By Gabe Mirkin, M.D.
I have often recommended that exercisers take a sugared, caffeinated soft drink for a performance boost during prolonged vigorous exercise. I still think this is good advice, but based on a study from the University of Buffalo, people who are exercising at a casual pace for an extended time may want to restrict their intake of soft drinks (Am J Physiol Regul Integr Comp Physiol, March 2019;316(3):R189-R198).
The study suggests that drinking a lot of a soft drink with sugar and caffeine during and after exercise in hot weather may damage your kidneys. Twelve healthy adults drank two liters of either a soft drink or water during four hours of exercise in hot weather (95 degrees), and one liter of the same beverage when they were finished exercising.
After taking the soft drink, the subjects had much higher markers of kidney damage compared to when they drank just water:
• higher blood creatinine (Their kidneys were not working to clear creatinine from bloodstream)
• higher urine NGAL (a biomarker of kidney damage)
• higher blood levels of uric acid (kidneys not clearing it from bloodstream)
• serum copeptin, (a marker of cell damage from high sugar levels caused by vasopressin)
The same type of kidney damage was shown previously in rats who drank large amounts of sugared drinks when they exercised (Am J Physiol Regul Integr Comp Physiol, 2016 Jul 1;311(1):R57-65). However, athletes who restrict sugared drinks in competitions or intense workouts lasting longer than an hour or in hot weather will find that they will lose maximum speed and endurance. When you exercise intensely for more than 50 minutes without taking a source of sugar, you slow down because you start to run out of the sugar stored in your muscles and liver.
Mechanism of Potential Damage
High blood sugar levels can cause chronic kidney damage by increasing the osmolality (concentration of minerals and sugar in blood), which causes your body to produce large amounts of the hormone, vasopressin. This can cause your body to retain fluid by blocking kidney function, and cause enzymes to convert sugar to sorbitol that can damage the kidneys and other cells throughout your body (Metabolism, Apr 1986;35(4Suppl 1):10-4).
Vigorous Exercisers Need Sugar
If you are exercising at a relaxed pace for up to two hours, you may not need to drink anything unless you are thirsty. However, if you are going to exercise intensely or compete in sports for more than an hour, you should take sugar and fluid just before you start and while you exercise. The limiting factor to how fast you can move your muscles is the time that it takes to move oxygen into muscles. During exercise, your muscles use primarily sugar and fat for energy, and to a lesser degree, protein. Sugar requires less oxygen than fat or protein do, so you can move faster when your muscles burn mostly sugar.
There is only enough sugar in your bloodstream to last three minutes at rest. To maintain blood sugar levels, your liver constantly releases sugar into your bloodstream, but your liver holds only enough sugar to last about twelve hours at rest and for up to 70 minutes during intense exercise. When muscles run out of their stored sugar supply, they hurt and become difficult to control.
If you exercise vigorously, I suggest that you treat the results of this new study with skepticism because fit endurance athletes have not been shown to be at increased risk for kidney damage. Realize that the amount the test subjects drank was very high (two liters during four hours of exercise and one liter afterward), and taking that much of a sugared soft drink might produce abnormal kidney test results whether a person was exercising or not. Also, taking in too much of any fluid during long endurance events can cause hyponatremia, a condition that can cause brain swelling and even death. This is more common in mediocre competitors who spend their time drinking rather than trying to keep up their pace.
Endurance exercisers who are concerned that sugared and caffeinated soft drinks might increase their risk for kidney damage may want to:
• alternate drinks of water with sugared drinks, with or without caffeine
• alternate water with fruit juice
• eat some sugary foods and drink plain water