So, you know the importance of your carbohydrates, fats and protein but not sure how you body actually uses them? Check out our extensive breakdown of how your body uses your macros for fuel.

Discover how your body uses your macros - Women's Health and Fitness Magazine.



Optimal fuelling for exercise depends on your type of training. “We generally tend to use a mix of both carbohydrates and fats to fuel exercise up to maximal intensities across most intensity levels,” says dietitian Margaret Mielczarek. “As exercise intensity increases, we shift from predominantly using fats to fuel exercise towards predominantly using carbohydrates to fuel exercise.”

Your need for pre- or post-workout carbs depends on duration and intensity at which you train; low-intensity workouts tend to use fat as their predominant fuel source, they don’t often need additional nutrition as fuel or for recovery. However some ‘topping up’ of glycogen stores may be necessary in the morning or afternoon when your blood glucose is at a fasted level. “Topping up your glycogen stores, which deplete slightly after an overnight fast, is important to ensure adequate supply of carbs for energy,” says Mielczarek. “And if training in the afternoon a while after eating, again you have a small snack to top up glycogen stores so that you have plenty of fuel to burn for energy.” According to the Australian Sports Commission, the amount of carbohydrates recommended for pre-training fuel is 70 grams for most athletes. The amount of glycogen you have already stored in your muscles is also dependent on your diet overall. Consuming a diet with adequate carbohydrates will ensure that there is plenty of glycogen available. “We typically recommend that about a quarter of your plate should be complex carbohydrates,” says Mielczarek. “It’s important to opt for complex carbohydrates such as brown rice, quinoa, sweet potato, pumpkin and corn, or even fruit, dairy and nuts.” 

With adequate carbohydrate intake, we usually have enough energy stored to last for 60 to 90 minutes without ‘bonking’ or hitting the wall. “Interestingly, the longevity of our carbohydrate stores is generally independent of intensity of exercise,” says Mielczarek. Daily carbohydrate needs should be adjusted depending on your training. According to the Australian Institute of Sport, low-intensity exercise requires around three to five grams of carbohydrates per kilogram of body mass. Moderate exercise for approximately an hour a day ups these needs to five to seven grams and endurance athletes training from one to three hours in a given day need around six to 10 grams. Pre-workout snacks generally consist of one to four grams of carbohydrates per kilogram of body mass. 

If training extends past the limits of your carbohydrate stores (snack or no snack), your body will eventually opt to burn fat. According to studies published in Sports Medicine journal, the rate depends on duration and exercise intensity, but as a general recommendation, exercise going for one to two hours should be supplemented with 30 grams of carbohydrate (for the second hour, naturally) and two to three hours of exercise with 60 grams per hour, which is technically the maximum rate the body can break down a single source of carbohydrates. A recent study published in Sports Medicine journal found that a carbohydrate mouth rinse could improve performance for exercise of shorter duration and higher intensity (for example, an hour at 75 per cent of VO2 max). The science: putting something in your mouth activates areas of the brain that respond to gustatory (taste) stimulation and act with an appropriate emotional, cognitive and behavioural response. 


Some athletes seek to become more ‘fat adapted’ by severely limiting carbohydrate intake. “Fat adaptation usually requires an athlete to go on a really low carb diet, less then 25 to 50 grams per day, to get their body to go into ketosis, thereby burning ketones as the fuel source,” says Mielczarek. This is the same principle behind the ketogenic diet, but unless your goal is rapid weight loss, fat adaption can be of limited use.”

“Becoming fat-adapted may have some disadvantages,” says Mielczarek. “The body has two energy systems it can use – carbohydrate breakdown and fat breakdown – so why wouldn’t we want to use both?

“Those who are going to great lengths to be fat-adapted (and only use fat as their energy supply by omitting all carbohydrates) are limiting their fuelling options and compromising their ability to sprint, surge or go at intensities which require quick breakdown of energy.” 

At lower intensities the body can efficiently use fat as a primary fuel source. “Carbohydrates provide fuel for working muscles because there is little oxygen available to utilise fat, as fat burning is an oxidative process,” says Mielczarek. Studies have shown that by consuming a higher fat diet, the body can become more used to using fat as fuel. A study published in the Journal of the International Society of Sports Nutrition found that a high-fat diet (more than 1.3 grams of fat per kilogram of body weight per day) not only performed better – a higher one-rep maximum bench press – but also had higher resting energy expenditure. Regardless of your goals, fats should comprise around 30 per cent of your daily caloric intake.


According to the Australian Sports Commission, an inactive person needs to eat around 0.8 grams of protein per kilogram of body mass, simply to maintain muscle. For athletes who are consistently repairing exercise-induced muscle damage, the requirements increase (this is across all sports, not just strength training). To both repair and grow muscles, the body needs around one to two grams of protein per kilogram of muscle mass. Because protein is about balance (rather than storage), it’s important to ingest good protein at every meal rather than just before and after the gym. The Gatorade Sports Science Institute recommends eating 20 to 25 grams of protein at every meal as well as immediately after exercise. Ingesting protein after training is important but there are benefits to ingesting protein prior to a session as well. “You want to start repairing muscle straight away, so it’s important that you have adequate protein in your system during training as well as after,” says Meilczarek. A recent study in Sports Medicine journal found that exercise could reduce blood flow to the gut, so nutrient absorption from recovery fuel can be compromised. To combat the effect, eat a small dose of protein prior to exercise. How much? One study from the Journal of Nutrition found that the ingestion of both protein and carbohydrates (0.15 grams of both per kilogram of body mass) prior to a bout of resistance exercise increased protein synthesis (anabolism) by 48 per cent during exercise, and an additional 19 per cent after exercise.

NEXT: Confused about how to count your macros? Check out our simple guide here.