It's a nice, comforting thought: eat a whole pizza tonight and head to the gym for 45 minutes the next morning to make up for it (which tends to not happen, let's be honest).
Unfortunately, out-exercising a bad diet doesn't work for a few reasons, the main reason being it takes a lot longer to work off the energy in junk food than we may realise.
To understand this better HuffPost Australia spoke to Jessica Spendlove, accredited practising dietitian and athletic performance dietitian for GWS Giants, Cronulla Sharks and Giants Netball.
Why can't you out-exercise a bad diet?
"Put simply, it is much easier to over-consume or eat excess energy than it is to burn it off," Spendlove explained.
"Weight gain is caused by an increase in energy intake compared to output, and most people may not really understand how energy dense foods can be and how much energy you actually burn during exercise.
"For example, a person could eat a 1,000 calorie meal in 5–10 minutes, but it would take them much longer than 60 minutes to burn off (probably closer to 90–120 minutes).
"Exercise and nutrition are both important aspects of overall health and weight loss, but when it comes to losing weight you will never be able to burn more than what you eat, which is why dietary choices are so important."
Here's a visual representation on why it's so hard to outrun a poor diet. Note these are approximate values.
Nutrients vs calories
However, it's not all about calorie content. Our bodies process healthy and unhealthy foods very differently. That is, 500 calories of chocolate and 500 calories of vegetables will have different effects on our bodies, which is why calorie counting often doesn't work as we're focusing on calories not nutrients.
"While the weight loss equation is, to an extent, energy in versus energy out, it is also about the source of the energy or the nutrient composition," Spendlove told HuffPost Australia.
I always encourage clients to focus on what they should be eating for their health and performance, rather than what they shouldn't.
Alcohol intake and excess saturated fat, sugar and carbohydrate intake are particular issues when it comes to weight loss.
"Foods which are high in sugar spike our glucose levels, which results in more insulin being secreted. Insulin is the hormone involved in moving glucose out of the blood stream, but while insulin is circulating around it makes it very difficult to lose body fat as insulin is a storage hormone," Spendlove said.
"Foods which are high in fat are naturally high in energy. This is simply because fat is double the energy of protein and carbohydrate for every gram, meaning the more fat in a meal, the higher the energy density of the meal."
"Alcohol is also an important factor. While we are consuming alcohol our body is unable to break down any energy consumed from fat, protein and carbohydrate, meaning anything we eat when we drink, we store," Spendlove said.
"For certain people with underlying medical conditions or even hormone imbalances, composition of what they are eating is particularly important."
What can we do instead?
If you're someone who is stuck in the 'eat junk food, train hard' cycle, here are three tips to follow.
1. Focus on what you should be eating
"I always encourage clients to focus on what they should be eating for their health and performance, rather than what they shouldn't," Spendlove said. "This means focusing on more vegetables, fruits, whole grains, lean protein and healthy fats.
"I find this helps clients reduce the attention on unhealthy foods and also helps meet nutritional requirements."
2. Make small changes
If you're used to eating lots of takeaway or junk foods, rather than going cold turkey make small changes which you can stick with for the long term.
"Make small, realistic changes to your routine and mindset. Extreme changes are never sustainable and don't work long term," Spendlove said. "Seek professional help if you are struggling to break the cycle on your own."
3. Treat treats as treats
Whatever your food vice is, whether it's doughnuts, ice cream or pizza, approach these foods as occasional treats -- not everyday foods.
"Focus on foods and meals you should be eating 80–90 percent of the time and then allow yourself an occasional treat. A healthy, balanced diet can include the occasional treat," Spendlove said.