Should you eat back exercise calories?

You just completed a hard workout, and your smartwatch says you burned 500 calories. On your rest day, your watch says you only burned 250 calories. So, do you eat 500 more calories on your workout day and cut back on your rest day? 

It’s not that you can never adjust your calorie or energy intake based on training; but, if you are adjusting your food intake based on the calories burned, this can be highly inaccurate. You should not use your smartwatch’s estimated exercise calories as a precise “calorie budget” that you eat back every day.

Why can this approach can backfire?

Your smartwatch is estimating energy expenditure, how many calories you are burning, but NOT measuring it. This estimate can have a substantial error due to the fact that GPS watches use algorithms based on heart rate, pace, body size, movement, etc. They can over or underestimate how many calories you’re actually burning, especially for activities like running, strength training, interval workouts, or mixed activities.  

A study by Stanford,  tested seven leading wrist wearables against gold-standard indirect calorimetry. Heart rate was found to be reasonably accurate, within 5%; however, energy expenditure was not. The best device missed energy expenditure by 27%, the worst by 93%. Every single one tended to overestimate.

Even when heart rate is measured somewhat accurately, your heart rate does not equate to the amount of calories burned. This number can vary greatly between individuals including the exercise intensity, fitness levels, heat, terrain, and other factors.

Using this estimated number as an absolute can be misleading. If your watch says you burned 500 calories, this does not mean you actually burned 500 calories, or that you need to eat exactly an additional 500 extra calories. This mindset can create either under-fueling or unnecessary over-fueling.

What are your nutrition needs based on?

Your nutrition needs are not based on workouts alone.  As an athlete, your fueling needs depend on the following:

• total daily energy expenditure (TDEE),

• non-exercise activity thermogenesis (NEAT)

• training volume and intensity,

• glycogen depletion,

• digestion,

• recovery and tissue repair,

• maintaining body temperature,

• hormonal and physiological processes,

• body size and composition,

• training adaptations, and

• overall goals as an athlete for sports performance.

Your body is burning calories all day long, not just when you are active.  Your workout calories are not separate from your total daily needs. Instead of thinking, “I burned 500 calories, so I need to replace it” after a workout, think more about how your body needs fuel to not only for recovery, but to help you as an athlete to adapt to the training demands and to support your overall activity.

What should my fueling needs be based on?

Fueling should be based on training demands, not just calories burned. For athletes, this is the type, duration and intensity of training. 

For example: An interval workout that burned 500 calories, may require different fueling that a 500 calorie easy workout.  The differences are based on glycogen use, carbohydrate requirements, recovery demands, the intensity and upcoming workouts.

Should I adjust my eating on rest or easy days?

Athletes who think “I didn’t burn enough today, so I shouldn’t eat as much”, can lead to under-fueling and/or under recovery. Recovery doesn’t stop when the workout ends.  The difference between fueling on a rest day vs a hard workout day is the additional calories required before, during and after the training to help with energy and recovery. 

Your body still requires energy on rest days for recovery from the previous workout session to maximize muscle and liver glycogen stores, as well as to help with muscle tissue repair, so your body is ready for training the next day. Under fueling on a rest day can compromise your recovery.

Real World Impact

A healthier approach is to think about what your training duration, intensity, upcoming workouts, hunger, recovery, performance, and overall fueling patterns rather than trying to precisely “eat back” exercise calories.

Smartwatch calorie estimations can either overestimate causing a calorie surplus; or an underestimation, resulting in under fueling, risking fatigue, poor recovery, and negatively impair sports performance.

Your smartwatch is a training tool, not a nutrition calculator.  Use it to better understand your training, but don’t let it estimate your calorie needs and how much you’re allowed to eat.  

References:

• Heydenreich J, Kayser B, Schutz Y, et al. Total Energy Expenditure, Energy Intake, and Body Composition in Endurance Athletes Across the Training Season: A Systematic Review. Sports Med Open. 2017 Feb 4:3-8. doi: 10.1186/s40798-017-0076-1

• Schcherbina A,  Mattsson CM, and Waggot D, et al. Accuracy in Wrist-Worn, Sensor-Based Measurements of Heart Rate and Energy Expenditure in a Diverse Cohort. J. Pers. Med. 2017, 7(2), 3.  https://doi.org/10.3390/jpm7020003

• Thomas DT, Erdman KA, and Burke LM. Position of the Academy of Nutrition and Dietetics, Dietitians of Canada, and the American College of Sports Medicine: Nutrition and Athletic Performance.  Journal of the Academy of Nutrition and Dietetics, 2016.116(3), 501-528. https://pubmed.ncbi.nlm.nih.gov/26920240/