It is said that weight loss is 80% diet. Indeed, the key to successfully losing weight is maintaining a calorie deficit relative to your TDEE over a sustained period of time. And the easiest way to achieve a deficit is by eating fewer calories.
TDEE, which is short for "total daily energy expenditure", is the total number of calories your body burns each day. This includes your basal metabolic rate (BMR), the calories you burn when digesting food and also the additional calories you burn through physical activity like exercise.
If you eat exactly the same number of calories as your TDEE, you will neither lose or gain weight. This is why TDEE is sometimes called your "maintenance calories".
While maintaining a calorie deficit will see you lose weight, because you are eating fewer calories than your body burns, there is another simple way to lose weight, too. Rather than eating less, you could simply increase your TDEE by doing more physical activity and exercise.
But what impact does physical activity have? And is it recommended to eat less and move more to lose weight, or should you focus on one or the other?
How activity impacts your TDEE
To best illustrate the effect that exercise has on your weight loss journey, let's first visualize the impact that physical activity has on your TDEE. If you run a maintenance calculation using Gym Geek's calorie calculator, you'll see that activity is defined using the Standard Activity Factors.
SAFs are numbers which multiply your BMR based on your activity level, resulting in your TDEE number. The SAFs range from x1.2 for sedentary to x1.55 for moderately active, and all the way up to x1.9 for extra active. Check out our SAF guide page for the definitions.
To start, we'll run multiple calculations using a hypothetical example. We've chosen a 30 year old man of typical height, weighing 169 lb to demonstrate the effect of exercise. For each activity level, we calculated this man's BMR and total TDEE, based on the Mifflin St-Jeor formula. Assuming 266 calories are burned by digesting food each day, we then worked out how many calories could be associated with physical activity.
Sedentary to moderately active
Most people's activity ranges between sedentary and moderately active. In fact, 85% of Gym Geek dieters describe themselves as either sedentary, lightly active or moderately active, while just 12.5% say they are very active.
Looking at the illustration above, you can see in all three cases that BMR and digestion account for most calories burned each day. Even if you are moderately active, which is 3-5 days of exercise per week, the calories burned through activity account for only 25% of the total.
With these numbers, you can see where the adage that "weight loss is 80% diet" comes from. If you want to maintain a calorie deficit of 500-1,000 calories per day, you must eat less.
The effect on your deficit
Let's suppose you are already lightly active, exercising 1-3 days per week. In the illustrated example, if you increase your activity to moderately active, your TDEE increases by 300 calories per day.
If you originally aimed to lose 1 lb a week, you'll need a total deficit relative to your TDEE of about 500 calories. But, if you now burn 300 calories more, your deficit effectively becomes 800. If you aimed to lose 2 lb a week, your deficit becomes 1,300.
In both cases, if you increase your activity level, your deficit increases. You might think this is a good outcome, but it can result in you losing weight faster than expected. Losing weight quickly can be not only dangerous, but counterproductive to your long term weight goals.
If you lose weight too quickly, you risk losing muscle as well as fat. You also risk becoming tired and suffering from rebound hunger, which can result in a plateau or difficultly keeping to your deficit.
That's why you should always recalculate your calorie deficit if you increase your activity level.
Eating back what you burn
For most people, the easiest way to track calories and lose weight is using the same calorie goal each day. This goal is calculated based on your current weight and typical activity level.
However, other people prefer to eat more on days where they are more active. This can make a lot of sense, since if you are doing more exercise on a particular day, you'll need a greater number of calories (particularly carbs) to keep your body fuelled.
If you want to vary your calorie intake daily, you can follow the pattern of "eating back what you burn". In this pattern, you first calculate your calorie goal as if you were sedentary. This is the baseline number of calories you will eat each day.
Then, as you do exercise, you'll need to track (or estimate) how many calories you burn each day. Many smartwatches and fitness trackers can do this calculation for you. Although all fitness trackers are inaccurate, they can give you a rough idea of the calories you burn.
Suppose for example that your calorie deficit is 500 calories per day and your baseline calorie goal is 1,500 calories. If you burn 600 calories through physical activity, you can "eat back" 600 calories to eat a total of 2,100 calories for the day.
Because 1,500 was calculated based on little to no activity, this new goal of 2,100 will still provide the same calorie deficit of 500 calories per day.
Cardio vs resistance training
When it comes to exercise, you can choose either cardio or strength training, or ideally a combination of both. However, if maximizing calories burned is your goal, then regularly doing a variety of cardio exercises is essential.
Although resistance training does burn some number of calories, cardio exercises like running and cycling burn far more calories. Cardio also promotes heart health and regularly doing cardio through the week will increase your overall fitness level over time.
The Physical Activity Guidelines recommend that healthy adults do at least 150 minutes of moderate aerobic activity or 75 minutes of vigorous aerobic activity each week.
Is resistance training worthwhile?
While resistance training won't burn many calories, there is an important reason to consider resistance training while you lose weight. When most people say they want to lose weight, what they really mean is that they want to lose fat rather than muscle.
The biggest reason people lose muscle while dieting is because they lose weight too quickly. If you’re on a large deficit (2 lbs per week), it’s almost inevitable you’ll lose some muscle. That’s one reason why we recommend losing weight gradually rather than rapidly.
But, even if you have a modest goal of say 1 lb per week, you can still lose muscle.
The only way you can avoid losing muscle is by doing some resistance training while you lose weight. Resistance training promotes the growth of muscle, increases metabolic rate and increases bone density.
That's why we recommend doing some cardio and resistance training when following a long-term weight loss plan.
References
Speakman, J. R., & Selman, C. (2003). Physical activity and resting metabolic rate. Proceedings of the Nutrition Society, 62(3), 621-634.
Ashtary-Larky, D., Ghanavati, M., Lamuchi-Deli, N., Payami, S. A., Alavi-Rad, S., Boustaninejad, M., ... & Alipour, M. (2017). Rapid weight loss vs. slow weight loss: which is more effective on body composition and metabolic risk factors?. International journal of endocrinology and metabolism, 15(3).
Piercy, K. L., Troiano, R. P., Ballard, R. M., Carlson, S. A., Fulton, J. E., Galuska, D. A., … & Olson, R. D. (2018). The physical activity guidelines for Americans. Jama, 320(19), 2020-2028.
