High-intensity interval training (HIIT) has become the “it” way to work out over the last few years, with studios and gyms capitalizing on the quick-but-effective way of training that alternates between bouts of intense activity, like sprints, with periods of less-intense active recovery or complete rest. Fans of this super-efficient workout swear by its ability to torch calories, burn fat, and build muscle, all in much less time than, say, a 5-mile moderate-intensity run.
Another purported benefit of HIIT is your body’s ability to stay in fat-burning mode long after your workout is finished, which is music to the ears of anyone who follows their Saturday sweat sesh with a boozy brunch. Known as the afterburn effect, this process is said to rev up your metabolism and torch calories for up to 24 hours after exercise. It’s a theory many HIIT-based studios tout as a major health benefit of their workout, promising you’ll reap the rewards of their one-hour workout for days.
But are these benefits too good to be true? Here’s what the science has to say about it.
The process is scientifically known as excess post-exercise oxygen consumption (EPOC). It refers to the oxygen your body needs to restore itself to the pre-workout state (your resting metabolism). Your body uses oxygen to produce fuel (scientifically known as adenosine triphosphate, or ATP) needed for your muscles to fire up during exercise, but it can also call on stored energy sources that don’t require extra oxygen. HIIT workouts typically utilize the latter more so than steady-state exercise, and will also require more oxygen post-sweat session—this is why these short bursts of intense, taxing exercise are associated with the afterburn effect. (Interesting right? Read more on understanding this science.)
“Those [oxygen and calorie burn] go kind of hand in hand after exercise,” says Tedd Keating, Ph.D., C.S.E.S., associate professor of exercise science at Manhattan College. “Your metabolism stays up for a period of time after exercise.” Although your body continues to burn calories after a workout, it’s usually only 6 to 15 percent of the total calories you burned while exercising. So if you burned 300 calories during your workout, your afterburn would only be about 18 to 45 calories. (Exercise isn’t the only way to maintain a high metabolism, btw. Eating a sufficient amount of protein post-workout will also help.)
EPOC is not limited to HIIT workouts—it’s present after all aerobic exercise—but the level of exercise intensity does play a role in just how many calories your body burns post-sweat. Keating reiterates this sentiment by saying that for low-intensity activity, say a treadmill jog, you can expect a lower afterburn effect (about 6 percent calorie burn, toward the lower end of the range), but with HIIT workouts and things like Tabata and speed drills, you could reap an afterburn closer to that 15 percent mark.
There are a few other factors that alter your individual afterburn rewards as well: weight, fitness level, and muscle mass all play a role. “People who are more aerobically fit, their bodies are going to be better fat burners as a whole,” says Keating. You can expect these people to have a more sustained afterburn effect.
The claim that your body continues to burn calories for up to 24 hours afterward is deceiving. Although people who are more physically fit may derive longer benefits from the EPOC effect, it’s usually only up to a couple hours, tops.
A study out of Colorado State University evaluated participants who performed sprint interval training and then measured their post-workout calorie burn. Although the results did show energy expenditure within 24 hours after the workout (it should be noted that your body is constantly burning energy/calories, even at rest), most of the calorie burning was seen during and just after exercise, with the numbers dropping significantly from there.
So does that mean the 24-hour EPOC effect is just a fairy tale? “Never say never. There might be a really heroic exercise program that can do it,” Keating says. “But your typical high-intensity sprint interval stuff that’s being recommended now, like four 30-second bursts or something gentler like 10 one-minute bursts, probably aren’t going to give people that big 24-hour afterburn.”
Even though HIIT is one of the most effective workouts to burn fat, the potential afterburn effect is just part of the puzzle, says Keating. During high-intensity workouts, your body produces more epinephrine and human growth hormone (HGH), which both have fat-burning capabilities; one study found that a 30-second sprint on the stationary bike resulted in a nearly 450 percent increase in HGH production. Other reported benefits of HIIT are improved insulin sensitivity and blood pressure, along with simply being an efficient way to work out.
Interval training is also better at preserving lean body mass, says Keating, and the more muscle you have, the more calories you’ll burn while at rest. Plus, as you get more fit, you’ll increase your VO2 max, which is a number that indicates how efficiently your body uses oxygen during exercise.
The bottom line: While HIIT exercise is a great way to get in shape, be wary of fitness studios promising to transform your body into a magical fat-burning machine long after you towel off. That doesn’t mean you should ditch your favorite boot camp (remember, HIIT training still improves endurance and body composition), but keep in mind that the afterburn effect isn’t a free pass to binge on junk food for two days.
