Lots of people use diet and activity trackers such as My Fitness Pal to log their food intake and exercise. After all, there's an old saying that "you can't manage what you don't measure." And yet it seems to be backfiring.
I get email after email from people using these trackers who can't understand why they're not losing weight. They're entering in every morsel of food and logging every activity. According to their trackers, they should be shedding losing two or three pounds a week. And yet the scale hasn't budged -- or they've actually gained weight!
The Problem With Net Calories
Here's how these trackers work: You start every day with a certain number of calories to spend. That number is based on your height, weight, age, sex, activity level, and your goals -- that is, whether you're trying to lose, gain, or maintain your current weight.
Calories are subtracted from your balance as you log your meals into the diet tracker over the course of the day. Ideally, you don't get to zero too early in the day. But if you do, there's a solution. Let's say it's 5 p.m. and I'm down to my last 400 calories. But wait! I can take an evening run, log it into the app and now I've got 840 calories to spend on dinner! How awesome is that?
The general principle here is sound: The more you move, the more you can eat. In practice, however, these "net calorie" calculations are inaccurate and misleading -- and they are suckering people into eating too many calories.
How Logging Exercise Leads You Astray
Although diet tracking apps can help you get an accurate picture of your calorie intake, they are much less reliable in determining how many calories you burn. Here are three ways they tend to get it wrong.
1. Your baseline may be too high. In order to calculate your baseline calorie requirements, you indicate your activity level: sedentary, lightly active, moderately active, or very active. This does not refer to how much you exercise -- we'll get to that in a moment. This is just about your daily activity level. And guess what? Most people select an activity level that's one or two categories higher than their lifestyle actually warrants. Unless you rope cattle 8 hours a day, your lifestyle probably does not qualify as "very active."
If you use a wearable fitness tracker like a Fitbit or Jawbone or even a low-tech pedometer or step counter, you can use that to help you select the proper category for your lifestyle.
- Fewer than 1,000 steps a day is sedentary.
- 1,000 to 10,000 steps or about 4 miles a day is Lightly Active.
- 10,000 to 23,000 steps or 4 to 10 miles a day is considered Active.
- More than 23,000 steps or 10 miles a day is Highly active.
If you walk or run for exercise, you can count those steps and/or miles toward your baseline activity level if you want, but then you can't enter them again as exercise. They've already been counted.
2. The calories burned from additional activity are often overestimated. You can use your diet tracking app to log other types of physical activities and exercise, such as a spinning class or yard work or ballroom dancing. However, you may not be burning anywhere near as many calories as it says you are. As with the readouts on the aerobic equipment at your gym, diet trackers may overestimate calories burned by anywhere from 10 to 25 percent. If you're using those overly optimistic estimates to justify an extra dessert, you're kidding yourself.
3. Some of those calories are being counted twice. If I spend the next hour sitting at my desk, I'll burn about 100 calories. Those non-active calories are already accounted for in my daily calorie allowance. If I spent the next hour on the stationary bike instead, I'd burn 500 calories. That's 400 more calories than I would have burned sitting at my desk.
But if I log my time on the bike into my diet tracker, it doesn't add 400 calories to my total allowance... it adds 500. Essentially, it counts those 100 baseline calories twice.
The more activities you enter in to your exercise diary, the more this double-dipping error compounds -- especially if you're logging a lot of low-intensity activities like housecleaning or yoga.
I once heard from a woman who said she burned 3,000 calories a day. She was only eating 2,500. She couldn't figure out why she was gaining weight. Sure enough, she was using an app to track her food intake and exercise.
According to MFP, her baseline calorie needs were about 1,800 calories a day. She then logged activities for almost every hour of her day: making beds, folding laundry, unloading the dishwasher, yoga class, walking the dog, grocery shopping, weeding the garden, playing the piano, cooking dinner, and so on. According to her tracker, all those activities were burning an extra 1200 calories a day -- which gave her a total "net calorie" allowance of 3,000 calories a day. She figured she could eat 2,500 calories a day and still lose weight.
In reality, all of her routine activities probably only burned a couple hundred calories above and beyond her baseline. Instead of eating 500 calories less than she burned each day, she was really eating 500 calories more than she burned each day. No wonder she wasn't losing weight.
How to Avoid the Net Calorie Trap
Diet trackers like My Fitness Pal can be a great tool for getting a handle on how many calories you are taking in. Activity trackers like Fitbit are a great way to keep track of how active you are. But to avoid the net calorie trap, I suggest that you keep the two separate. Don't log your exercise and other activities into your diet tracker or sync your wearable fitness tracker to your diet log.
If you're trying to lose weight, calculate your baseline calorie needs, using the guidelines I suggested above, and set your daily calorie goal for 300-500 calories less than that baseline. If you do nothing else, you should lose half to one pound a week. If you also add 30 to 60 minutes a day of moderate to vigorous exercise (without increasing your food intake to compensate), you'll lose a little faster.