High-Intensity Interval Training for Metabolic Health

intense training using tire

If you are looking for a free, accessible, and effective way to improve metabolic health in a short amount of time, look no further. HIIT is an efficient option for those of us who are constantly on the go. Now that we’ve got your attention, let’s explain this powerful tool for metabolic health.

What is HIIT? 

high-intensity exercise and blood sugar

HIIT stands for high-intensity interval training. It is a type of exercise that incorporates short bursts of high-intensity exercise interspersed with rest. The high-intensity segments of the workout can last between 5 seconds and 8 minutes in duration with a heart rate between 80-95% of our max. For those of us who are unfamiliar with our maximal heart rate, 220 minus your age is a good starting point to estimate your maximal heart rate. For example, for a 34-year-old adult, their estimated maximal heart rate would be 186 beats per minute (220-34= 186). HIIT can be incorporated into almost any mode of exercise.  Whether your go-to is a bodyweight circuit in your living room, a run down the street, or an indoor bike ride, HIIT can be incorporated into any of these.

When it comes to a specific protocol, the following are examples of formats of HIIT exercise that have been scientifically shown to improve metabolic health: Tabata, 10-by-1, and 4-by-4. Tabata involves 20 seconds of intense exercise with 10 seconds of rest. This circuit is repeated using your favorite exercises. For example, if you are looking for a quick at-home workout, try 20 seconds of burpees followed by 10 seconds of rest. Then 20 seconds of jumping jacks, with 10 seconds of rest. And continue this pattern for 20 minutes. Another option would be a 10-by-1 workout, which is a 20-minute routine with 10 rounds of 1 minute of high-intensity followed by 1 minute of low-intensity. 4-by-4 involves 4 rounds alternating 4 minutes of high-intensity work with 3 minutes of rest. These are just the heavily researched options; it does not mean that you can’t modify or create your own version and have it be equally effective. Mix it up, and have some fun.

What does HIIT have to do with blood sugar? 

high-intensity exercise for metabolic health

When we exercise, we predominately use energy from two sources: fat or glucose. Fat already stored in our body is the primary way we power low-intensity exercise and glucose is the main fuel source for high-intensity exercise.  High-intensity exercise in particular activates a metabolic pathway called the adenosine monophosphate-activated protein kinase (AMPK) pathway. This signals a transporter (GLUT4) to pull glucose from the circulation into the cell to be used as fuel. The amount of sugar we have in circulation at the start of our workout is determined by what food we ate previously and a bunch of other factors like hormones, sleep, stress, etc. that are beyond the scope of this article. As we power through our workout, we will eventually utilize all of this circulating glucose. Luckily, our body is incredibly adept and it saw you crushing those burpees and said “we better release some of our stored sugar so you can continue to workout at this high-intensity.” Our bodies mobilize our stored glycogen in the liver and muscles, breaks it back down into glucose, and releases it into circulation. Glycogen that we have stored in muscles can only be used by the cells in the muscle where it is stored. Thus, if we only do bicep curls, we can only use glycogen stored in the bicep to refuel that muscle (leaving large storage of glycogen in larger muscle groups unused). However, glycogen stored in the liver, can be released into circulation and transported throughout the body. This is one of the reasons why our blood sugar often rises during high-intensity exercise. 

blood sugar and exercise

HIIT also impacts our blood sugar by increasing the production of hormones called catecholamines. These catecholamines can raise the production of glucose by 7 or 8-fold. This can occur even if we only use 3 or 4 times as much glucose. Our body is not able to see the countdown clock on the treadmill or predict when we plan to end our workout. This results in our body sometimes overestimating the amount of glucose that we need to finish our exercise, leaving some blood sugar left unused in circulation. However, these blood sugar spikes are not harmful to metabolic health, they are necessary in order to power our activity. In fact, the rise in blood sugar that accompanies high-intensity workouts is a healthy response that improves insulin sensitivity. During exercise, when we need glucose to power our physical activity, the job of insulin is put on hold and insulin production is inhibited. This is because when we use glucose as fuel, our workout reduces the amount of insulin needed to bring down our blood sugar because exercise is overtaking the job of insulin (to bring down blood sugar levels). In this way, HIIT can be a way to help individuals with insulin resistance.

The advantages of HIIT even persist after the workout is completed. The longer or higher the intensity of the workout, the more glucose we use. The more glucose we use, the longer it will take the body to refill our body’s stored glucose (called glycogen) and restore normal blood sugar levels. During this time, the body is consistently pulling blood sugar from circulation and working to refill our glucose storage reservoir, lowering the amount of blood sugar left floating around in the blood. Additionally, HIIT has been shown to improve insulin sensitivity for up to three days after our workout. Enhanced insulin sensitivity allows us to more efficiently pull glucose from circulation and put it to use in our cells as fuel.

How can HIIT improve our health? 

intense exercise to improve health

HIIT is a powerful tool that we can use to enhance our metabolic health. HIIT has a wide range of benefits including improved aerobic capacity (how well the heart and lungs get oxygen to the muscles), blood pressure, resting heart rate, glucose metabolism, insulin sensitivity, and fasting glucose. When it is combined with a healthy diet, HIIT can significantly improve body composition. The high-intensity nature of HIIT means that a greater number of calories are burned during our workout. This can give a boost to those of us who are looking to lose weight.

In the previous section of this article, we mentioned how during a HIIT workout insulin gets to take a break from its job of reducing blood sugar. After a HIIT workout, our insulin sensitivity is improved. These insulin-related benefits play a huge role in our health. When there is too much insulin, the body can develop a resistance to this hormone, and develop a condition aptly named ‘insulin resistance.’  Research has shown that 40% of adults have insulin resistance, many of which are unaware that their metabolism is not operating as it should. This resistance to insulin significantly increases the risk of developing type 2 diabetes, obesity, high blood pressure, high cholesterol, and cardiovascular complications.  Thus, HIIT is a free, drug-free, time-efficient, and easily accessible way to significantly impact our health.

How can I implement HIIT into my exercise program? 

how to do HIIT

The Department of Health and Human Services recommends that the average adult perform at least 150 minutes of moderate-intensity physical activity or 75 minutes of vigorous aerobic activity each week. While HIIT may not be right for everyone (or every week), it is an efficient way to reach our 75-minute goal of vigorous activity each week. The 75 minutes do not need to be done all at once either.  Three 25-minute workouts can be incorporated into a lunch break or while watching your favorite Netflix show at night. If you are trying HIIT for the first time, ease into it. Add a short 10-minute workout into your normal routine the first week. Then gradually increase the amount of HIIT you incorporate into your week. Remember to give balance to your physical activity whenever possible. Mix up the muscles you work and always give yourself adequate time to recover. When possible, give yourself at least one day between HIIT workouts. As you get used to the intensity, longer workouts or workouts on consecutive days can be implemented, if you desire. Exercise does not have to be a chore, grab a friend, a family member, a trainer, or join a group exercise class to help keep yourself accountable to your health goals and make it enjoyable. 


Author: Dr. Colleen Gulick