Can Exercise Make Up For A Bad Diet?
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Can Exercise Make Up For A Bad Diet?

It is possible to burn off the excess calories consumed by eating a bad diet by doing more exercise, but we wouldn’t recommend it. Trying to out train a bad diet is not only very difficult and unsustainable, but unhealthy.

However, whether exercise can “make up” for a bad diet will also depend on your goals. Do you want to lose fat, gain strength and improve your general health, or is your only goal to lose weight?

Can you lose weight with a bad diet?

Technically yes, you can lose weight even if you have a bad diet. This is because you will lose weight if you are eating less calories than your body uses.[1] But you would need to exercise a lot and you would likely feel hungry and tired throughout. To understand why it would be so difficult, it’s helpful to understand how your body uses your calories.

Your body’s energy usage comes down to your:

Basal Metabolic Rate (BMR): the minimum number of calories your body needs to accomplish its most basic functions, such as breathing, cell production, nutrient processing and circulation.[2] This accounts for about 70% of your energy usage.

Thermic effect of food (TEF): About 10% of the energy you use is to digest, absorb and metabolise the food you eat.

Physical Activity: Roughly 20% of your energy usage is through your day-to-day movement and exercise. This is the energy you burn when making yourself breakfast, walking around your workplace and in your planned exercise, such as at the gym.[3]

Add the calories used for these purposes together and you get your total daily energy expenditure. This is the number of calories your body needs to maintain its composition. To lose weight, you need to either use more calories than your body needs through exercise, or eat less than your body needs. You can calculate your TDEE using this calculator.

But you’d have to really exercise a lot more to out train a bad diet. This is because your planned exercise only accounts for a very small amount of energy burned. For example, a moderately active 70kg female’s total daily energy expenditure would be around 2,300 calories.

However, in one 45-minute strength training session she might burn around 200 calories. This means that a strength training session would only account for less than 10% of her daily energy expenditure.

To put the calories in perspective, 200 calories is the equivalent of eating an apple and a banana, or a single small slice of pizza. Even if she worked out especially hard in a long gym session, she’s unlikely to burn enough calories to offset a poor diet.

What happens if you try to ‘out train’ a bad diet

Although you can lose weight eating junk food if you’re in a calorie deficit regardless of what you’re eating, you won’t get the same results as if you were eating a healthy, balanced diet. This is because your body needs nutrients, which are broken down into macronutrients and micronutrients. In essence, macronutrients (protein, carbohydrates and fat) are how your body gets its energy. Micronutrients are vitamins and minerals, which help your body perform important functions such as strengthening your immune system, healing and support your nervous system.[4] If your body isn’t getting the energy and nutrients it needs, this is what will happen…

You’ll lose muscle, not fat

Protein is an essential nutrient for functions such as building and repairing muscle and making hormones and enzymes. Many processed foods are low in protein and high in sugar and fat. If you aren’t getting enough protein but you’re in a calorie deficit, you may still lose weight, but it will likely be from muscle, instead of fat.

You won’t build muscle

If you want to build muscle, you need to be in a calorie surplus of about 5%, doing strength training and eating adequate protein. Using our example of a 70kg female, this would mean eating another 120 calories. If you’re not eating enough protein, your body won’t have the nutrients it needs to recover and build muscle mass. This means that without protein, your muscle-building efforts at the gym will go to waste.

You’ll feel lethargic

Have you ever noticed how when you eat poorly, you’re more inclined to choose the couch over the gym? A bad diet with plenty of refined carbs and sugar like bread, pasta and cakes will lead to a drop in blood sugar. This is what makes you feel tired, grumpy and since the energy is used quickly, hungry for more.[5]

If you’re going into a gym session with low energy, you’re likely to struggle through it. This means you won’t feel like pushing yourself to get more out of your session. You also probably won’t enjoy it and you might be more likely to make excuses as to why you shouldn’t go next time.

Your mood and mental clarity will suffer

What you eat directly affects the function of your brain and ultimately, your mood. Your brain is primarily fuelled by glucose, which your body gets from eating carbohydrate-rich foods. If you eat slow-release carbohydrates, you will experience longer-lasting energy and a more level mood.[6] Protein is also essential for brain function because it supports production of serotonin, the “happy hormone”. Furthermore, eating high fibre foods such as fruit and vegetables support the good bacteria in our large intestine, which has been linked to managing mood and stress levels.[7]

Your risk of disease increases

If your body isn’t receiving the nutrients it needs to perform at an optimal level, you will increase your risk of developing many chronic diseases, such as type 2 diabetes, certain cancers, and cardiovascular diseases.[8] A balanced diet also plays a role in maintaining a healthy immune system. A diet lacking in one or more nutrients can impair the production and activity of immune cells and antibodies. This can leave you susceptible to getting sick more often and more severely.[9]

What should I do in the gym if I overeat?

While it’s possible to exercise off excess calories consumed through a bad diet, it won’t leave you feeling good. It is not only ineffective, but it’s unhealthy. Although the best approach will always be to eat a healthy, balanced diet with plenty of protein and whole foods, it’s okay to splash out and enjoy yourself from time to time.

So next time you have a big meal or a big weekend, don’t “punish” yourself in the gym or stress out about “undoing” any hard work. Remember that that the key to a sustainable, heathy and enjoyable lifestyle is balance.