How Long Does It Take for HIIT Results? What 10 Studies Say

HIIT, short for high intensity interval training, is often touted as a way to lose fat and improve your cardiovascular fitness with workouts that are very short but extremely hard.

Read on, and I’ll show you what sort of results you can expect to see after weeks and months of regular HIIT training.

Here’s what’s covered:

• What is HIIT and what are the benefits?
• What sort of results can you expect after 2-12 weeks of HIIT?
• How long does it take to lose weight with HIIT?

How Long Does It Take For HIIT Results?

If you want to improve your cardiovascular fitness and exercise performance, studies show that HIIT can deliver results in 2-3 weeks. While it can certainly contribute to the calorie deficit required for weight loss, most research shows that the fat burning powers of HIIT are relatively modest.

What Is HIIT?

HIIT is short for high-intensity interval training. The idea is that you get your heart rate up by working extremely hard for a short period of time, usually somewhere between 30 seconds and 4-5 minutes. 

This is followed by a recovery period where you ease up on the intensity and allow your heart rate to drop.

Then you repeat the process, usually somewhere between 5-10 times, depending on your goals, fitness level and the time you have available to train.

HIIT Workout Results: The Benefits of HIIT

HIIT is a time-efficient way to improve both your cardiovascular fitness and metabolic health. You get many of the same benefits of longer workout routines in a relatively short amount of time.

Compared to lower intensity exercise, HIIT also burns more calories for a given workout duration. That is, energy expenditure during 30 minutes of HIIT will be greater than the same duration of exercise performed at a lower intensity (i.e. 60-70% of your maximum heart rate).

How Long Should a HIIT Workout Be?

A HIIT workout, including warming up and cooling down, is going to take somewhere between 10 and 45 minutes.

A bout of long interval training (i.e. 4×4 intervals), for example, will typically take around 45 minutes, depending on how much time you spend warming up and cooling down.

A bout of sprint interval training, on the other hand, can often be completed in around 10 minutes.

What Results Can You Expect After Two Weeks of HIIT?

Studies show that sprint interval training, a form of HIIT involving repeated bouts of all-out exercise, can increase the oxidative capacity of skeletal muscle, as well as improve endurance performance, in just two weeks.

In one study, subjects performed six sessions of HIIT over a 2-week period [1]. 

The routine involved 4-7 bouts of “all-out” cycling lasting 30 seconds, separated by 4-minute rest periods, where the subjects cycled at a lower intensity.

The result?

After doing HIIT 3 times a week for just a couple of weeks, the results show that time trial performance improved by an average of almost 10 percent.

In another trial, HIIT generated identical improvements in time trial performance compared to lower intensity, long duration endurance exercise, despite the fact the HIIT workouts required a lot less time to complete [2].

Here’s how the researchers sum up their findings:

“Given the large difference in training volume, these data demonstrate that sprint interval training is a time-efficient strategy to induce rapid adaptations in skeletal muscle and exercise performance that are comparable to continuous training in young active men.”

HIIT for Weight Loss: How Long Does It Take To Lose Weight?

Do HIIT 2-3 times a week, paired with a solid diet that puts you in a calorie deficit, and you might lose around 2-4 pounds of body fat in your first two weeks.

Weight loss does tend to scale up with body weight, in the sense that the more weight you have to lose, the faster you tend to lose it. 

That is, smaller individuals with less fat to lose will see results near the lower end of that range, while larger individuals with more fat to lose will see results near the higher end.

The numbers you see on the scale may be even larger, because of the difference between weight loss and fat loss.

In the early stages of dieting, weight loss tends to happen more quickly than fat loss, mainly because of losses in fluid and glycogen (the name for carbohydrate stored in your body).

However, most of that weight loss will have been generated by the diet, rather than HIIT training per se.

To repeat a point I made earlier, no matter what form of exercise you do, your ability to lose fat hinges on being in a calorie deficit. 

While high-intensity exercise, be it HIIT, strength training, or even just regular steady-state cardio done at a high intensity, can contribute to the energy deficit required for weight loss, it’s not going to produce radical changes in body composition by itself.

