A common refrain directed toward those interested in losing weight is that ‘it’s as simple as balancing Calories In vs. Calories Out.’ On paper, this makes sense: Expend more energy than you consume, and your body will be in an energy deficit. Expend less energy than you consume, and your body will store the excess as fat. However, this advice is far too simplistic to be valuable for the following reasons:

1. You don’t accurately know your Calories In

Yes, it is possible to track of all food and drink that one consumes. Unfortunately, the labels on our foods are not that accurate. The US FDA only requires that the Calorie composition of food be within 20% of the label (21 CFR 101.9). Researchers have confirmed that most packaged snack foods and frozen meals have been found to exhibit accuracy within this range (Jumpertz et al. 2013), although the labels almost always underpredicted the actual number of Calories. Of course, many foods do not have nutritional labels (e.g., fruits, vegetables) and as a result, they are also nearly impossible to quantify accurately.

It may be surprising to hear, but the research behind the caloric composition of foods can be an inexact science. For example, it was found that almonds actually contain ~30% fewer Calories than was originally thought (Novotny et al. 2012). How is this possible? It turns out that different foods have different ‘digestibility’ – how well Calories are extracted and utilized as opposed to expelled from the body as waste. It was found that the digestibility of fats from whole nuts, like almonds, differs compared to fats from other food sources.

But even if the accuracy of the caloric content of all of our foods were of a much higher accuracy than that which is even possible, these small errors would amplify for each item we consume thereby increasing the uncertainty over longer periods of time.

2. You don’t accurately know your Calories Out

That’s right, we never truly know how many calories we are burning throughout the day. One’s Total Daily Energy Expenditure (TDEE) is a combination of multiple components. To simplify somewhat, we’ll primarily consider these three contributions: 1. BMR (Basal Metabolic Rate), 2. Non-exercise movement (walking to your car, cooking meals, doing chores), and 3. Exercise.

One’s BMR is the baseline amount of energy required simply to maintain essential physiological processes. This is not a static number and can increase or decrease over time. Specifically, it has been shown to decrease as we age – however, this decrease may be largely due to sarcopenia, which is an age-related loss of muscle mass (Shimokata and Kuzuya 1993). Regular resistance training may be able to mitigate this decline. In addition to aging and muscle mass, what you eat can have a significant impact on your BMR. For example, it has been shown that very-low carbohydrate diets reduce one’s BMR less than similar diets with the same caloric intake but larger ratio of carbohydrates (Ebbeling et al. 2012). Therefore, you may be able to quantify your BMR in a laboratory, but this is number is a moving target that can change based on age, physical activity, and diet.

As of late, activity trackers and other wearable devices have surged in popularity – perhaps partly in an attempt to assist to quantifying one’s non-exercise movement. However, even the best of these trackers have been demonstrated to have questionable accuracy in determining what activity is being performed. There are dozens of references to cite for this – but the inherent value of these trackers is in the motivation they can provide, not in how accurately an accelerometer on one’s wrist can detect a myriad of potential activities.

Even given all of the inaccuracies discussed thus far, many individuals place far too much importance on the ‘number of Calories burned’ during exercise. Even if we assume that there were an accurate method for determining caloric expenditure during physical activity, the vast majority of your body’s energy usage in a 24 hour period is NOT exercise (~500 Cal) but everything else (~1500-2000 Cal)!

Finally, we have specific, focused physical activity which is usually referred to as exercise. The American College of Sports Medicine reports predictive equations for cycling, walking, running, and stepping (Riebe et al. 2018), but we don’t currently have an accurate methodology for determining energy expended during many other tasks such as playing sports, lifting weights, and interval training. In any case, many of these predictive formulas are just that, predictions, and simply cannot account for all of the necessary variables to be of high accuracy.

Recommendation

Effective weight loss is not a hopeless task. The point here is not to ignore energy expenditure and energy intake. Just the opposite. Some people get lost in the details and lose sight of the big picture. Track your weight (not hourly, or even daily, but perhaps weekly) and consider if you are generally moving in the right direction based on your goals. Be considerate of what you eat, and try to incorporate more physical activity – both cardiovascular and strength training – into your daily routine.

This evidence suggests that the advice of calorie counting, either in or out, is far too simplistic to be accurate. However, an increased understanding about diet and exercise is important. Diets without exercise is not optimal; Exercise without consideration of what you eat is also not optimal. Do both, in moderation. Eat generally healthy, but don’t be obsessed with Calories. Try to be more physically active and always consider what and how much you eat, but don’t be obsessed with numbers that are inherently inaccurate.

References

Riebe D et al. (2018) ACSM’s Guidelines for Exercise Testing and Prescription. 10th ed. Philadelphia: Wolters Kluwer.

Code of Federal Regulations: 21 CFR 101.9 – Nutrition labeling of food (g)(5). <https://www.govregs.com/regulations/expand/title21_chapterI_part101_subpartA_section101.9&gt;

Ebbeling C et al. (2012) Effects of dietary composition on energy expenditure during weight-loss maintenance. JAMA. 307(24):2627-34.

Jumpertz R et al. (2013) Food label accuracy of common snack foods. Obesity. 21(1):164-9. <https://onlinelibrary.wiley.com/doi/full/10.1002/oby.20185&gt;

Novotny J et al. (2012) Discrepancy between the Atwater factor predicted and empirically measured energy values of almonds in human diets. Am J Clin Nutr. 96:296-301. <https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3396444/&gt;

Shimokata H, Kuzuya F. (1993) Aging, basal metabolic rate, and nutrition. Nihon Ronen Igakkai Zasshi. 30(7):572-6.