I often get asked, “What is the best time of day to exercise?” Like most things related to physical activity—especially with diabetes as an added variable—the answer often is, “It depends.” What is your goal for being active? Are you trying to better balance your blood glucose, or is weight loss your goal? Do you take insulin? What is your normal diet? How much time do you have? Which activities? There are so many questions that likely need answering before you may be able to ascertain the best time for you personally to be active.
A recent article on CNN (1) proclaimed, ”Exercising before breakfast burns more fat, study says.” That article starts out by asking, “Should you eat before or after exercise in the morning?” It then states, “In 30 obese or overweight men, those who exercised before breakfast burned twice the fat as men who ate breakfast before they worked out.”
Before even reading the CNN article or looking up the actual research it quoted, I already had issues with their approach. You have to understand that the fuel your body uses during exercise matters less when it comes to weight loss than the total number of calories you use. Most calories burned during moderate or harder exercise come from carbohydrate sources simply due to the body’s more efficient use of that fuel compared to fat (or protein) (2). Your muscles use fat when forced to or when activity is light—but using more fat during exercise doesn’t mean you lose more body fat, which is how the press always interprets it. What’s more, fat is the primary fuel your body uses during recovery from exercise, which lasts far longer each day than your workouts, no matter what time of day you exercise.
Delving into the research itself (3), they actually found that active muscles did use more intramuscular fat in these subjects during prebreakfast training—but after breakfast blood glucose spikes were unchanged after six weeks of moderate cycle training regardless of which time they trained. They released less insulin for breakfast eaten after training, but that is hardly a surprising finding given than eating a meal makes most people (who can) release insulin. Burning more fat during prebreakfast exercise also did not cause those men to lose more weight than those exercising afterward (again, this is unsurprising).
For anyone with diabetes, fasted exercise tends to prevent the typical drop in blood glucose during moderate aerobic exercise—due to higher levels of cortisol and other glucose-raising hormones released when fasting—but many people with diabetes find that exercising in the morning causes their blood glucose to rise, which can also be an issue. I know many people with type 1 diabetes who love to exercise before breakfast, but I personally hate exercising then because I am more insulin resistant for hours afterward and battle to lower my blood glucose. Research agrees with me and has shown this to be especially true for resistance or high-intensity exercise done in the morning in adults with type 1 or type 2 diabetes (4,5).
In all honesty, the best answer when asked what is the best time of day to exercise is: “When you have the time!” In other words, exercise whenever it fits best into your daily life—because no matter when you are active, it’s always better to do something than nothing at all when it comes to your overall health and long-term blood glucose management. Just make sure to adjust your diabetes regimen as needed to keep your blood glucose in check.
LaMotte, S. Exercising before breakfast burns more fat, study says, October 18, 2019, https://www.cnn.com/2019/10/18/health/exercise-breakfast-fat-burn-wellness/index.html
Kerksick CM, Arent S, Schoenfeld BJ, Stout JR, et al. International society of sports nutrition position stand: nutrient timing. J Int Soc Sports Nutr. 2017;14:33. doi: 10.1186/s12970-017-0189-4
Edinburgh RM, Bradley HE, Abdullah N-F, Robinson SL, et al. Lipid metabolism links nutrient-exercise timing to insulin sensitivity in men classified as overweight or obese. J Clin Endocrinol Metab, 2019 Oct 19. doi: 10.1210/clinem/dgz104 [Epub ahead of print]
Toghi-Eshghi SR, Yardley JE. Morning (Fasting) vs Afternoon Resistance Exercise in Individuals With Type 1 Diabetes: A Randomized Crossover Study. J Clin Endocrinol Metab. 2019;104(11):5217-5224. doi: 10.1210/jc.2018-02384.
Savikj M, Gabriel BM, Alm PS, Smith J, et al. Afternoon exercise is more efficacious than morning exercise at improving blood glucose levels in individuals with type 2 diabetes: a randomised crossover trial. Diabetologia. 2019;62(2):233-237. doi: 10.1007/s00125-018-4767-z.