What Results Can You Expect After Six Weeks of HIIT?

After six weeks of HIIT, expect significant gains in cardiovascular fitness. In fact, studies show that regular training will increase your VO2 max, a popular way to measure cardiovascular fitness, by around 8% in around six weeks [3].

VO2 max is a way of measuring aerobic power. It tells you how much oxygen your body can use at a maximal level of effort and is usually expressed in milliliters of oxygen per kilogram of bodyweight per minute.

Some studies show even more impressive results.

In one trial, subjects using a method known as the Tabata Protocol saw their VO2 max shoot up by 15% after six weeks of training [4].

However, the Tabata Protocol describes a very specific type of sprint interval training, in that it uses 20-second bouts of all out sprinting on a stationary bike separated by just 10 seconds of rest.

I’m not talking about a level of intensity that gets you a little out of breath — this type of training is brutally hard.

“If you feel OK afterwards you’ve not done it properly” says Izumi Tabata, the Japanese scientist who put the protocol to the test back in the 1990s. 

“The first three repetitions will feel easy but the last two will feel impossibly hard. In the original plan the aim was to get to eight, but some [subjects] only lasted six or seven.”

And it’s not like the subjects taking part in Tabata’s study were complete novices, unaccustomed to the demands of high intensity exercise.

They were all physically active young men, and were members of varsity table tennis, baseball, basketball, soccer, and swimming teams.

Over the years, the Tabata Protocol label has been watered down, often being applied to any form of training that involves 20 seconds of work separated by 10 seconds of rest. 

Don’t be taken in. 

Many of the so-called Tabata workouts promising to make all your physical fitness dreams come true with less than 10 minutes of exercise are nothing like what was done in the original study.

What Results Can You Expect After Three Months of HIIT?

In one study, 10 minutes of HIIT, done three times a week for three months, delivered many of the same benefits as 150 minutes of traditional moderate intensity exercise [5].

After warming up for two minutes, the participants pedaled fast for three 20-second intervals. Each interval was followed by two minutes of slow pedaling. The workout finished with a three-minute cool-down.

From start to finish, the workout lasted 10 minutes. Only 60 seconds of that time was spent working hard.

By the end of the three-month study, subjects in the continuous moderate intensity group had ridden for 27 hours, with each cardio workout done at around 70% of their maximum heart rate. However, the interval group spent a little over five hours on the bike.

Despite a five-fold lower time commitment, various measures of cardiometabolic health improved to a similar extent in both the HIIT and steady-state groups.

• VO2 peak, a measure of aerobic fitness, increased by 15-20% in both groups.
• Insulin sensitivity, a marker of blood sugar control, improved to a similar extent in both groups.
• Muscle mitochondrial content also increased similarly after HIIT and steady-state exercise.

How Effective Is HIIT for Weight Loss?

Most research comparing HIIT and steady-state cardio show little difference in weight loss between the two types of exercise [9].

Research published in the British Journal of Sports Medicine reports that after 30-35 workouts (around three months if you’re training three times a week), the average amount of fat lost with HIIT training was 3.5 pounds (1.6 kilograms), compared to 2.5 pounds (1.1 kilograms) with steady-state cardio [6].

Granted, the HIIT workouts didn’t last as long. But still, the overall difference in results – one pound of additional fat loss over a 10-12 week period – was relatively small.

In a similar review published in the journal Obesity Reviews, researchers could find “no evidence to support the superiority of either high-intensity interval training or steady-state cardio for body fat reduction.”

A separate research team came to much the same conclusion after doing their own analysis of the research [10].

HIIT and moderate-intensity continuous training (MICT) are similarly effective, they say, and both elicit “modest improvements, and of similar magnitude, in body fat levels and waist circumference in overweight and obese adults.”

They do note, however, that HIIT is a more efficient alternative to steady-state cardio, delivering similar fat-loss benefits with less time spent in the gym.

One of the main selling points of HIIT is its so-called “afterburn effect,” which refers to the fact that your body is still burning calories in the hours and days after a workout is over. 

Unfortunately, the size of the afterburn effect, as well as the extent that post-exercise changes in metabolism and fat burning contributes to weight loss, is not as great as was once believed.