Smoothies, a fixture of juice bars and cafes around the country, are made with fresh ingredients and taste great. But can they help you lose weight? That depends on what goes into them and how you use them, but it’s definitely possible to gradually and healthfully shed pounds with the aid of low-calorie smoothies.
Adding smoothies to your diet can be a healthy move, but it won’t necessarily help you lose weight unless you use those smoothies to replace higher-calorie foods. You need to consistently burn more calories than you eat to lose weight over time, so try drinking a low-cal smoothie when you’d usually have dessert or using it as a daily meal replacement
To make a filling smoothie that’s still relatively low in calories, start with a high-protein base. According to a research review published in the “American Journal of Clinical Nutrition” in 2008, protein is more filling than either carbohydrates or fats, making it a smart choice for weight loss. A 6-ounce container of nonfat, plain Greek yogurt makes a neutral base and has only 100 calories with 18 grams of protein. Soft, silken tofu will make a thicker smoothie. A generous 200-gram slice has 110 calories and nearly 10 grams of protein.
When building your smoothie, use fresh or frozen fruits. Canned fruit and fruit juices often contain added sugar, which contributes calories and raises your risk of several chronic health conditions. For a thick smoothie, throw in a small frozen banana, which adds just 90 calories and blends up to a consistency like that of soft serve. To boost fiber content, add berries. One cup of raspberries has just 65 calories and 8 grams of fiber, and the same amount of blackberries has 60 calories and 7.5 grams of fiber. According to a research review published in 2005 in the journal “Nutrition,” dietary fiber consumption has an inverse association with body weight and body fat mass.
Blending veggies into your smoothie may sound odd, but it’s a great way to ramp up the nutrient content of your drink and keep the calorie count low. Nonstarchy vegetables have about one-third the calories of fruit, and they’re packed with filling fiber. Dr. Ben Kim recommends using a ratio of 40 percent fruits and 60 percent leafy greens, but if the taste is too bitter for you, you can start with just a small handful of greens and gradually increase the amounts until you’re more accustomed to including them. One cup of fresh spinach has just 7 calories, and the same amount of kale has 30 calories.
Body composition is the proportion of fat and fat-free mass in your body. A healthy body composition is one that includes a lower percentage of body fat and a higher percentage of fat-free mass, which includes muscle, bones, and organs.
Body composition is measured to assess your health and fitness level. Often, you will have body composition measured at the start of a weight loss or fitness program and checked periodically to monitor your progress.
Your body is composed of two types of mass: body fat and fat-free mass.
Body fat percent is a measurement of body composition telling how much of the weight of your body is fat. The percentage of your body that is not fat is fat-free mass. There are normal ranges for body fat, which differ for men and women.
Weighing yourself on a regular bathroom scale does not truly assess your body composition because a regular scale cannot tell how much of your total weight is comprised of water, fat, or muscle.
To know if your body composition is healthy, you should get an estimate of your body fat percentage. You can do so by taking simple measurements and entering them into a body fat percentage calculator.
The American Council on Exercise (ACE) gives these ranges of values for different populations:
ACE Body Fat Percent Norms for Men and Women
|10% to 13%
|2% to 5%
|14% to 20%
|6% to 13%
|21% to 24%
|14% to 17%
|25% to 31%
|18% to 24%
Athletes tend to have lower body fat, which may be beneficial for performance in sports such as running and cycling. But having an extremely low body fat percent is a health problem. The female athlete triad increases the risk of injury and health issues. It includes eating disorders, amenorrhea, and decreased bone mass with an increased risk of stress fractures and osteoporosis.
If you are overweight or obese, you have an excessive amount of body fat and a high body fat percentage. You can improve your body composition by gaining lean body mass through building muscle and bones, and through losing excess body fat.
There are several ways to get an estimate of your body fat percentage at home, at the gym, or from your doctor:
Your body composition can be influenced by factors you can’t control:
If your body fat percent is too high, you may want to try to decrease it to improve your health, athletic performance, and wellbeing. You may also be able to decrease your risk of disease. If your body fat percent is below the level of essential fat, you may also want to make changes to bring it up to that level as that will reduce your health risks.
Your body composition and body fat are important measurements when you are on a weight loss program. You could be successful in losing fat and gaining muscle without seeing your weight go down. Tracking your weight loss and fitness efforts with body composition measurement is a good way to see your progress. It is easier than ever with the wide availability of body fat scales.
American Council on Exercise. Tools and Calculators.
Fahey TD. Fit & Well: Core Concepts and Labs in Physical Fitness and Wellness. New York: McGraw-Hill Education; 2017.
National Heart, Lung, and Blood Institute. Assessing Your Weight and Health Risk.