Impact of Exercise on Cardiovascular Risk Factors: Diabetes Mellitus

Abstract

Individuals with any type of diabetes can gain health benefits from regularly participating in physical activity that extend well beyond increasing insulin sensitivity and lowering blood glucose levels, and only minimal risks are present in some individuals. As various types of training can benefit cardiometabolic health, the exercise prescription for patients with diabetes should be tailored according to the timing of their medications, presence of diabetes complications, and individual fitness and glycemic management goals. A comprehensive exercise program performed consistently and progressively will help persons with diabetes better manage their disease, limit the impact of cardiovascular disease risk factors, improve their overall quality of life, and potentially limit premature mortality. In some cases, pre-exercise assessment and exercise testing may be warranted to ensure safe and effective participation. Athletes with diabetes may very well engage in competitive sports, although certain precautions are important. In athletes using insulin, fluid and food intake during exercise are important to consider, along with following an insulin regimen compensatory for the type, order, timing, and duration of physical activities to avoid hypoglycemia and hyperglycemia and negative long-term consequences.

Case Study Wrap-Up

Case Vignette: A 54-year-old man presents for an assessment of his physical fitness. Two years ago, he experienced a non-ST-elevation myocardial infarction of the anterior wall, which necessitated placement of a drug-eluting stent in his left descending artery. He also reports that his GP told him that he has “elevated blood sugar levels” for which he received lifestyle advice. His family history is unremarkable for cardiovascular diseases and he reports no angina or other cardiovascular disorders. The patient smokes one pack of cigarettes per day (and has done so for 30 years). As for physical activity, he reports engaging in 30–45 min of jogging once a week. His present medications include 100 mg of acetylsalicylic acid, a beta-blocker (bisoprolol 5 mg 1–0-0), an angiotensin converting enzyme (ACE)-inhibitor (ramipril 2.5 mg 1–0-0), and a statin (simvastatin 20 mg 0–0-1).

On physical examination, his body mass index (BMI, 31.1 kg/m²), waist circumference (105 cm), and blood pressure (145/95 mmHg) are elevated. Auscultation of heart and lungs reveal no pathologic findings and no signs of heart failure are noted. The ECG reveals a sinus rhythm with 65 beats per minute, normal indices, and no Q-waves. A maximal exercise stress test reveals a reduced exercise capacity (75% of age predicted, or 151 Watts), but it is negative for ischemia and other abnormalities.

Blood tests show a fasting plasma glucose of 6.6 mmol/l (119 mg/dl), HbA1c of 6.9%, and dyslipidemia despite use of a statin: total cholesterol, 7.0 mmol/l (270 mg/dl); low density lipoprotein (LDL)-cholesterol, 4.0 mmol/L (155 mg/dl); high density lipoprotein (HDL)-cholesterol, 2.2 mmol/l (85 mg/dl); and triglycerides, 1.7 mmol/l (148 mg/dl). Urinary analysis shows some microalbuminuria and glucosuria.

1.1 Questions

• 1. In addition to optimization of his medications (change of statin, upped titration of ramipril, and addition of antidiabetic medications) and smoking cessation, what exercise advice would you give this patient?

• 2. Would you recommend blood glucose monitoring before, during and after exercise? What precisely do you recommend?

• 3. Would you perform a graded exercise stress test on this patient prior to his starting a more formal exercise program?

• 4. During routine exercise stress testing conducted on this patient, he becomes symptomatic with fatigue, dizziness, and a dry mouth. What is the most likely cause of his symptoms?

1.2 Answers

• 1. All adults with diabetes are recommended to undertake at least 150 min per week of moderate to vigorous aerobic exercise training, although up to 300 min likely bestows additional health benefits. Given his underlying CVD (including a prior myocardial infarction and stent placement), his recommended intensity should be light to moderate, at least to start. He may choose to continue jogging (at a moderate pace) and include other types of aerobic training spread throughout the week, allowing no more than 2 days to lapse between bouts of activity. In addition, he should add in moderate resistance training exercises at least twice weekly (on nonconsecutive days) to enhance retention of his muscle mass. Engaging in flexibility exercises and balance training at least 2–3 days per week should also be recommended to someone his age (see Table 38.1), along with staying more active overall on a daily basis and breaking up sedentary time with frequent bouts of activity.

