Glucose Tolerance After HIIT Workouts: What Science Says

Shana Spindler PhD
Shana Spindler PhD
June 12, 2021

The timer hit three minutes, and I collapsed into an exhausted heap, muscles quivering against the cool gym floor.I had just finished my first “HIIT” fitness class. High IntensityInterval Training has taken center stage in gyms across the country. HIIT workouts, known for their repeated pattern of high intensity exercise followed by a period of rest, are touted for aiding weight loss and improving cardiometabolic health.

The first time I tried HIIT was in 2016, when my husband deployed to the Middle East. I was at home with two young children, and I needed the ability to carry a toddler and open a jar of pickles on my own. At the time, my local gym offered an eight-week HIIT program (and free childcare!), so I figured why not? Over the course of those eight weeks, I improved my endurance, strength, and mental fortitude, leading to my first ever unassisted pull up—and an easier time with the heavy lifting at home. I was hooked on HIIT.

But in a study published in the journal Cell Metabolism last month, researchers in Sweden found an upper limit to the benefits of high intensity training. They report that excessive HIIT exercise impairs mitochondria function and glucose tolerance in healthy individuals. Before questioning my HIIT workouts, I wanted to gather more information about this popular approach to exercise and its health benefits.

Trainer weigh-in on HIIT workouts

To learn more about HIIT-style training, I reached out to Paul Roberts, personal trainer and owner of Sand and Steel Fitness, a gym that specializes in weight loss and functional strength training. Roberts isn’t your average gym owner. The 41-year-old has a Bachelor of Science degree in Biomedical Engineering from Johns Hopkins University, a law degree fromRutgers University School of Law, and over 30 personal training and CrossFit certifications. After losing his father to a heart attack while scuba diving (Roberts’ father struggled with obesity for years), Roberts dedicated his life to helping others get healthy.

“The short answer: HIIT is good,” Roberts says. “It occupies a space of weight loss and strength. People think it’s more magic than it is, but jog and then sprint when the ball comes near you. Humans are used to this ‘high-low’ stuff.”

HIIT workout regimens vary, but in general they include repetitions of one to four minutes of high intensity exercise (the kind that cannot be sustained for a long time) followed by a period of rest. For example, HIIT can be three minutes of fast running followed by one minute of walking, repeated five or six times. In the HIIT class I attended, we cycled through full body movements like burpees, mountain climbers, push-ups, and squats, for a total of 30 minutes.

“Compared to sitting on a bench, HIIT does burn a lot more calories,” Roberts explains. “You can lift weights and build muscle while improving conditioning.”

Energy sources are different for HIIT versus endurance exercise

An important difference between HIIT and endurance training is the body’s energy source for HIIT workouts. HIIT is considered anaerobic, meaning it’s an exercise in which the body derives energy without the use of oxygen. In contrast, endurance training is an aerobic exercise. Aerobic energy production is slower and cannot alone meet the fast energy needs of HIIT.

“HIIT uses the glycolytic pathway,” Roberts explains. In the glycolytic pathway, your body breaks down glucose into molecules that can power muscle contractions for quick energy needs (there is an even faster phosphagen system, but I won’t go into that here). Once a steady state exercise is reached, such as in endurance training, the body incorporates the aerobic system, using both carbohydrates and fat as an energy source. To be clear, the body uses several energy production systems during exercise, but the faster anaerobic pathways for energy are critical for HIIT workouts.

If you wear a continuous glucose monitor (CGM),you might notice that your blood sugars rise after HIIT. Roberts says that during exercise “your body increases the amount of free glucose to provide energy.” He continues: “Blood glucose goes up with any kind of training, not just HIIT training. With longer endurance training, your body uses up the glucose. But with HIIT training, your body increases free glucose faster than you can use it.”

The effect of HIIT on glucose sensitivity

This brings us back to our original question: what does research reveal about the effects of HIIT on our health, especially for those who are tracking blood sugar levels?

In a 2018 clinical trial on patients with Type 2 diabetes, HIIT exercise had comparable or better effects on fitness, fat loss, and blood sugar regulation compared to endurance training, even with 45% less time spent in HIIT. In another 2018 study of HIIT for individuals with Type 2 diabetes, researchers found significant improvements in insulin sensitivity following functional high-intensity training, a form ofHIIT that integrates aerobic and resistance training using varied, real-world movements.

