For our purposes here, high-intensity cardio falls between about 80 to 90 percent of your maximum heart rate (MHR) or, if you're not using heart rate zones, about a 6 to 8 on a 10-point perceived exertion scale. What this translates to is exercise at a level that feels challenging and leaves you too breathless to talk in complete sentences. But you're not going all out, as in sprinting as fast as you can. There's no doubt that some high-intensity training work can be helpful for weight loss as well as improving endurance and aerobic capacity.
Once again, it's about hormones. At night your insulin sensitivity decreases, meaning your body must release more insulin than usual to put any carbohydrates you eat at night to use in the body. And by now you know that higher insulin levels can decrease fat-burning and enhance fat storage. In addition, the body naturally produces a fat-liberating hormone called growth hormone within the initial 90 minutes of sleep.
At its core, burning fat comes down to the process of lipolysis—the breaking down of fat lipids, explains Seedman. This happens in the mitochondria of the muscles, or the powerhouses of the cells, responsible for generating the energy our cells need to do their jobs. Exercise has been shown to improve mitochondria function, which then promotes fat breakdown, Seedman adds. Plus, working out helps regulate pretty much all the hormones that optimize fat loss.
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