  At least initially, this patient’s blood glucose should be monitored closely before, during and after exercise. Depending on which diabetes medications this patient is prescribed, monitoring blood glucose levels may remain important around exercise given that beta-blocker therapy can mask the onset of hypoglycemia. Done before and after exercise, such monitoring may also provide positive feedback regarding the glycemic benefits of regular exercise training, which may result in greater subsequent long-term adherence to his exercise training prescription. This is particularly important since exercise is a cornerstone for the management of all types of diabetes and its potential health complications.

• 2. Although routine monitoring around exercise is not recommended for all adults with type 2 diabetes (given that most have a low risk of developing exercise-related hypoglycemia), when such patients begin an exercise program, their blood glucose responses to exercise should be checked. Monitoring and recording blood glucose levels before and after exercise, at least initially, is important because it may:

  • Allow for early detection and prevention of hypoglycemia or hyperglycemia

  • Help determine appropriate pre-exercise blood glucose levels to lower risk of glycemic imbalances resulting from activities

  • Identify patients who may benefit from continued monitoring around exercise

  • Provide information for modifying prescribed exercise based on glycemic responses

  • Allow for better adjustment of diabetes regimens to manage all activities

  • Motivate patients to remain more active to better manage their diabetes

• 3. Likely yes, but it depends. Since this patient is already somewhat active, a stress test is only advisable if he plans to start new activities more vigorous than his current daily ones, which include weekly jogging (which he has been undertaking without any problems or symptoms arising). As stated, previously sedentary older individuals (≥ 40 years) with diabetes and anyone ≥30 years of age with a high cardiovascular risk should likely undergo medical screening and obtain medical clearance prior to performing vigorous intensity exercise. In addition, this patient does meet several criteria for a possible pre-participation exercise stress test, including his age, cigarette smoking, dyslipidemia, and known coronary artery disease (see section on “General Indications for Exercise Stress Testing in Patients with Diabetes”).

• 4. Routine pre-session blood glucose measurement is 11.2 mmol/l (200 mg/dl), which compares well to his previous pre-session measurements. He starts to train on a cycle ergometer. After 10 min of ergometer training his heart rate starts to rise, although his workload is constantly decreasing from 75 to 45 W. The patient also reports feeling dizziness and a dry mouth. No ECG changes are evident, however.

  His immediate blood glucose is 19.4 mmol/l (350 mg/dl), indicating an increase in hyperglycemia in this patient. Some of his symptoms correspond to possible symptoms of hyperglycemia, which can include fatigue, hyperventilation, and dry mouth, along with polyphagia, polydipsia, polyuria, blurred vision, weight loss, poor wound healing, dry or itchy skin, impotence (male), recurrent infections such as vaginal yeast infections, groin rash, external ear infections (swimmer’s ear), cardiac arrhythmias, stupor, and coma. Moreover, physical exhaustion causes glucose production in the liver (glycogenesis and glycogenolysis) plus enhanced free fatty acid release by adipose tissue and reduced muscle uptake of glucose and, consequently, may contribute to hyperglycemia in exercising patients with diabetes.

  After a few minutes of rest, the patient’s symptoms resolve, but his blood glucose remains elevated at 19.4 mmol/l (342 mg/dl) 15 min later. A urinary analysis reveals moderate ketone bodies. When asked further, the patient reports having had a demanding week at his job and furthermore some domestic problems, resulting in significant insomnia during the prior week. To reduce his mental stress, he had exercised on a cycle ergometer for 2 h before this scheduled testing. He is instructed not to perform any exercise for the rest of the day and to continue to monitor his blood glucose levels frequently. The next day the patient’s urine is rechecked and found to be free of ketones. His blood glucose is 9.6 mmol/l (174 mg/dl) and he reports feeling completely recovered after a day of rest. He reports that his GP has recommended that he start on diabetes medications immediately to lower his blood glucose levels and that he plans to do so.