A study from Japan found that exercising in the late afternoon (4-6 PM) helps to control blood sugar, cholesterol and triglyceride levels better than exercising in the morning (9-11 AM) (Front Endocrinol, July 19, 2022).
Twelve healthy young men walked on a treadmill for one hour at 60 percent of their maximal oxygen uptake (VO2max) on Monday, Wednesday and Friday. They were tested during and after a week of exercising just in the morning or just in the afternoon. Blood sugar and fat control was much better in the afternoon exercisers.
• Continuous 24-hour monitoring of their blood sugar levels showed that total blood sugar levels were lower in the afternoon exercise time.
• Blood sugar levels after meals were lower after afternoon exercise. Most cell damage from high blood sugar comes from a high rise in blood sugar 1-2 hours after eating a meal.
• Triglyceride levels were lower after afternoon exercise. This is very important because your blood sugar level rises after you eat, and if it rises too high, sugar sticks to cell membranes and damages them. That’s why diabetes can damage every cell in your body. When your blood sugar level rises, your pancreas releases insulin to keep blood sugar levels from rising too high. Insulin lowers blood sugar by driving sugar from the bloodstream into the liver. However, if the liver is full of sugar, the liver does not accept more sugar and all the extra sugar is converted to fatty triglycerides. Having a blood triglyceride level greater than 150 usually means that your blood sugar rises too high after meals and you are already diabetic or prediabetic.
• Blood levels of the good HDL cholesterol were higher after afternoon exercise. When triglycerides rise too high, you are at increased risk for clots. To protect you from a high rise in triglycerides, your good HDL cholesterol carries the triglycerides from your bloodstream into your liver, so a high rise in triglycerides causes a drop in blood levels of the good HDL cholesterol. The lower your good HDL, the more likely you are to suffer a heart attack.
High blood levels of sugar and triglycerides, high total cholesterol, and low HDL (good) cholesterol are very strong markers for increased risk for heart attacks, diabetes, and obesity (Clin Chim Acta, (2014) 431:131–42; Curr Med Res Opin, (2014) 30(8):1489–503; Curr Vasc Pharmacol, (2011) 9(3):309–12; Expert Rev Endocrinol Metab, 2020;15(3):147–57).
Why You Should Exercise Just Before or Just After You Eat
Hundreds of articles show that exercise helps to prevent and treat obesity, diabetes and heart attacks (Med Sci Sports Exerc, 2007;39(8):1423–34; Med Sci Sports Exerc, 2011;43(7):1334–59). When blood sugar rises after meals, your pancreas releases insulin which lowers blood sugar by driving sugar from your bloodstream into your liver and muscles, the only places in your body where you can store significant amounts of sugar. As soon as the liver fills up with sugar, it cannot accept any more sugar, so all the extra sugar is converted to fatty triglycerides. Exercise empties sugar from your liver and muscles, so if you exercise before or after you eat, your liver and muscles can accept more sugar, which means that both blood sugar and triglycerides levels will be lower and the good HDL cholesterol will be higher. This helps to protect you from storing extra fat in your body, becoming diabetic, and forming plaques in your arteries that can break off to cause a heart attack.
Everyone who can safely do so should try to exercise regularly to help prevent diabetes, heart attacks and obesity. Having high blood sugar, high triglycerides or low good HDL cholesterol increase risk for these conditions. The most effective time to lower all these blood risk factors is to exercise just before or after you eat. The least effective time to exercise is just before you go to bed at night. You will get better protection by exercising before or just after you eat your last meal of the day.