The Athlete with Diabetes

General Principles

  • Diabetes mellitus (DM) is a chronic group of metabolic diseases characterized by hyperglycemia resulting from defects in insulin secretion leading to an absolute insulin deficiency due to beta-cell destruction (type 1), by defects in insulin action because of progressive insulin secretory loss along with increasing insulin resistance (type 2), or by other causes such as pregnancy (gestational), neonatal diabetes or maturity-onset diabetes of the young (monogenic diabetes), diseases of the exocrine pancreas, or drug- or chemical-induced causes.

  • A majority of patients with DM have type 2 (90%–95%). Athletes with DM range in sports participation from youth to competitive Olympics and professionals.

  • Of the 210,000 individuals 19 years of age and younger diagnosed with diabetes in the United States, about 187,000 have mainly type 1 diabetes (T1DM) and 23,000 have type 2 diabetes (T2DM) or other types. The ratio of T1DM and T2DM changes with age in the general population.

  • Each sport and the type of exercise have their own effects on DM management. Numerous factors affect glucose levels, including stress, level of hydration, rate of glycogenolysis and gluconeogenesis, and secretion of counter-regulatory hormones.

  • Management includes excellent clinical care, continuous patient self-management, patient education, and longitudinal support to prevent long-term complications (renal failure, blindness, peripheral vascular disease, and peripheral neuropathy).

Anatomy and Pathophysiology

  • T1DM has two subtypes: type 1A and type 1B. Each type can occur at any age, but typical onset is before the age of 30 years, with peak incidence during adolescence.

  • Type 1A is an autoimmune disease characterized by cellular antibodies that may form against islet cells (ICA), insulin (IAA), and glutamic acid decarboxylase (GAD65). Type 1B is an idiopathic, nonautoimmune disease state with loss of beta-cell function and is not human leukocyte antigen (HLA)–associated but is an inherited form of diabetes.

  • Both types are caused by loss of insulin secretion because of progressive loss of insulin production.

  • These physiologic changes lead to increased hyperglycemia, weight loss, possible ketoacidosis, and possible death if insulin is not administered.

  • T2DM occurs in most patients older than 40 years and is characterized by defects in both insulin secretion and resistance to insulin action.

  • Type 2 patients may present with diabetic ketoacidosis (DKA), especially in certain ethnic minorities (Latino or African Americans).

  • Impaired insulin secretion, increased hepatic glucose production, and decreased muscle glucose uptake lead to increased levels of insulin production and eventual insulin resistance.

  • Both genetic (family history or familial hyperlipidemia) and environmental (sedentary lifestyle or inappropriate diet with increased caloric intake) factors are involved in the development of insulin resistance. Insulin resistance is associated with obesity, hypertension, and hyperlipidemia and may precede the onset of diagnosed DM by 10–20 years.

  • T2DM occurs in athletes with an increased body mass for their particular sport (football lineman or rugby players) or those who do not remain fit (e.g., certain baseball players).

  • Patients with T1DM and T2DM benefit from regular exercise:

    • Exercise decreases the insulin resistance of peripheral tissues and alleviates the defect of insulin-stimulated glycogen metabolism in skeletal muscles.

    • It improves postprandial hyperglycemia and possibly postprandial insulin secretion.

  • For patients with T2DM, both aerobic and resistance exercises can benefit glycemic control.


  • The incidence of DM is increasing worldwide. It is estimated that 463 million people are living with diabetes in the world. It is predicted this figure will increase to 642 million people by 2040.

  • Currently, in the United States, approximately 30 million patients have DM, with about 23 million diagnosed with T2DM.

General Medical Concerns

  • In comparison with inactive individuals, the additional demands of training and competition affect glucose homeostasis in athletes with DM, thus creating additional challenges.

  • Challenges include the athlete’s safety during athletic participation, adequate glucose monitoring, and diet and insulin adjustments for safe and effective athletic performance.

  • The most significant concern for athletes using insulin or insulin secretagogues (sulfonylureas and meglitinides) is the potential for exercise-related hypoglycemia; it can occur during, immediately after, 6–15 hours after exercise, or up to 48 hours after exercise.

  • Most competitive athletes learn to manage their DM during training and competition by trial and error while sharing personal experiences with other athletes.

  • Medical concerns include unsafe dietary patterns, using nutritional supplements with no benefit or even detrimental effects, and using illegal drugs.

  • Other concerns include the female athlete triad (amenorrhea, osteoporosis, and eating disorder), rapid weight loss to “make weight” in the respective wrestling or gymnastic competition, and excessive consumption of a single macronutrient (carbohydrate, protein, or fat) in certain athletes such as football players.

