes is a metabolic disorder that impairs the body’s ability to either produce or effectively use insulin, a key hormone for moving glucose from the bloodstream to the working muscles. Diabetes is currently ranked as the seventh leading cause of death in the United States, with type 2 diabetes accounting for 90 to 95% of currently diagnosed cases. See how exercise can improve insulin efficiency along with exercise program recommendations for diabetic clients. Diabetes Mellitus (DM) represents a group of metabolic diseases that are collectively characterized by hyperglycemia, a condition defining elevated blood glucose levels attributed to some defect in insulin production or in insulin recognition by cells. Although most diagnosed cases are either type 1 or type 2, four types of diabetes exist based upon their etiologic causes (1).
- Type 1 Diabetes is caused by factors that include our own autoimmune destruction of the insulin-producing (beta-) cells within the pancreas, or due to some genetic variant or inherited gene, and accounts for approximately 5 – 10% of all diagnosed cases (2). These individuals with type 1 diabetes rely upon exogenous insulin (i.e., pumps, injections) and are more prone to ketoacidosis, a potentially life-threatening condition. Ketoacidosis is a metabolic state associated with inadequate ketone regulation (i.e., excessive accumulation) in the blood that lowers blood pH substantially – it reflects a build-up of incompletely metabolized fatty acids or amino acids due to a lack of available carbohydrates in the cell which are generally needed to metabolize these substrates as fuel.
- Type 2 Diabetes is attributed to an increase in insulin-resistance at the cellular level or due to a gradual secretory deficit from the pancreas, and accounts for approximately 90 – 95% of all diagnosed cases. The causes are varied and sometimes unknown, but dietary choices (e.g., sugar), inactivity and obesity, especially excessive abdominal fat, have all been verified as triggers. Some type 2 diabetics may need exogenous insulin.
- Gestational Diabetes is a condition that approximately 9 – 10% of pregnant women develop around the 24th week of pregnancy due to changing hormonal levels needed to support fetal growth that impairs insulin function. The mother is unable to produce adequate levels of insulin needed to promote normal glucose uptake in the cells and therefore relies upon exogenous insulin (3). For most, this condition corrects itself post-partum.
- Diabetes due to other specific origin (e.g., drug induced, environmental toxins) – given the idiopathic (unknown) nature of some diabetes cases, some causes of diabetes remain unexplained and attributed to a possibility of events.
An estimated 29.1 million Americans have diabetes, representing 9.3% of our population, but recent emphasis on screening, early detection and preventative measures have decreased the number of new cases each year from 1.9 million (2010) to under 1.7 million cases (2012) (4). Unfortunately, the prevalence of this disease in seniors aged 65 and older remains very high at 25.9% of that age group (11.8 million people), and with this age group (i.e., Baby Boomers) continuing to expand, this is becoming a growing concern. Diagnosed cases of diabetes however, do not include individuals identified with prediabetes, a condition characterized by blood glucose levels that are higher than normal, but not high enough for a diagnosis of diabetes. This also explains those individuals who may have little-to-no capacity to produce/secrete insulin, yet do not have any genetic variant, or the autoimmune antibodies to the beta-cells in the pancreas. People with prediabetes (aka impaired glucose tolerance, impaired fasting glucose) demonstrate an increased risk for developing diabetes as well as many forms of heart disease. It is estimated that 86 million Americans age 20 and older currently have this condition, and regrettably this number has increased from the 79 million in 2010 (3).
Health Risk Screening:
For any individual contemplating starting an exercise program, a thorough health risk assessment (HRA) that includes a screening for diabetes and pre-diabetes is essential to safeguard against potential harm. Any person with a previous diagnosis of diabetes is automatically classified as a high risk individual according to the American College of Sports Medicine (ACSM) disease risk stratification criteria (high risk = individual with a known or medically-diagnosed cardiac, pulmonary, metabolic or renal disease). This risk categorization does not apply to a person with prediabetes who might only be classified as a moderate risk. High risk individuals require a medical exam and medical clearance prior to initiating any moderate- or vigorous-intensity exercise program:
- Moderate-intensity = 40 – 59% VO2R* or 3 – 5.9 METs**
- Vigorous-intensity = ≥ 60% VO2R or ≥ 6 METs
* VO2R or %VO2 Reserve is defined as the difference between resting and maximal VO2, and correlates better to % Heart Rate Reserve (%HRR) which is considered a more accurate and appropriated predictor of exercise intensity than % Maximal Heart Rate (%MHR) (5)
** 1 MET is defined as the amount of oxygen consumed while sitting at true rest and is equal to 3.5 ml O2 per kg body weight per minute (3.5 mL/Kg/min) (6).
