Macrofactor: How to Lose Weight in 2023 as an Endurance Athlete

Let me tell you how to reach and maintain your target weight as an endurance athlete.

Weight loss is a difficult subject for endurance atheletes. Eating disorders, obsession with intake and crash diets are big problems in athletes historically. While the idea of fitness for all body types is a great inclusivity goal, it’s true that for most endurance sports, body weight and composition are among the most important factors in performance.

I’ve been training seriously as a cyclist for 4 years now. By nature, I’m an endomorph, meaning that I tend toward body roundness. After graduating from high school, I was never able to wear 31 inch waist shrink to fit Levi 501’s and remember one dieting episode while in medical school when my weight was as low as 137 pounds. As someone who is 5 foot 5 inches tall, my usual weights in the 150 lb range are in the overweight range on the standard BMI chart.

I did make some progress with simple food logging using Lose It which provides a comprehensive food database. Regardless of calorie target, honest food logging reveals where exess calories are coming from and provides feedback for better eating and nutrition.

Even riding thousands of miles a year on the bicycle, my weight has been remarkably stable. For endurance athletes, the challenge is fueling the efforts that need hundreds of calories for energy yet create a minor deficit that creates a negative caloric balance. THe problem with simple food logging is handling the daily expendature variations for big effort days, regular training and rest days. Without good control, the body adjusts intake for stable weight at its setpoint. Which for me as a cyclist has long been unnessesarily high. There’s no real reason why I need to walk around with 25% of my body weight as fat that needs to be accelerated and dragged up hills on my super light carbon fiber bicycle.

The solution I’ve found is an iPhone app called MacroFactor. I discovered Macrofactor from this post at Lifehacker The Best Paid Diet App. That title is not really accurate because MacroFactor is really a weight management system, not a diet app.

The new approach taken by the system is to control caloric intake based on weight trajectory. So if one wants to maintain a weight, the app will find the daily calorie intake that produces a flat weight curve without gain or loss. If the goal is weight gain, it will increase calorie target until the desired positive weight slope is achieved. And the same for weight loss. Since expenditure is varying day to day, week to week and month to month, the process is dynamic, adapting to the rate of change. The algorithm used isn’t explicit, but after using it for a few months, I can see that it is primarily looking at slope over time to predict current expenditure.

Here’s a screen capture of the last 3 months of my weight data:

You can see that the dark blue trend line doesn’t run through the center of the data like a moving average or a smoothing technique like a loess curve. Instead, it looks like the line is showing the slope in the underlying data, often sitting above the line as it trends down. And yes, I’ve been able to get back down toward that goal weight in just a few months. A few flat sections as well- when I had recovered from COVID and the last week or so.

Now here’s what the. expenditure curve looks like over the same period.

The initial suggested daily burn was way too high and as my weight was not trending down at the targeted rate, the algorithm reduced it day by day until the desired rate of decline was in view. Actually, it overshot a bit and then, when my weight leveled off, it brought the expenditure down to a pretty low level of 1800 kcals/day. This oscillation is one of the problems anticipated with a simple feedback loop where the output is controlled by a lagging input. Like a simple thermostat, temperature oscillates above an below the target temperature as the thermostat cycles the furnace on and off. But the oscillation is around the correct value, providing a homeostatic control over time. We’re talking about one or two hundred calories a day- a 10 or 15 percent error range.

The app tracks macronutrients in a nice display as well with targets for protein, fat and carbs. So as it shifts the calorie allowance based on weight change, it pushes me to keep protien intake up. So I base all of my meals around protien often supplemented with a protein shake, fueling the exercise sessions with all the allowed carbs. I’ve shifted body composition without being underfueled and without being too hungry between meals.

For me, one of my favorite aspects of the program is how non-judgemental it is. If I go over the day’s caloric intake target to fuel a big workout, the app doesn’t care. In fact, the whole approach not only discourages cheating, it encourages complete entry of all dietary intake. After all, the more I say I eat, the more it allocates as calorie expenditure, since all it knows is what I say I eat and what I weigh. Expenditure is based only on the relationship between calories in and weight.

The monthly subscription is $11.99 a month, but has certainly been worth it for me. I believe that even once I’m at a goal weight and stable I’ll be happy to support the developers and see how training and life influence my metabolism over the longer term..