Whether you consider a six-pack a goal or a beverage is beside the point when it comes to extra fat at your waistline. Abdominal adiposity — excess stomach fat — is associated with increased mortality risk, regardless of body mass index.
The skinny on fat
To understand HIIT’s role in health and longevity, you need to first understand that all fat is not created equal.
Two types of belly fat, subcutaneous fat and visceral fat, accumulate in your abdomen, but they look and act very differently. Subcutaneous fat is stored just underneath the skin, says Tom Holland, a Connecticut-based exercise physiologist and fitness consultant.
It’s visceral fat, however, that you should be concerned about. Nestled deeper in your abdomen, adjacent to your organs, it’s “almost like an endocrine organ” that poses serious health risks, Holland says.
Unlike belly fat, visceral fat is “metabolically active,” says endocrinologist Reshmi Srinath, director of the Weight and Metabolism Management Program at Mount Sinai Hospital. It produces molecules known as adipokines that can increase inflammation in various organ systems. Such inflammation is associated with chronic conditions, such as insulin resistance, higher glucose levels, diabetes, heart disease and fatty liver disease.
Your waistline measurement is generally an accurate predictor of excess visceral fat and the health risks associated with it. According to Srinath, who is board certified in endocrinology and obesity, women whose waists measure 35 inches or more and men whose waists measure 40 inches or more are at increased risk of conditions including heart disease, diabetes, hypertension, hyperlipidemia and obstructive sleep apnea.
But although a larger waist circumference and excess visceral fat generally “go hand in hand,” Holland says, this isn’t always the case. According to Evan Jay, a physician assistant and athletic trainer at Redefine Healthcare in New Jersey, some thin people who don’t appear to carry extra weight in their abdomen do have visceral fat. Meanwhile, there are people with larger waistlines who don’t have excess visceral fat.
The only way to know for sure what kind of fat you’re carrying in your stomach is through imaging, which, Srinath says, isn’t typically done in clinical practice. Instead, in addition to looking at waist circumference, your health-care provider will note clinical markers indicative of visceral adiposity, Jay says. These include low HDL cholesterol, high triglycerides, high blood pressure and high fasting blood glucose, all of which are associated with excess visceral fat.
HIIT to the rescue?
Can HIIT reduce stomach fat? The answer is yes, according to a 2018 meta-analysis, which looked at 39 studies involving 617 subjects. “HIIT significantly reduced total (p = 0.003), abdominal (p = 0.007), and visceral (p = 0.018) fat mass,” the study’s authors said. What the authors did not say is that HIIT reduced fat better than other forms of exercise. And, in fact, studies have not found a difference in fat loss between HIIT and moderate-intensity continuous exercise in laboratory trials or in the real world.
“From a clinical perspective, there’s really no difference” between HIIT and moderate exercise, Srinath says. “The real benefit of HIIT,” she adds, is its efficiency.
Holland agrees. The “true beauty of a HIIT workout,” he says, is how little time it takes compared with moderate-intensity exercise, such as brisk walking, leisurely swimming or biking on a flat surface at a conversational pace. “When you look at a lot of studies,” he adds, “you’re getting the same results with high-intensity interval training in half the time.” And that, he says, helps people maintain their routine. He explains: “It’s, ‘Hey, I’m getting the same results — and in half an hour rather than an hour. I’m more likely to be consistent.’ ”
HIIT training also has potential cardiovascular health benefits. A 2019 meta-analysis reviewed 22 researched articles comparing HIIT training with moderate-intensity training. The authors found that when subjects’ total energy expenditure was equal, HIIT and moderate training produced similar reductions in weight, body fat, total cholesterol and cardiorespiratory fitness, all of which reduce the risk of cardiovascular disease. However, the HIIT subjects spent less time working out than their counterparts; their sessions were about 10 minutes shorter than the moderate exercisers’ sessions.
Also, because HIIT workouts tend to be shorter, participants may be more likely to avoid “compensatory eating,” says Holland. In other words, you might be less prone to “rewarding” yourself with a bigger portion or a dessert after a 25-minute HIIT session versus a 45-minute jog.
In fact, because fat loss depends on an overall caloric deficit, exercise alone won’t necessarily drive results. According to Jay, fat reduction has more to do with your meals than your workouts. Anybody interested in starting a HIIT program who hasn’t started looking at their diet, he says, “should take all that energy and redirect it.”
What to expect from HIIT
None of this is to say that you shouldn’t give HIIT a try. There are many ways to do it; the type of exercise doesn’t matter, as long as you get your heart rate into the “red zone,” Holland says. Common moves include compound (multi-joint) movements, such as variations on burpees, squats and lunges, “due to their relative difficulty and concomitant high heart rate response.”
If arthritis or other mobility issues preclude you from high-impact movements, you can turn any low-impact workout, such as cycling, swimming or using the elliptical, into a HIIT workout. All you need are a timer and some motivation.
According to the American College of Sports Medicine, the work periods should range from five seconds to eight minutes and be performed at 80 percent to 95 percent of your maximum heart rate. The time commitment varies, but it can take as little as 20 minutes.
If you don’t know your maximum heart rate or don’t have a heart-rate monitor, Holland suggests going by feel, or rate of perceived exertion (RPE). On a 0 to 10 RPE scale, shoot for eight, nine or 10 during your work intervals. To maintain the intensity throughout the session, he suggests shorter intervals, ranging from 20 seconds to a minute. “You’re outside your comfort zone, but you’re not staying there very long.”
Pace yourself, so by the time your work interval is complete, you’re out of breath. “And right at that point where you finally catch your breath, it starts again,” Holland says. Your ratio of work to recovery time can be adjusted to accommodate your fitness level, but it should generally range from 1:4 to 1:1. This might look like a minute of burpees followed by two minutes of recovery or 20 seconds of jump squats followed by a minute of recovery.
Although you might be tempted to minimize your recovery time in pursuit of a more intense workout, that strategy can be counterproductive. “If you cut [your recovery] too short,” Srinath says, “you can’t go at a high intensity for the next interval.”
Holland says a 25-minute HIIT session might include five one-minute rounds of high-intensity exercise, each followed by two minutes of recovery, sandwiched between a five-minute warm-up and a five-minute cool-down.
Regardless of what moves and time intervals you use, it’s important to vary your routine. As you get fitter, your body becomes more efficient — and you burn fewer calories. “Our bodies are very smart machines,” Holland says, so “we need to mix it up.”
To avoid injury and burnout, Srinath suggests incorporating HIIT into your routine two to three times a week; on other days, you can insert strength training and low-impact aerobic exercise, such as walking or swimming. Although the workout is appropriate for any age, if you’ve been diagnosed with a heart condition, you should exercise with medical supervision, Srinath says.
HIIT is an excellent choice for anyone who has time constraints and wants to optimize their long-term health — in other words, just about everyone, including beginners. No matter what your HIIT workout looks like, it will be done before you know it.
Pam Moore is a Boulder-based freelance writer, speaker, marathoner, Ironman triathlete and certified personal trainer. Visit her at pam-moore.com.