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.
My Recommendations
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
Sugar for Prolonged Hard Exercise
Caffeine Boosts Endurance and Strength
It would be helpful to define “sugar”. Most health professionals recommend avoiding processed sugar. There are many sources of naturally Occurring sugar, like fruits and dates that don’t have the negative side effects of added sugar. Very surprised anyOne would recommend drinking a sugared soft drink, I don’t think any of the pro water bottles have sugar in them..
Greetings from Captain Obvious!
Plenty of other reasons besides the kidney to AVOID surgery soft drinks for your main exercise hydration in the heat.
It is well known that too high a sugar content in a drink INHIBITS acute rehydration, as was the case in this study. Plasma volume immediately post-exercise was DEcreased in the soda group but not the water-only group. Functionally- FOUR cases in the soda group could not complete the full exercise protocol (despite the caffeine ‘boost’) vs only one in the water group (cross-over experiment so each subject exercised with H2O or soda on different sessions at least a week apart).
Also- this study used a hi-fructose soda, and many have GI side effects from taking large amounts of fructose. I would be a hurtin’ cowboy if I drank 3 liters of soda in one day…..exercising or not 🙁
Nothing wrong with a bit of soft drink here or there during a long hot day’s ride, but not the best choice as the day’s sole beverage.
I’d like to ask the study authors if they considered other components of soft drinks (specifically phosphoric acid) as possible causes of the apparent kidney damage. Three liters per day is a pretty high dose!
Three liters of Coke? Wow! And for subjects exercising at “a casual pace”. I don’t see anything here that will dissuade me from my mid-ride bottle of Coke in a 4 hour, 65 mile ride. I’ve been doing this since the 1970s with zero kidney damage 🙂
I also think that 3 liters of soda is a huge dose, but I don’t drink soda except on rare occasions.
But that does lead me to wonder if carrying a small flask of coffee that is strong with caffeine like espresso, then sweetened with sugar, could be use to take a hit from every 1/2 hour to an hour on the bike and drinking regular water or some sort of cycling drink like Nuun for hydration?
Odd, many studies showing that high fructose consumption is a factor in kidney disease but no mention here. Since it has to be processed by the liver it would also seem to be useless during exercise, Hence sports drink and energy gels generally contain glucose and glucose polymers but little too no fructose. Not sure why anyone would think it’s a good idea to drink it during exercise.
“Everything in moderation” still works.. My 16oz coke in the last half of a hard hot 50 mile ride gives the lift needed to enjoy the finish. The damage done by frequent soft drink consumption is without question, but an occasional well timed sugar/caffeine boost works for many.