The intensity of interval training does appear to matter when it comes to health benefits. In a 2019 clinical trial with prediabetic young adults, high intensity interval training was more effective than low intensity interval training at reducing fasting blood glucose levels and progression to Type 2 diabetes.

But according to the Swedish study I mentioned earlier, there is an upper limit to HIIT’s benefits. Too intense for too long, and glucose tolerance begins to suffer. In their study, excessive HIIT training included 152 total minutes of high intensity intervals at 95% VO2 max (the maximum ability for your heart to pump oxygen around your body) in a week.  

In addition to monitoring glucose, it is important to prevent injury.

“Both athletes and those looking to improve their health through exercise should carefully monitor the response to training, as too much exercise might have negative effects,” the authors of the Swedish study wrote. “Using changes in glucose tolerance or careful tracking of glucose homeostasis using CGM [continuous glucose monitoring]could be a minimally invasive and a novel approach to optimize the amount of exercise associated with the greatest benefits.”

In addition to using a CGM during HIIT workouts, maintaining proper technique is another important tip for injury prevention. As a personal trainer, Roberts pays special attention to his clients’ form as fatigue sets in during HIIT workouts.

“Injuries occur when there’s overtraining because people introduce compensations that hurt their shoulders and knees,”Roberts says. “Work as hard as you can without introducing any faults in the exercise.”

I was always careful about proper form during HIIT, but I’ll admit that I’m out of practice. With the COVID-19pandemic, I haven’t joined a HIIT class in quite some time. Seeing the benefits of HIIT-style exercise on important measurements of health is certainly an inspiration to “HIIT” it hard—just maybe not too hard.

References

Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers

Getin the Zone: Use Energy Systems to Train with Purpose

Fuel Choice During ExerciseIs Determined by Intensity and Duration of Activity

The effect on glycaemic control of low-volume high-intensity interval training versus endurance training in individuals with type 2 diabetes

Functional high-intensity exercise training ameliorates insulin resistance and cardiometabolic risk factors in type 2 diabetes

Effects ofDifferent Dosages of Interval Training on Glycemic Control in People With Prediabetes: A Randomized Controlled Trial

The timer hit three minutes, and I collapsed into an exhausted heap, muscles quivering against the cool gym floor.I had just finished my first “HIIT” fitness class. High IntensityInterval Training has taken center stage in gyms across the country. HIIT workouts, known for their repeated pattern of high intensity exercise followed by a period of rest, are touted for aiding weight loss and improving cardiometabolic health.

The first time I tried HIIT was in 2016, when my husband deployed to the Middle East. I was at home with two young children, and I needed the ability to carry a toddler and open a jar of pickles on my own. At the time, my local gym offered an eight-week HIIT program (and free childcare!), so I figured why not? Over the course of those eight weeks, I improved my endurance, strength, and mental fortitude, leading to my first ever unassisted pull up—and an easier time with the heavy lifting at home. I was hooked on HIIT.

But in a study published in the journal Cell Metabolism last month, researchers in Sweden found an upper limit to the benefits of high intensity training. They report that excessive HIIT exercise impairs mitochondria function and glucose tolerance in healthy individuals. Before questioning my HIIT workouts, I wanted to gather more information about this popular approach to exercise and its health benefits.

Trainer weigh-in on HIIT workouts

To learn more about HIIT-style training, I reached out to Paul Roberts, personal trainer and owner of Sand and Steel Fitness, a gym that specializes in weight loss and functional strength training. Roberts isn’t your average gym owner. The 41-year-old has a Bachelor of Science degree in Biomedical Engineering from Johns Hopkins University, a law degree fromRutgers University School of Law, and over 30 personal training and CrossFit certifications. After losing his father to a heart attack while scuba diving (Roberts’ father struggled with obesity for years), Roberts dedicated his life to helping others get healthy.

“The short answer: HIIT is good,” Roberts says. “It occupies a space of weight loss and strength. People think it’s more magic than it is, but jog and then sprint when the ball comes near you. Humans are used to this ‘high-low’ stuff.”

HIIT workout regimens vary, but in general they include repetitions of one to four minutes of high intensity exercise (the kind that cannot be sustained for a long time) followed by a period of rest. For example, HIIT can be three minutes of fast running followed by one minute of walking, repeated five or six times. In the HIIT class I attended, we cycled through full body movements like burpees, mountain climbers, push-ups, and squats, for a total of 30 minutes.

