How to Use a Power Meter to Lose Weight
If you don’t race, you may wonder whether you would benefit from using a power meter. But a power meter can be an effective tool to help with a goal that isn’t strictly related to racing: weight loss.
What’s more, the learning curve required to use it for that purpose is a lot simpler than for training. At the heart of a power meter’s data is a value called the kilojoule (often abbreviated as kJ). A joule is a measure of energy, or work, like a calorie. There are roughly four kJs per food calorie. But, says Allen Lim, a physiologist, cookbook author, and founder of energy food company Skratch Labs, “the human body is only 20- to 25-percent efficient at transferring energy to work; the rest gets lost as heat.”
That's a good thing for people who count calories, because it means the figure of kJs that your power meter captures and displays on the head unit is almost exactly equal to the number of calories burned (if anything, it slightly underestimates calorie burn). And the power meter makes cycling unique; in no other sport is there a reliable, accurate way to directly measure work. And aside from what it can offer you for training, that ability to measure work makes a power meter perhaps the best tool available for monitoring calorie burn and losing weight.
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How Much Do You Need to Eat?
To start, you need to know your total daily caloric needs. That’s a combination of two factors: your baseline, or resting, metabolic rate (RMR); and the energy you expend in activities, from mowing the lawn to riding.
There are various ways to measure RMR, says Nanna Meyer, associate professor of health sciences at the University of Colorado, Colorado Springs. “A lab test is the most accurate, but that’s not practical for most people,” says Meyer, who is also a senior staff dietitian at the US Olympic Committee and has worked with Olympic teams and BMC Racing.
Next best: formulas to estimate RMR. They’re all off by a little bit, but Meyer likes the Harris-Benedict (revised) and Cunningham equations. Cunningham “has proved more accurate for athletes because it relies on lean body mass,” says Meyer, but you need a pretty good idea of your body fat percentage.
Here are the equations:
Harris-Benedict (revised)
Men: 88.36 + (13.4 x weight in kg) + (4.8 x height in cm) – (5.68 x age)
Women: 447.59 + (9.25 x weight) + (3.1 x height) – (4.33 x age)
Cunningham
500 + (22 x lean body mass in kg)
Whatever result either of those equations produce has to be combined with energy burned in activities to get a number called Total Energy Expenditure (TEE), which is calculated by multiplying your RMR by your activity level. Multipliers range from 1.2, for a mostly sedentary person, to 1.9, for a person who has an extremely active job—say, a roofer or backcountry ski guide.
But you don’t need to rely solely on estimation if you have a power meter, because it will precisely track your calorie use on rides; you’ll know, day to day, how many extra calories you burn. You still have to do some estimating, though, because the power meter doesn't monitor calories burned in other activities. When you estimate TEE, use a multiplier that discounts your rides so you’re not double-counting. If you’re riding one to two hours a day but are otherwise a desk jockey, use the 1.2 multiplier.
All of that may seem complicated, but you’re really only keeping track of two numbers: your TEE, minus riding, and the kJs from whichever ride you did that day. Total the two, and that’s how many calories you need to maintain your current weight. Similarly, losing weight is basically “a math equation,” says Hunter Allen, a noted coach and author of Training and Racing with a Power Meter. To lose weight, you have to burn more than you consume.
A pound of fat contains roughly 3,500 calories, which has been cited by nutritionists for decades as a baseline for weight loss: Cut 500 calories a day, and you can lose a pound a week. Turns out, it’s not quite that simple; the human body responds dynamically to weight loss, so you may need more of a deficit, particularly later in your effort, if you’re trying to lose a lot of weight.
You can get a deeper view of your eating and calorie needs by keeping a food diary, says Meyer. For macronutrients, like fat, carbs and protein, it doesn’t need to be exhaustive or lengthy—just diligently record all food for a few weekdays and a weekend day. Repeat it periodically to track changes. Want to know what all that means in calories? Meyer suggests the MyFitnessPal app as one of the better calorie trackers available.
Lose Fat, Become a Better Rider
You know by now that there’s no such thing as spot weight loss. But you can target fat loss in general, and the technique to do it has the added benefit of making you a better rider. It’s called fasted training.
The idea is simple: In the morning, simply go ride. No breakfast, says Allen, although the caffeine in a small cup of black coffee can help release free fatty acids (FFAs) that your body will burn as fuel.
