Has your exercise performance been less than you’d hoped recently? There are many different things that can cause fatigue, but here are some potential causes (and solutions) to consider.
Inadequate rest time: A really simple answer to your exercise issues is that you may be getting through your workouts well, but then fail to perform when you have races and events simply because you didn’t take enough rest time to restore glycogen, repair muscle damage (caused by every workout), and fully recover. It’s critical to cut back on your workouts (“taper”) for at least 1-2 days before a big event. During that time, you also want to keep your blood glucose in good control so your glycogen levels will be as full as possible on race/event day.
Blood glucose and glycogen stores: Another thing to consider is your blood glucose control. It's harder for your body to restore your muscle glycogen (stored carbs) between workouts unless you're eating enough carbs and have functioning insulin available. Doing longer and harder workouts can deplete glycogen stores, and you may simply just not be restoring them fully fast enough due either to your carb intake or your blood glucose management. Your carb intake doesn't have to be tremendous—probably just 40% of your total calories coming from carbs will suffice—but you may need more if you’re not eating enough calories. Your blood glucose absolutely needs to be in good control for your muscles to store all the carbs you need to exercise optimally, so make sure your insulin is adequate and working effectively.
Iron levels: For starters, having low iron stores can cause you to feel tired all the time, colder than normal, and just generally lackluster. You can get a simple blood test done to check your hemoglobin (iron in red blood cells) and your overall iron status (serum ferritins). It’s possible to be iron deficient without having full-blown anemia. If your body’s iron levels are low (due to diabetes or non-diabetes causes), taking iron supplements can help, along with eating more red meat since it has the most absorbable form of iron.
Magnesium deficiency: Most people also have issues with magnesium deficiency, especially if you take insulin or your blood glucose levels are not well controlled. Magnesium is involved in over 300 enzyme-controlled steps in metabolism, including protein synthesis, muscle and nerve function, blood glucose control, and blood pressure regulation. If you’re deficient in this mineral, your exercise will be compromised and you may even experience some muscle cramping (unrelated to dehydration). It’s always good to eat more foods with magnesium in them—such as nuts and seeds, dark leafy greens, legumes, oats, fish, and even dark chocolate—but taking a supplement (magnesium in the aspartate, citrate, lactate, and chloride forms is absorbed better than magnesium oxide and sulfate) may help. Low magnesium can also lead to potassium imbalances, which can also affect your ability to exercise well.
B vitamin intake: For people with diabetes, thiamin deficiency is also a likely culprit for exercisers, especially if they’re not eating properly. In general, the eight B vitamins are integrally involved in metabolism and even red blood cells formation. Thiamin (B1) in particular can be depleted by alcohol intake, birth control pills, and more. People who take metformin to control diabetes can also end up deficient in vitamins B6 and B12, both of which are essential to nerve function and muscle contractions. Taking a generic B complex vitamin daily can help you avoid these issues, and excesses of most of the B vitamins are harmless (and end up in your urine).
Insulin delivery method: While insulin pumps can help manage blood glucose acutely, they deliver rapid-acting insulin analogs like Humalog, Novolog, and Apidra, and these altered insulins are metabolized in the body differently than the long-acting basal one called Lantus. Rapid ones have little to no insulin-like growth factor (IGF) affinity, and most adults are reliant on IGF to stimulate muscle growth and repair rather than human growth hormone (which is only higher in youth). Lantus does stimulate IGF one, though, so you may want to talk with your doctor about combining insulin pump use (for meal boluses) with Lantus (for basal insulin coverage) to get more IGF activity to promote muscle repair. (Go with Lantus, though, as Levemir is less effective at raising levels of bioactive IGF.)
Thyroid issues: Many people with diabetes also have thyroid hormone imbalances. Having lower levels of functioning T3 and T4 can cause early fatigue and poor exercise performance, among other things. However, it may not be enough to just check your main thyroid hormones (TSH, T3 and T4); you may also want to consider getting your thyroid antibodies checked if your thyroid hormones levels are normal and nothing else is helping your exercise (specifically check for antibodies to thyroid peroxidase), especially if you have celiac disease.
Still stumped? If you’ve been through this whole list and had everything check out okay, then consider other possible issues like your hydration status, daily carb intake (adding even just 50 grams per day to your diet may help), other possible vitamin and mineral deficiencies (vitamin D, potassium, etc.), statin use (some statins taken to lower blood cholesterol cause unexplained muscle fatigue), frequent hypoglycemia, and hypoglycemia-associated autonomic failure.