Just how efficient (or fast) your body is at converting its fuel is known as metabolic rate, and it’s determined by a range of factors.
“Our bodies burn calories in so many ways and it is part nature – [ie,] genetic – and it’s part nurture as well,” says Cleo. “The energy just to keep the body functioning at rest is called basal metabolic rate (BMR), and that actually uses 50 to 75 per cent of your calories for the day. Our digestion uses about 10 per cent of the calories we eat, then we’ve got exercise [and] the physical activity that we do.”
Put simply, if you eat more calories than your body uses, the surplus goes into fat storage; if you eat less, you’ll lose weight. But the story doesn’t end there: age, muscle mass, sleep and hormones impact metabolic rate, so if you suspect yours is on the slower side, there are ways to, ahem, ‘rev’ it.
“Metabolism is always changing to match the lifestyle triggers that we’re giving it,” says exercise physiologist Drew Harrisberg. “It’s so disempowering to think you either have a fast one or a slow one, when actually it’s [in] this constant dynamic state of flux.”
Sorting the metabolism-tweaking facts from fiction, however, can be tricky. Allow us to guide you.
1. Don’t skip meals
If you think of your metabolism as a simple kilojoule burning machine, it’s easy to assume that giving it less food to burn would speed it up. Not so, confirms Cleo.
“This is one of the reasons I always tell people, we’re dieting ourselves heavier,” she says. “Our bodies are programmed to sense a lack of food as starvation and so, in response, it decreases the metabolism, which means we burn fewer kilojoules over time. Just by decreasing how much you eat, you can decrease your metabolism by five to 10 per cent.”
It’s why crash diets can leave you feeling lethargic – your body slows its processes to preserve fuel.
“You feel more tired, your breathing is slower, you just don’t feel motivated to get up and move. That’s actually your metabolism. That’s the reaction your body is having.”
If you are trying to lose weight, prevent your metabolism from nosediving by “reducing your kJs in a healthy and moderate way”, says Cleo.
2. Focus on muscle gains
Tweaking your body composition in favour of muscle is a smart metabolism boosting strategy.
“The body’s level of muscle mass is the biggest factor determining an individual’s [basal] metabolic rate,” explains professor Grant Brinkworth, co-author of the new CSIRO Protein Plus Nutrition and Exercise Plan. “And because resting metabolic rate contributes the greatest proportion of total daily energy expenditure, adopting strategies that increase it will have the greatest impact on increasing energy metabolism.”
Bottom line: your body has to work harder to maintain muscle tissue when you’re at rest than it does fat tissue. It’s why men have a metabolic advantage over women – we naturally begin to lose muscle mass after the age of 30, but you can prevent and even reverse some of that loss by doing regular resistance workouts, strength training, and eating protein (more on that later).
3. Show your gut some love
One overlooked piece of the metabolism puzzle may be – drum roll, please – gut health. A University of Iowa study suggests unhealthy microbiome shifts could significantly slow basal metabolic rate, adding up to an average weight gain of 13kg over a year! According to researchers from Boston Children’s Hospital, friendly gut bacteria may produce fatty acids that optimise metabolic function, while the bad bacteria consume the same fatty acids, slowing it down. More research is needed but in the meantime, try feeding your gut’s good guys with probiotic and prebiotic whole foods such as natural yoghurt, bananas and fibre-rich oats.
Fill up that drink bottle and add some ice cubes while you’re at it: water has its own metabolic effect known as water-induced thermogenesis.
"We’ve seen anything from a five to 30 per cent increase in the number of kilojoules we burn for about an hour after drinking half a litre of water,” reveals Cleo.
Cold water is great – your body needs to expend more energy warming it slowly to body temperature – but any agua is better than none.
5. Chow down on protein
The stuff you eat provides energy, but digesting it requires energy – an outcome known as the thermic effect of food (TEF). The mighty macro protein has a double metabolic effect in that it builds muscle and gives your digestion a workout.
“Compared [with] fat and carbohydrate, protein requires more energy to help digest and absorb and therefore increases the overall thermic effect of food,” notes Brinkworth.
Scientists haven’t yet pinpointed the perfect dose of protein for peak TEF, but Brinkworth says emerging research suggests that at least 25g of high-quality protein in each meal can help build muscle mass, so it could be a good starting point. “This amount could promote a higher level of energy expenditure from increased metabolism, both at rest and while you are eating
6. Boost your shut eye
One of the (many) perks of a good night’s sleep is a well-functioning metabolism. In fact, they’re so closely aligned that just four nights of restricted sleep can lead to a state of ‘metabolic grogginess’, a term University of Chicago researchers coined to describe the body’s reduced ability to regulate fat metabolism and blood sugar. Skimp on sleep and you’ll also see a hike in the stress hormone cortisol – which in turn messes with your hunger and satiety hormones and ramps up cravings for sugar and carbs, a knockout punch for your metabolism.
“Getting that rest reduces stress hormones and just balances everything nicely,” says Cleo.
7. Hold the tea
Antioxidant-rich green tea and warming spices such as chilli, cinnamon and ginger are often credited with revving metabolism. It’s true they have a thermic effect but don’t get carried away by the hype. You’d need to drink five cups of green tea a day to score a three per cent hike in metabolism (which would burn about 200kJ). It’s a similar story for spices, which likely offer a temporary kick but an overall insignificant effect.
“It’s important to remember that the amount of energy expenditure from eating food only contributes a small portion to the total amount of daily energy expenditure, and that consuming fewer kilojoules overall has a much bigger impact in increasing an energy deficit to promote weight loss,” says Brinkworth.
In other words, to hit your body goals, it’s best to focus on reducing kJs sensibly (obvs) and improving your BMR by building muscle mass. When it comes to metabolism, slow and steady just might be the winner after all.
8. Don't stress about grazing
Weight-loss lore: eating several small meals across the day beats three square ones, ’cos it keeps your metabolism revving. In fact, it really doesn’t matter either way.
“Whenever we eat, we have a temporary spike in our metabolism and [burn about] 10 per cent of what we’re eating in the digestion process. But, what matters is not how many meals you eat but the amount of food,” Cleo explains. “If you’re eating three bigger meals a day, your metabolism will have a significant increase three times. If you’re having six small meals, it’ll increase slightly six times. Overall, it’s still going to [burn] roughly 10 per cent of what you eat.” If grazing works for you, keep it consistent. “Our bodies like a predictable rhythm,” Cleo adds.
9. HIIT it
All movement (even fidgeting!) will hike your metabolism to varying degrees, but high-intensity interval training has a special advantage in that it keeps it elevated for up to 48 hours after your sweat session.
“If you do a really intense workout where your heart rate is at 85 to 90 per cent of its maximum, it creates so many by-products, such as lactic acid, that your body has to work hard to clear in the next 24 to 48 hours,” Harrisberg explains.
You may have heard of it as the ‘after-burn effect’, but its fancy name is excess post-oxygen consumption (EPOC). And, thankfully, it’s not restricted to fast-and furious HIIT. Strength training is a great option.
“Weights workouts produce a good EPOC effect, especially if you ‘feel the burn’ and do it for a good 45 minutes to an hour,” Harrisberg adds.
Bonus points for building lean muscle while lifting.