Anaerobic interval training (short, sharp sprints) demands metabolic adaptation, meaning the body is forced to burn fat to maintain high intensity. As a fringe benefit, it also increases energy use for more than 24 hours post-workout.
In a 2008 study, participants exhibited 12 per cent more fat burn during exercise in a six-week program orbiting anaerobic intervals. In a study the year before, active women who engaged in interval training experienced 36 per cent more use of fat for fuel during workouts in as little as two weeks and another study found that women who participated in regular 20-minute cycling intervals for 15 weeks encountered an average 2.5 kg of fat loss. Better, most fat was stripped from the abdomen and legs. The control condition, steady-state aerobic exercise, resulted in zero fat loss.
The biological explanation is anchored by preferential fuel substrates, which depend on both type of training and duration. While aerobic exercise favours carbs first and kickstarts mechanisms that can result in muscle decline or catabolism, high-intensity exercise such as kettlebell swings and sprints burn a greater percentage of fat while conversely piquing pathways that spur muscle growth. Intervals also elicit greater post-exercise oxygen consumption (EPOC) than a steady half-hour treadmill power walk. Anaerobic exercise also raises levels of growth hormone (GH), which promotes fat burn and muscle building beyond levels seen in aerobic exercise. Another fat-burning hormone, adiponectin, is also released by fat tissue during high-exertion muscle contractions.
To reap the perks of a fat-burning workout, 40 minutes is the magic threshold. To really work the science, add a subsequent anaerobic session – weights or other – to tap into the dearth of energy reserves needed for glycolysis and force the body to burn fat. According to MedFitness, the best strategy for maximum fat burning is 30 minutes of weights followed by cardio, which will primarily use fatty acids for fuel, reducing body fat. Bonus: since most fuel is now FFAs, muscle protein is largely spared. “If you lose weight too fast, you’re likely to lose a quarter to a third of your weight loss as muscle, and that directly compromises your metabolic rate,” O’Neill says.
You’ve heard the adage, ‘abs are made in the kitchen’. But if you don’t want to confine yourself to boiled chicken and sweet spuds, that’s not the end of the ab story. You’ve basically got six hours to burn up food energy before it starts to make its way to your midriff/hips/butt.
“Six hours is when we start to store that body fat, but if by lunch you’ve overeaten and starting to store fat, you’ve still got half a day to fix it by either going to the gym and burning it off or reducing the amount of food you’re eating later in the day,” says Matt O’Neill, exercise physiologist and dietitian from Metabolic Jumpstart.
The key now is burning glycogen, which if allowed to accumulate to overload proportions, will be stored as fat, and burning maximum kilojoules to negate any energy surplus. (While an extra 400 kJ here or there won’t equal fat gain, energy excess does compound and will over time result in extra fat.) Sub-30 minute aerobic exercise can prevent new fat stores but not erode the old ones. In other words, doing aerobic exercise first exhausts creatine and glycogen reserves without burning much fat.
Cortisol is notoriously known as the stress hormone that cruelly also facilitates fat storage – particularly around the abdomen – and promotes muscle catabolism. While corporate types and business owners are in the firing line, physiological stress – and that includes exercise – can also pique cortisol. One study found that aerobic athletes had significantly more cumulative cortisol secretion in their hair than non-athletes. In a cortisol showdown, anaerobic training elicits lower cortisol spikes than aerobic moves because while short, sharp sprints and lifts do stimulate cortisol, they also invite growth hormones such as testosterone, a cortisol antagonist.