“Compared to sitting on a bench, HIIT does burn a lot more calories,” Roberts explains. “You can lift weights and build muscle while improving conditioning.”

Energy sources are different for HIIT versus endurance exercise

An important difference between HIIT and endurance training is the body’s energy source for HIIT workouts. HIIT is considered anaerobic, meaning it’s an exercise in which the body derives energy without the use of oxygen. In contrast, endurance training is an aerobic exercise. Aerobic energy production is slower and cannot alone meet the fast energy needs of HIIT.

“HIIT uses the glycolytic pathway,” Roberts explains. In the glycolytic pathway, your body breaks down glucose into molecules that can power muscle contractions for quick energy needs (there is an even faster phosphagen system, but I won’t go into that here). Once a steady state exercise is reached, such as in endurance training, the body incorporates the aerobic system, using both carbohydrates and fat as an energy source. To be clear, the body uses several energy production systems during exercise, but the faster anaerobic pathways for energy are critical for HIIT workouts.

If you wear a continuous glucose monitor (CGM),you might notice that your blood sugars rise after HIIT. Roberts says that during exercise “your body increases the amount of free glucose to provide energy.” He continues: “Blood glucose goes up with any kind of training, not just HIIT training. With longer endurance training, your body uses up the glucose. But with HIIT training, your body increases free glucose faster than you can use it.”

The effect of HIIT on glucose sensitivity

This brings us back to our original question: what does research reveal about the effects of HIIT on our health, especially for those who are tracking blood sugar levels?

In a 2018 clinical trial on patients with Type 2 diabetes, HIIT exercise had comparable or better effects on fitness, fat loss, and blood sugar regulation compared to endurance training, even with 45% less time spent in HIIT. In another 2018 study of HIIT for individuals with Type 2 diabetes, researchers found significant improvements in insulin sensitivity following functional high-intensity training, a form ofHIIT that integrates aerobic and resistance training using varied, real-world movements.

The intensity of interval training does appear to matter when it comes to health benefits. In a 2019 clinical trial with prediabetic young adults, high intensity interval training was more effective than low intensity interval training at reducing fasting blood glucose levels and progression to Type 2 diabetes.

But according to the Swedish study I mentioned earlier, there is an upper limit to HIIT’s benefits. Too intense for too long, and glucose tolerance begins to suffer. In their study, excessive HIIT training included 152 total minutes of high intensity intervals at 95% VO2 max (the maximum ability for your heart to pump oxygen around your body) in a week.  

In addition to monitoring glucose, it is important to prevent injury.

“Both athletes and those looking to improve their health through exercise should carefully monitor the response to training, as too much exercise might have negative effects,” the authors of the Swedish study wrote. “Using changes in glucose tolerance or careful tracking of glucose homeostasis using CGM [continuous glucose monitoring]could be a minimally invasive and a novel approach to optimize the amount of exercise associated with the greatest benefits.”

In addition to using a CGM during HIIT workouts, maintaining proper technique is another important tip for injury prevention. As a personal trainer, Roberts pays special attention to his clients’ form as fatigue sets in during HIIT workouts.

“Injuries occur when there’s overtraining because people introduce compensations that hurt their shoulders and knees,”Roberts says. “Work as hard as you can without introducing any faults in the exercise.”

I was always careful about proper form during HIIT, but I’ll admit that I’m out of practice. With the COVID-19pandemic, I haven’t joined a HIIT class in quite some time. Seeing the benefits of HIIT-style exercise on important measurements of health is certainly an inspiration to “HIIT” it hard—just maybe not too hard.

References

Excessive exercise training causes mitochondrial functional impairment and decreases glucose tolerance in healthy volunteers

Getin the Zone: Use Energy Systems to Train with Purpose

Fuel Choice During ExerciseIs Determined by Intensity and Duration of Activity

The effect on glycaemic control of low-volume high-intensity interval training versus endurance training in individuals with type 2 diabetes

Functional high-intensity exercise training ameliorates insulin resistance and cardiometabolic risk factors in type 2 diabetes

Effects ofDifferent Dosages of Interval Training on Glycemic Control in People With Prediabetes: A Randomized Controlled Trial