Type 2 diabetes mellitus (T2DM) currently affects approximately 20% of the population [1]. The Centers for Disease Control and Prevention estimate that by 2050, approximately 1 in every 3 adults will be diagnosed with the disease [1].

This disease is characterized by insulin resistance, or the inability to properly use insulin (hormone released from the pancreas) to control blood glucose levels. This is different than type 1 diabetes (an autoimmune disease) that targets pancreatic B-cells and compromises their ability to produce insulin [2]. Let’s take a look at the pathophysiology of T2DM and the impact of exercise, specifically resistance training in managing this disease.

The following are key points in the clinical diagnosis of Type 2 diabetes mellitus:

• A person’s fasting blood glucose (FBG) levels are greater than or equal to 126 mg/dL or blood glucose levels greater than or equal to 200 mg/dL after an oral glucose tolerance test (OGTT) [3].

• Diagnosis of “pre-diabetes” can be made when FBG are between 100 and 125 mg/dL or blood glucose levels are between140 and 199 mg/dL after an OGTT [3].

• Hemoglobin A1C (HbA1c) levels (referred to as the A1c test) used to assess a person’s glycemic response over time and is frequently measured in research evaluating the effect of exercise on T2DM [4]

• HbA1c is a type of hemoglobin that reacts with plasma glucose to form a glycoprotein and is elevated in individuals with diabetes [5].

• American Diabetes Association now recognizes HbA1c levels greater than 6.5 mg/dL as a clinical biomarker for the diagnosis of T2DM [6].

Benefits of Resistance Training

Managing and preventing T2DM have been shown through both aerobic and resistance exercise programs. Resistance training is purported to help decrease the risk of T2DM by increasing muscle mass, utilization of glucose, and control of enzymes involved in glucose metabolism [7]. Resistance training, independent of aerobic exercise habits, was recently demonstrated to be associated with a 34% decreased risk of developing T2DM [8].

However, combining both aerobic exercise and resistance training showed the greatest reduction in risk of type-2 diabetes (59%). The evidence that resistance training alone has such an impact on T2DM is encouraging for those individuals who find it difficult to adhere to an aerobic exercise schedule.

The primary goal of resistance exercise in the management of T2DM is to improve glycemic control [4, 6]. Secondary goals include increased lean body mass, increased strength, and reduction in associated comorbidities. Appropriate intensity, progression, and program length are critical for influencing glycemic control [9].

It’s recommended by the American College of Sports Medicine and American Diabetes Association that medically cleared persons with controlled T2DM begin a resistance training program at a moderate intensity (50% 1 repetition maximum [1RM]) and progress to a more vigorous intensity (75-80% 1RM) over time [9].

These recommendations were based on progressive high-intensity resistance training programs that were able to significantly decrease HbA1c levels in people with T2DM [10]; a decrease in HbA1c is a clinically significant sign of improving glycemic control.

The following are proposed research-based mechanisms by which resistance training improves glycemic control:

• Improved glycemic control is because of increased muscle mass-and subsequent increased muscle glycogen-seen in individuals with T2DM who perform resistance training [10].

• Better insulin sensitivity and control of membrane transporters (e.g., GLUT4) attributable to resistance training are responsible for improved glycemic control in these people [11].

Regardless of the proposed mechanism, resistance training helps manage T2DM due to the increased utilization of glucose. More glucose is stored in muscle tissue resulting in less blood glucose available to bind with hemoglobin (i.e., lower HbA1c levels).

The following are key objectives in regards to exercise complications and medical management:

• Clients with T2DM should be evaluated by a medical professional before beginning a new exercise program. This evaluation should include pertinent information about the client’s level of glycemic control, current medications, exercise contraindications, and other precautions associated with an exercise program [12].

• Exercise-induced hypoglycemia is one of most common risk factors for individuals with T2DM, especially those taking insulin and/or other blood-glucose controlling medications [6, 9]

• An absolute contraindication for exercise is blood glucose less than 70 mg/dL.

• Hypoglycemia releases many hormones (e.g., glucagon, epinephrine, cortisol, growth hormone) which can lead to a variety of physiological responses such as hunger, sweating, nervousness, dizziness, confusion, anxiety, weakness, and increased pulse [13].

• Hypoglycemic individuals are advised to consume 15-20 g of carbohydrates (fruit, fruit juice, or sport drink/bar) until glucose values stabilize [9].

• Hyperglycemia is considered a characteristic of T2DM and not considered a risk factor. Individuals with blood glucose levels more than 300 mg/dL, without ketones, can still participate in exercise as long as they are properly hydrated and feel well [9].

• Comorbidities that are present in people with T2DM include macrovascular disease, hypertension, dyslipidemia, retinopathy, neuropathy, and nephropathy.

• Trainers should be aware that acute bouts of resistance training can significantly elevate blood pressure, heart rate, and cardiac output, especially during the eccentric phase of exercise or when the Valsalva maneuver is used.

• High-intensity exercise is contraindicated for those clients with proliferative retinopathy because of possible retinal hemorrhaging.

• Clients with loss of sensation (peripheral neuropathy) in their feet should not conduct high-impact activities including running, jumping, and heavy walking and some resistance training exercises [12].

It’s important to realize that the severity of T2DM comorbidities will vary among clients, and fitness professionals should refer to appropriate resources for additional risks and contraindications related to each health condition.

The following are key summary points:

• The benefits of resistance training are research-based to be useful in managing hyperglycemia for clients with T2DM. The end result leads to a better utilization of blood glucose within the muscle cell.

• Trainers are responsible for implementing safe, effective, and individualized fitness plans for their clients to maximize gains. Trainers need to be aware of a client’s medical history including medications and disease comorbidities.

• The resistance training that is a part of Metabolic Precision is designed to be progressive high-intensity programs and is therefore, ideal for implementing with clients that have Type 2 diabetes.

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Dr. Paul Henning is leading scientist at the Military Performance Division, United States Army Research Institute of Environmental Medicine. Paul's an avid bodybuilder, lives the MP lifestyle and is a proud member of the MP Team. Read more from Dr Henning here.

1. Prevention., C.f.D.C.a. Diabetes Successes and Opportunities for Population-Based Prevention and Control at a Glance 2011. 2011 August 12, 2012]; Available from: http://www.cdc.gov/chronicdisease/resources/publications/AAG/ddt.htm.
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11. Holten, M.K., et al., Strength training increases insulin-mediated glucose uptake, GLUT4 content, and insulin signaling in skeletal muscle in patients with type 2 diabetes. Diabetes, 2004. 53(2): p. 294-305.
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