  • In sports with weight categories (wrestling, boxing, and weightlifting), insulin is often omitted so that athletes can lose weight before weigh-ins; the consequence is poor glucose control and the risk of ketoacidosis.

  • All athletes with a diagnosis of type 1 or type 2 diabetes should have a personalized diabetic care plan for their exercise, practices, and games that is supported by the team physician and athletic trainer.

  • SGLT2 inhibitors promote glucosuria and a have a modest diuretic effect. Dehydration is a potential concern, but no dose adjustments are recommended before initiating an exercise routine.

Athletes With Type 1 Diabetes Mellitus

  • Appropriate hydration and maintenance of glucose levels can maximize performance. The excitement of competition can increase catecholamine release, resulting in hyperglycemia, but adjustment of insulin dosing is usually unnecessary.

  • For these athletes, hypoglycemia usually occurs within 45 minutes of starting aerobic exercise unless treatment regimen adjustments are made to prevent it.

  • Athletes with T1DM learn their personal requirements from training and recognize that they always require a certain amount of insulin supplementation.

  • Glucose monitoring before, during, and after exercise can establish an athlete’s usual glycemic responses and allow for maintaining euglycemia with appropriate adjustments in insulin dosing and carbohydrate ingestion.

  • When an athlete with T1DM is insulin deficient, hyperglycemia occurs with a risk of further elevation in glucose levels, which may exacerbate or precipitate ketoacidosis. Moreover, osmotic diuresis with relative dehydration occurs as well.

  • Morning endurance sports are less likely to cause hypoglycemia because of the physiologically elevated diurnal cortisol and growth hormone levels. Events that occur later in the day require adjustments in food and insulin.

  • Certain athletes with T1DM intentionally avoid achieving good blood glucose (BG) control before their competitive event to have increased lipid utilization, which prevents exercise-induced hypoglycemia.

Athletes With Type 2 Diabetes Mellitus

  • The duration the athlete has been diagnosed with T2DM and the specific sport will determine management. Endogenous insulin production or secretion early in the disease requires little, if any, exogenous insulin. These athletes maintain the ability to physiologically decrease or increase endogenous insulin secretion and are generally able to achieve optimal glucose levels in the desired range.

  • With progression of the disease, endogenous insulin secretion diminishes and exogenous insulin administration becomes necessary and must be adjusted to prevent hypoglycemia.

  • It is not uncommon for such athletes to decrease exogenous insulin requirement by ≥50% with competition.

  • Athletes with T2DM who use oral insulin secretagogues and sensitizers may need to decrease their dosing as training and insulin sensitivity increase and body fat decreases, resulting in an overall increase in lean body mass.

Sport-Specific Issues

Endurance Athletes (Distance Cyclists and Runners)

  • Appropriate glucose management before, during, and after exercise is crucial to care of an athlete with DM.

  • Endurance runners often strive for optimal prerun BG levels of 120–180 mg/dL, use minimal insulin, and estimate a glucose reduction of 10–15 mg/dL/mile.

  • When running for 30–60 minutes, self-monitoring of BG or continuous glucose monitoring (CGM) during the training period will delineate the athlete’s predicted response.

  • Distance cyclists and runners may choose to set the basal rate of insulin infusion via continuous subcutaneous insulin infusion (CSII) or insulin pump at a lower rate or may select a decreased long-acting basal insulin dosage (not uncommon to have to decrease by 50%). Carbohydrates are then gradually ingested to match energy utilization with exercise. By creating a steady-state balance between exercise requirements, basal insulin infusion, and ingestion of energy (carbohydrates), glucose levels are held constant (e.g., 130–150 mg/dL) over several hours.

  • When a cyclist or runner with T1DM encounters a demanding section of a course, they will either adjust the basal rate of insulin downward or ingest additional carbohydrates to maintain optimal glucose levels.

  • For runners and cyclists who are prone to hypoglycemia, performing a series of anaerobic sprints or resistance exercise before the aerobic endurance event may help prevent subsequent hypoglycemia.

  • Running sports pose special problems for athletes with DM and include skin breakdown or other lesions because of vascular compromise or neuropathy.

  • Athletes should travel with at least two pairs of well-fitting shoes that (1) are well broken in, (2) have no areas of material weakness or breakdown, and (3) cause no discomfort with exercise. In addition, (4) athletes should have their feet examined by the certified athletic trainer (ATC)/healthcare provider on a weekly basis, (5) avoid switching to a new pair of shoes on competition day, and (6) avoid using any alcohol-based lotion on the feet that may cause skin drying.

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