High risk individuals also require supervised exercise testing before beginning any moderate- or vigorous-intensity exercise program. The term ‘supervised’ includes administration of testing by a non-physician health care professional trained in clinical exercise testing and working under the supervision of a licensed practitioner (e.g., doctor). In 2014, ACSM also released additional recommendations that any individual with diabetes, regardless of how well their condition is managed, who qualifies with at least one of the following should also undergo exercise testing before initiating any physical activity (1):
- Over 35 year of age, or
- Diagnosis of type 2 diabetes for over 10 years, or
- Diagnosis of type 1 diabetes for over 15 years, or
- Elevated total cholesterol score > 240 mg/L (6.62 mmol/L), or
- Elevated systolic blood pressure > 140 mm Hg or diastolic blood pressure > 90 mm Hg, or
- Current smoking, or
- Family history of coronary artery disease within a first degree relative (father, mother, brother, sister, son, daughter), or
- Presence of microvascular disease, or
- Autonomic neuropathy.
An individual with prediabetes is not a high risk, but may qualify as a moderate risk pending other compounding risk factors (e.g., blood pressure, lipid profile). This person is able to initiate a moderate-intensity exercise program without a prior medical exam and can also do so without any exercise testing. However, before initiating any vigorous-intensity exercise programming, they must undergo a medical exam beforehand.
Various criteria are used to detect the presence of diabetes or prediabetes and are presented in Table 1-1 below. While fasting blood glucose continues to remain the most popular diagnostic tool, it is limited by the fact that it only represents one timestamp of an individual’s day, albeit it an important timestamp reflecting levels after an overnight fast when they should be very normal. Glycosylated hemoglobin (HbA1C) is a newer measure which is a form of hemoglobin that reflects the average blood glucose concentration over a prolonged period of time (i.e., 2 – 3 months). Sustained periods of elevated blood glucose results in greater amounts binding with hemoglobin molecules, which can become a cause of many negative health events like inflammation, diabetes, cardiovascular and kidney disease (nephropathy), and retinal eye damage (retinopathy). HbA1c scores under 5.7% are considered healthy; scores between 5.7 and 6.4% are associated with higher risks for developing diabetes; while scores at 6.5% or higher indicate diabetes (1). An initial goal for diabetics is to maintain HbA1C levels below 7%. Some health experts believe HbA1C is a more appropriate diagnostic tool given how it examines sustained blood sugar levels.
Table 1-1: Diagnostic Blood Glucose Levels for Prediabetes and Diabetes
|Status||Fasting (8-hour) Blood Glucose|
|Healthy Scores||Less than 100 mg/dL (less than 5.55 mmol/L)|
|Prediabetes *||100 – 125 mg dL (5.55 – 6.94 mmol/L)|
|Diabetes **||Greater than 125 mg/dL (greater than 6.94 mmol/L)|
* Also diagnosed using an oral glucose tolerance test (OGTT) where 2-hour impaired glucose tolerance score ranges between 140 and 199 mg/ dL.
** Also diagnosed using an oral glucose tolerance test (OGTT) where 2-hour impaired glucose tolerance score is equal or greater than 200 mg/ dL.
Managing diabetes effectively requires a structured lifestyle program that includes education, dietary modification and regular physical activity to improve overall health (prevent and/or treat diabetes complications, dyslipidemia, cardiovascular disease, hypertension, and nephropathies). For type 2 diabetics, it should also aim to target an initial weight loss of 5 to 7% (a primary recommendation for many as obesity is a contributing factor). For type 2 diabetics and individuals with prediabetes, regular physical activity improves insulin sensitivity; glucose tolerance and uptake; and reduced HbA1C levels. For type 1 diabetics or any type 2 diabetics requiring insulin, regular physical activity and the subsequent improvement to insulin sensitivity does not positively impact pancreatic function, but it does reduce the quantities of exogenous insulin required.
