HIIT is not superior to moderate-intensity continuous exercise for fat loss. HIIT is not superior to moderate-intensity continuous exercise for increasing lean muscle mass.
One of the most popular misconceptions is that High-Intensity Interval Training (HIIT) keeps your metabolism revved for hours after exercise. High-intensity resistance exercise burns about 8.83 kcals/minute. (1) This is slightly less than someone bicycling at a moderate effort (9.8 kcal/min) and jogging a 10-minute mile (12.3 kcal/min). (2) Metabolic rate after exercise, or what the scientific community calls EPOC (Exercise Post Oxygen Consumption), returns to baseline very quickly after exercise, usually less than an hour.
EPOC is the measure of oxygen consumption after exercise, which is a measure of metabolic rate. Many exercise factors affect EPOC, including exercise intensity and duration. The duration of EPOC is elevated after exercise is determined by ATP resynthesis, heart rate, ventilation, body temperature, hormone levels (i.e., adrenaline), lactate, glycogen replacement, etc.
HIIT can certainly increase metabolic rate compared to moderate and low-intensity exercise, but it’s not the massive amount that one would expect. A landmark study in 1997 compared running with either HIIT training (i.e., 20 max -1-minute intervals) to moderate-intensity exercise (i.e., 30 minutes of continuous exercise) over nine hours after exercise. The HIIT training resulted in an energy expenditure of about 57 calories greater over the nine hours.
A 2021 meta-analysis compared HIIT to moderate-intensity continuous exercise and found that HIIT post-exercise burned ~32 calories, whereas moderate-intensity exercise burned ~24 calories. (3) Thus, HIIT technically does burn more calories post-exercise, but don’t think that because you performed a HIIT gives you the green light to eat whatever you want. The calories burn post-exercise, or the afterburn effect is relatively small.
Since there are very small changes in 24-hour fat oxidation or fat burning that occurs with exercise, it’s been suggested that exercise causes a redistribution of nutrients toward muscle for fuel replenishment and cell regeneration and away from adipose tissue, causing increases in lean mass with decreases in body fat. (4)
HIIT VS. LOW-INTENSITY EXERCISE EFFECTS ON VISCERAL ADIPOSE TISSUE
High-Intensity Interval Training or HIIT can be near maximal exercise efforts with two to three-minute rest periods. Cycling classes often use sprint cycling for 30 seconds or less with two to five-minute rest periods. Although both HIIT and low-intensity exercise burn calories, there are physiological differences in adipose tissues’ response to each.
Adipose tissue can be divided into visceral adipose tissue (VAT) and subcutaneous adipose tissue (SAT). VAT is adipose tissue located around surrounding organs, whereas SAT is located under the skin. Although most people want to get rid of subcutaneous fat, VAT is more dangerous. VAT is directly related to type II diabetes, cardiovascular disease, several types of cancer, and mortality. (5, 6) Some of the more interesting aspects of how the two forms of fat affect overall health came from studies that came from liposuction surgery. Subjects who had liposuction surgery had 9.1 kg or 4.1 pounds of fat. Still, they had no health improvements such as improved insulin sensitivity or changes in glucose which occurs with regular weight loss. (7) Thus, subcutaneous fat is not causing the major health-related issues; it’s the fat surrounding the internal organs.
IS HIIT MORE EFFECTIVE FOR FAT LOSS?
Exercise training can reduce VAT and SAT without changing body weight. Exercise intensity has an important role in the reduction of VAT. VAT is much more important for overall health and wellness than SAT. In a review of the literature, high-intensity aerobic exercise without dietary restriction resulted in larger reductions in VAT than moderate and low-intensity. Moderate-intensity had a greater impact on reducing VAT than low-intensity exercise. Another interesting finding was that training duration didn’t significantly reduce VAT. (8) This indicated a certain intensity threshold for reducing VAT. Despite less fat oxidation occurring during HIIT training than in low-intensity training, HIIT training can offer several beneficial health benefits, such as reducing VAT and SAT in a shorter time and reducing body fat. (9)
REVIEW OF HIIT STUDIES
A 2020 review of the literature of 32 studies found the following when comparing HIIT modalities, duration, frequency, and intensity. (10)
a) Exercise duration greater than 150 minutes had no greater effect on reducing VAT.
b) Both aerobic exercise and HIIT were effective in decreasing VAT.
It should be mentioned that this meta-analysis included many overweight and obese subjects with metabolic diseases, such as diabetes. In a meta-analysis of healthy young adults, it was found that resistance exercise reduced body fat by 1.4% and reduced visceral fat as well. (11) VAT also has been linked to insulin resistance; it is suggested that resistance exercise reductions in VAT could be due to improved insulin sensitivity. One study found that resistance exercise resulted in a 10.3% reduction in VAT and an 11.2% reduction in SAT, and insulin sensitivity increased by 46.3%.(12)