There are two basic types of aerobic training, long-slow distance (LSD) and high intensity interval training (HIIT). With the long-slow distance, you exercise at a constant level of intensity, usually based on your age and fitness level, for a set amount of time. The HIIT training is characterized by short bursts of high intensity exercise, as shown by a higher pulse rate and exercise intensity, interspersed with brief recovery periods, where you slow down, and let your pulse drop down. The main advantage of doing HIIT training as opposed to the more conventional LSD type of aerobics, is that you get the same, or better results with far less investment of training time. Indeed, studies show that just 6 HIIT training sessions over a 2-week period resulted in the same changes in muscle oxidative capacity as doing continuous moderate intensity aerobics that required 3-fold as much training time, and 9-times more training volume. A recent study showed that doing HIIT of 10x 60 second intervals at 90% of maximum heart rate led to an immediate increase in insulin sensitivity as measured by a lower resting glucose level in diabetics.
So it appears that you can get the same, or even superior benefits with HIIT compared to conventional LSD training. The notable advantage of HIIT is far less time in the gym. In addition, from a bodybuilding perspective, you also are less likely to slip into an overtraining state from doing HIIT compared to hours of conventional aerobics.
A current issue of aerobic training is whether you should exercise in a fasted state, or eat something prior to training. Some believe that exercising in a fasted state permits more fat oxidation, especially when done first thing in the morning. The idea here is that glycogen levels are low in the when you awaken, and thus it's easier to tap into fat stores when you exercise at that time. One study found that 6 weeks of conventional aerobic exercise in the fasted state produced changes that resulted in greater muscle oxidative enzymes (required for fat oxidation or "burning."), and also increased glucose and fatty acid transport capacity. Young men who engaged in fasted aerobics didn't gain weight despite consuming a higher fat and calorie intake.
Based on these findings, a new study had 16 overweight, obese women engage in HIIT for 6 weeks. They used the 10x 60 seconds HIIT protocol, during which they raised their heart beat levels to 90% of maximum for 60 seconds, followed by a recovery period in which they slowed down (they were on stationary bikes) for another 60 seconds. They did 10 bouts of this per session, three times a week for a total of 18 sessions. But eight of the women consumed a meal prior to the exercise session, while the other eight did the exercise in a fasted state. The women who ate consumed a meal an hour prior to exercise, while the fasted women ate their last meal before exercise the evening before, but did eat a meal an hour after the exercise. The meals consisted of 439 calories, with 74% of the calories derived from carbohydrates.
The results showed that both groups showed similar beneficial changes, and that eating the meal prior to training had no effects on these changes.Specifically, the women showed reduced fat in their thighs and abdominal regions. And they got this from only 30 minutes of exercise a week. HIIT may be more efficient at lower body fat levels because of increased release of hormones that promote fat mobilization, such as epinephrine and norepinephrine. HIIT also leads to a higher post-exercise oxygen consumption, which means a higher resting metabolic rate compared to conventional aerobics. One recent study also suggested that HIIT produces a greater decrease of appetite after training, which means less total food consumption.
One change that didn't occur was an increase in insulin sensitivity. This effect more often happens in men, and is related to a greater depletion of existing glycogen stores. When women exercise, they are more efficient at preserving glycogen levels. In fact, they use up to 50% less glycogen then men during high intensity exercise. In addition, about 25% of people just don't get any change in insulin sensitivity following exercise, and another 15% show a decline in insulin sensitivity. But since abdominal fat, especially the deep-lying visceral fat, is related to insulin sensitivity, and since all the women in this study did lose significant amounts of abdominal fat, the odds are that their insulin sensitivity was improved, but the effect was more subtle.The women also showed lean mass gains in their legs, which never occurs with conventional aerobics.Gains in lean mass, or muscle, are known to boost insulin sensitivity.
So for those who lack the time to engage in long aerobic sessions in an effort to reduce excess body fat levels, HIIT may be the best way to go.
Gillen, JB, et al. Interval training in the fed or fasted state improves body composition and muscle oxidative capacity in overweight women.Obesity 2013;21: 249-2255.
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