When programming for diabetics or even individuals with prediabetes, health and fitness professionals should adhere to the following basic FITT guidelines (1, 2):
- Frequency: As exercise frequency is critical to improving insulin sensitivity, these individuals should aim to perform some activity between 3 to 7 days a week (1). The American Diabetes Association (ADA) recommends aerobic activity a minimum of 5 days a week. As the effects of improved insulin sensitivity may only last 24 – 48 hours between sessions, this helps explain the need for greater frequencies (i.e., frequency equals improved insulin sensitivity). Program with no more than 48 hours between each activity bout. As many type 2 diabetics are often deconditioned and overweight, they may struggle to complete exercise 5 to 7 times a week. Fitness practitioners therefore, should implement a manageable and systematic plan that considers, and fits the abilities and lifestyle of their clients first.
- Intensity: Moderate-intensity exercise is probably more appropriate for these individuals, especially for overweight or obese individuals. ACSM recommends initially maintaining intensities between 40 – 59% VO2R (40 – 59% HRR) or an RPE between 11 and 13 (Borg 6 – 20 scale) which constitutes moderate-intensity (1). Gradually, they recommend greater emphasis on more vigorous-intensity activity, especially if weight loss is a goal. The ADA recommends moderate-to-vigorous intensities and defines moderate as levels where you can talk, but not sing, whereas intense activity is defined as a level where you can only speak in short phrases before needing a pause to take a breath (2). Newer research utilizing lower-volume, higher-intensity exercise like HIIT training however, also demonstrates improvements in diabetic patients (7). In their study, Gibala and colleagues had subjects perform 10 sprints, each lasting 60 seconds at 90% of their measured maximal heart rate, alternating with 60s of recovery between each interval. This protocol was performed three time a week for a total of 2 weeks and showed positive results. Professionals should always remember however, that this form of exercise with high-risk individuals should only be performed under proper medical supervision.
- Duration (Time): Considering how most individuals will participate in moderate-intensity activity, ACSM recommends a minimum of 150 minutes of accumulated activity every week, with each bout lasting no less than 10 minutes in duration (1). ADA also recommends a total of 150 minutes a week, but suggests it should be spread over a minimum of 3 days each week (2). Benefits are considered to be dose-related, implying that a greater accumulation of time provides additional health benefits. This is certainly more relevant to the type 2 diabetic than the type 1 diabetic who needs to improve insulin sensitivity. However, as illustrated in the study by Gibala and colleagues, lower-volume, higher-intensity (i.e., 30-minutes of intervals a week) is also effective.
- Type: Conventional wisdom suggests rhythmic and continuous activity like cardio that emphasizes the larger muscle groups as this supports improved insulin sensitivity, but newer research also supports interval-type activities as well (e.g., sprint intervals, resistance training). Resistance training should only be recommended in the absence of any contraindications that might be exacerbated by this modality (e.g. neuropathies). Resistance training should be performed at least 2 times a week and any formats that can simultaneously provide some cardiorespiratory overload is preferred (e.g., circuit-style format). However a combination of both cardio and resistance training is also recommended for diabetic patients. For example, the individual could perform 10-muinutes of moderate-intensity cardio, followed by a 5-10 minute resistance training circuit, then repeat this cycle throughout their training session. The resistance training program should emphasize 8-10 major muscle groups in circuit format if training fewer than 3 times a week, or become more regionalized (i.e., grouping muscles or body segments) when training more frequently.
Hypoglycemia during exercise is a significant concern, especially with those taking insulin or oral hypoglycemic agents that increases insulin secretion and pushes excessive amounts of glucose to enter the cell. Hypoglycemia is defined as a blood glucose level below 70 mg/dL (< 3.80 mmol/L). Symptoms of hypoglycemia include shakiness, dizziness, sweating, headache, sudden mood or behavior changes, tingling of mouth and fingers, clumsiness, mental confusion, and hunger. These individuals should follow a practice of monitoring blood glucose before and for several hours following exercise. Exercise timing, exercising under proper supervision or with a partner, reducing insulin dosages, or even temporarily disconnecting insulin pumps should all be considered as advised by the individual’s doctor. Furthermore, increasing carbohydrate consumption before exercise to prevent hypoglycemia during and following exercise, and also keeping a small supply of high-glycemic load foods nearby is suggested (8). When blood glucose needs to be quickly raised, recommend the consumption of 15 – 20 grams of a high-glycemic sugar or carbohydrate food (e.g., sports drink, sweets).