In fact, studies show that the post-exercise calorie burn (known as excess post-exercise oxygen consumption, or EPOC for short) lasts for hours, rather than days [7 , 8].

Compared to strength training, HIIT isn’t a particularly effective way to build muscle mass either. 

If you’re completely deconditioned and out of shape, HIIT is likely to stimulate some kind of increase in muscle mass.

In novices, virtually any stimulus represents an unusual challenge to muscle tissue, which will adapt to that challenge, in part at least, by getting bigger.

But if you’ve got a few years of training behind you, your muscles will already have adapted to such a low level of stress, and HIIT is unlikely to provide a significant stimulus for growth.

How Often Should You Do HIIT?

Once, I got an email from a guy who’d seen one of these “I did HIIT everyday for a month and here’s what happened” videos on YouTube, and decided that he wanted to add daily HIIT to his existing resistance training program.

However, HIIT is a very demanding form of exercise. Doing too much of it can seriously impair your ability to recover and grow.

Back in the day when I was an interval training fundamentalist, the only cardio I did was 20 minutes of HIIT first thing in the morning three times a week. I considered anything other than HIIT as nothing more than a wasted opportunity for progress.

Given the superiority of HIIT over steady-state cardio, I thought to myself, why spend time doing something that takes longer and is less effective?

What I learned the hard way is that high-intensity interval training is very tough on the body, especially if you combine it with heavy strength training and a calorie-restricted diet.

While HIIT training helped me drop fat as well as maintain a reasonably high level of conditioning, I noticed that it had an adverse effect on my strength levels in the gym. And I would often feel burned out, tired and irritable.

If you want to exercise every day, rather than daily HIIT workouts, you’re far better off with some kind of low-intensity exercise, such as walking, or even riding a bike.

If you want to do HIIT while you focus on building muscle, limit it to a couple of short sessions a week.

Frequently Asked Questions

How much weight can I lose in a month with HIIT?

Combine regular HIIT with a diet that puts you in a calorie deficit, and you’ll lose anywhere between 4-8 pounds of fat in a month. However, most of that weight loss will have been generated by the diet, rather than the HIIT.

Is HIIT 3 times a week enough?

Research shows that doing HIIT 3 times a week is enough to boost your VO2 max (a measure of cardiovascular fitness), improve exercise performance, as well as enhance various measures of cardiometabolic health, such as insulin sensitivity and skeletal muscle mitochondrial content.

What happens if I do HIIT everyday?

Doing HIIT every day doesn’t give your body a chance to recover, and will ultimately lead to a physiological state known as overreaching, a term used to describe temporary overtraining.

Some of the symptoms include persistent tiredness, heavy, sore muscles, and a reduction in exercise performance. Most people are better off doing HIIT 2 or 3 times a week, rather than every day. Your body needs time to recover from and adapt to any training stimulus, which it can’t do if you’re performing intense exercise on a daily basis for extended periods of time.

Should you do HIIT on an empty stomach?

Doing HIIT on an empty stomach is no more effective for fat loss than doing it after a meal. Ultimately, training in a fed or fasted state is a decision that can be left to personal preference. As far as improving your body composition is concerned, there’s no great harm or benefit to one or the other.

Can you lose weight just doing HIIT?

HIIT can certainly contribute to the calorie deficit required to lose weight. However, just doing HIIT without making any changes to your diet isn’t going to deliver much in the way of weight loss. No matter what form of exercise you do, be it HIIT, low-intensity steady-state cardio or resistance training, your ability to lose weight depends on you being in a calorie deficit.

Can you lose fat and gain muscle at the same time using HIIT?

HIIT can certainly make a contribution to the calorie deficit required to lose fat. But as I mentioned earlier, getting your diet right is the most important thing when it comes to fat loss.

As for gaining muscle, certain types of HIIT (such as sprint interval training on an exercise bike) can have a small impact on muscle size in the thighs. But overall, HIIT isn’t going to do much for muscle growth, especially in the upper body. You’ll need to do some form of resistance training to make your muscles grow.