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The key is pace and time. The idea behind fasted training is to teach your body to burn fat, not carbs in the form of muscle glycogen, as fuel. To do that, you need to stay at less than 75 percent of your Functional Threshold Power (FTP), says Allen; go much higher in intensity, and you’ll dip into your glycogen stores and bonk. If you’re new to fasted training or to cycling in general, start with about 60 minute-efforts; over time, you’ll be able to increase your time to as much as 2.5 hours.
In addition to trimming body fat, you’re teaching your body to use multiple energy sources when riding. By using fat, you leave more of your muscle glycogen stores intact for later in a ride, which means you’ll feel fresher even in that third hour, and able to do short, hard efforts like dropping your buddy on that climb.
Fasted training is not an every-ride workout. You should do it only one to two times a week. If you only ride at low intensities, Meyer points out, your body will adapt to burning fat at the expense of its ability to burn carbs. Keep your top-end power by mixing fasted training with shorter, more intense workouts. Take a bit of food on fasted training rides as insurance against a bonk.
For his athletes, Allen likes a workout style he calls the “kitchen sink.” Specific rides vary, but each one lasts three or more hours and has, true to its name, a little bit of everything. An example:
•Before your ride, do a “zero offset” on your power meter to ensure it’s properly calibrated—it’s like checking that a scale reads zero before weighing yourself.
•Start with 90 minutes to two hours of fasted training. (70 to 75 percent of FTP.)
•Bring food. (a simple PB&J on whole wheat offers the right mix of macronutrients) and eat at the end of the fasted training portion.
•Do some combination of high-intensity work. Examples:
•Three to five all-out sprint efforts. (~10 seconds, three minutes recovery in between, at steady-state effort–85 to 90 percent of FTP.)
•Four threshold efforts. (10 minutes, 100 to105 percent of FTP, with five to 10 minutes of recovery in between.)
•Big-gear bursts. (Slow to almost a stop and then do a sprint in a big gear [52x16, for example]. Wind it up to 80rpm, then back off the intensity to 80 percent of your FTP for five minutes; repeat five times.)
•Continue to eat a little bit as needed during the last hour of effort—anything from a gel to a rice cake. Drink as much water or low-calorie sports drink (like Nuun, Osmo or Skratch Labs) as you need.
What to Eat, and When
Unless you’re doing a fasted training session, aim to eat a regular meal one to three hours before you ride, or a small snack (100 calories) within 30 minutes of starting, says Meyer. Most riders eat more on a ride than they need to, says Lim. The lengthy introduction to his second cookbook, Feed Zone Portables, offers a detailed look at athlete physiology and nutrition. The key takeaway: For a lot of rides that last less than two hours, you probably don’t need food at all, says Lim, and on any ride, you’re probably eating more than you need. “When I ask recreational riders how they eat on rides, they take in maybe 10-percent less food than some of the pros I work with, but they’re riding half as far or fast,” he says. This is where the power meter helps again: The kJs don’t lie.
A rider putting out 200 watts for an hour will burn roughly 700 calories. For rides over two hours, says Lim, aim to replace about half your calories burned per hour. This might seem perilous if you’re used to fueling a two-hour ride with two bottles of drink mix, a gel, and an energy bar. But everything you eat has to be subtracted from the total kJs burned on the ride.
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Post-ride hours present another food trap for many riders, both because we’ve been drilled to utilize our glycogen window and because it’s easy to think of food as a reward for working out. “The motivation has to extend past the training session to the recovery period because you can screw it up with the wrong choices,” says Meyer.
You do need food post-ride. But you don’t need that much of it. Meyer suggests a simple fruit smoothie, with whole-milk yogurt for fat and protein to add a level of satiety. Engineered supplements like protein shakes aren’t necessary, she says; whole foods are the best choice. Lim suggests “calorically undense foods” like whole fruits. The key: to properly refuel with the right portions of the right foods, which help you feel full enough to resist the temptation of the potato chip bag.
Weight loss may be mathematically simple, but it’s not easy. Lim and Meyer both caution against focusing too hard on the mechanics—the food tracking, the relentless attention to numbers.
A power meter is a valuable tool partly, says Lim, “because it helps people to quickly learn something that takes a lot of time and work otherwise: How long does it take to burn 500 calories?” But the key is how it fits into your overall plan.
Be mindful of social and behavioral cues—anything from being raised to clean your plate to the size of the plate itself. “I don’t think of foods as good or bad,” says Lim. All of these tools are just that: tools. What matters most is using them to instill healthy, sustainable eating and exercise habits.
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