Conversely, hyperglycemia can become a concern for individuals who are not in glycemic control with their diet and medication. This rarely occurs in healthy individuals because of hormonally-mediated responses, but this response is essentially lost in diabetics – subsequently the symptoms associated with hyperglycemia should be monitored at all times. These symptoms include polyuria (increased urine output), fatigue, weakness, excessive dry mouth, increased thirst and the presence of acetone breath due to ketone accumulation in the blood. As mentioned previously in this article, if insufficient glucose is available to the cells (i.e., indicated by elevated glucose levels in this case), incompletely metabolized fatty acids and amino acids accumulate that elevate ketone body levels. These ultimately convert to acetone if not metabolized and acetone in blood passes to sweat, breath and urine, producing a fruity-sweet smell. Monitoring ketone levels through urine can be completed using a simple urine strip test provided by a healthcare provider. Although exercise generally lowers blood glucose levels, if pre-exercise glucose levels exist above 240 mg/dl, urine ketone levels should be checked, and if present the individual should contact their healthcare provider immediately.
Dehydration resulting from polyuria can occur frequently with hyperglycemia. These individuals should always be regarded as a high risk for dehydration and for developing heat-related illnesses during exercise – always monitor symptoms for any heat-related illness. Hydration before, during and following exercise (rehydration) is an important consideration for hyperglycemic individuals.
Diabetics with retinopathy are also at risk for retinal detachments and vitreous hemorrhaging if participating in vigorous exercise where systolic and mean arterial pressures increase significantly (1). This risk can be reduced by implementing exercise programs that produce smaller elevations in blood pressure (i.e., moderate-intensity exercise and avoiding heavy resistance training or excessive upper body resistance training).
For those individuals with peripheral neuropathies, certain types of exercise (e.g., running, walking and standing) can exacerbate foot issues like blisters and ulcers. Proper foot care is an essential preventative strategy and includes ideas such as:
- Using softer shoe inserts.
- Keeping feet dry at all times with polyester or blended socks, or using silica gel or ventilated midsoles – change socks during a workout as needed.
- Incorporating more non-weight bearing activities.
Although diabetes is a serious metabolic disease best monitored by medical professionals, the growing prevalence of this disease has dictated a need for fitness professionals to become part of this healthcare continuum. Although the benefits of exercise in managing diabetes has been clearly established, the fitness professional needs to develop and implement programs with careful consideration as many complications exist that have serious consequences. Understand the specific nature of your client’s disease, then follow all recognized guidelines and instructions prescribed by their doctor to ensure the delivery of safe and appropriate programs that will improve their overall condition.
- American College of Sports Medicine (2014). ACSM’s Guidelines for Exercise Testing and Prescription (9th). Philadelphia: Lippincott, Williams & Wilkins.
- American Diabetes Association (2012).Standards of medical care in diabetes – 2012. Diabetes Care, 35(suppl 1):S11-S63.
- American Diabetes Association (2015). 2014 National Diabetes Fact Sheet. Retrieved 10/24.
- DeSisto CL, Kim SY, and Sharma AJ (2014). Prevalence Estimates of Gestational Diabetes Mellitus in the United States, Pregnancy Risk Assessment Monitoring System (PRAMS), 2007–2010. Preventing Chronic Disease, 11:130415. http://dx.doi.org/10.5888/pcd11.130415.
- Swain DP, and Leutholtz BC, (1997). Heart rate reserve is equivalent to %VO2 reserve, not to %VO2max. Medicine and Science in Sports and Exercise, 29(3):410-414.
- Jetté M, Sidney K, and Blümchen G, (1990). Metabolic equivalents (METS) in exercise testing, exercise prescription, and evaluation of functional capacity. Clinical Cardiology, 13(8): 555-565.
- Little JP, Gillen JB, Percival ME, Safdar A, Tarnopolsky MA, Punthakee Z, Jung ME, and Gibala MJ, (2011). Low-volume high-intensity interval training reduces hyperglycemia and increases muscle mitochondrial capacity in patients with type 2 diabetes. Journal of Applied Physiology, 111(6):1554-1160.
- Sigal RJ, Kenny GP, Wasserman DH, Castenada-Sceppa C, and White RD, (2008). Physical activity/exercise and type 2 diabetes: a consensus statement from the American Diabetes Association. Diabetes Care, 29(6):1433-1438.