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Health & Fitness New research challenges the long-held ‘metabolic window of opportunity’ theory

The moment you step off the treadmill or lower the barbell for the last time, the clock starts ticking. Will you make it back to your locker in time to scarf down your recovery shake or energy bar before it’s too late? Or will you miss out on some of the benefits of your workout by leaving your tired muscles starved of fuel until you get home for dinner?

That stark choice has been drummed into generations of gym-goers since the 1980s thanks to a theory called the “metabolic window of opportunity,” which holds that your muscles are uniquely primed to recover and grow if you feed them immediately after exercise. Miss the window and you’ve missed a chance to get fitter and stronger. Some fitness researchers have even argued that when you eat is more important than what or how much you eat.

But not everyone agrees. In a recent article in the Journal of Orthopaedic & Sports Physical Therapy, prominent sports nutrition researchers Brad Schoenfeld of Lehman College in New York and Alan Aragon of California State University, Northridge, reassess the evidence for the window theory. Postworkout refuelling is certainly important, they conclude – but the emphasis on a narrow window is misplaced.

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There are actually two distinct components of the window theory. The first involves refilling the glycogen fuel stores in your muscles after prolonged endurance exercise. In 1988, researchers at the University of Texas showed that taking in carbohydrates immediately after exercise nearly doubled the rate at which these stores were replenished compared to taking in the same amount of carbohydrate two hours later.

The second element is the “anabolic” window for building new muscle. Similar research found that ingesting protein immediately after a strength-training workout caused new muscle to be formed more rapidly than ingesting the same protein an hour or two later.

Schoenfeld was at one point a true believer in this idea: “I even included a chapter in one of my books that extolled the need to take in protein quickly after training,” he says.

But in the new article, he and Aragon argue that research on the topic turns outs to be conflicting and sometimes flawed. For example, many studies compared postworkout protein to a protein-free placebo, meaning that the control group received less protein overall in the hours following exercise.

They also point to recent evidence that getting 25 grams of protein before a workout (and then eating nothing else for three hours afterward) produced the same muscle gains as getting the supposedly optimal postworkout dose of 25 grams of protein.

“Since the anabolic effects of a meal last up to about six hours, the window becomes much larger when preworkout eating is considered,” Schoenfeld says.

Postworkout protein is still important, he emphasizes – it’s just that the timing isn’t as crucial as once thought, and it depends on when you last ate before the workout. In practice, he and Aragon suggest aiming for no more than four to six hours between your preworkout and postworkout meals. So if you eat a protein-rich lunch at 1 p.m. and lift weights at 4 p.m., you don’t need anything else before dinner at 6 p.m.

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The carbohydrate window, it turns out, is also wider than previously thought, Schoenfeld adds. It’s true that taking in carbs immediately after a workout speeds up how quickly your muscles refill with glycogen – but this only matters if you’re planning to exercise again within the next eight hours or so.

“As long as you meet your daily carbohydrate needs for fuelling,” Schoenfeld says, “you will ultimately wind up with the same glycogen repletion over a 24-hour period.”

When you add up the pros and cons, making an effort to refuel after a workout remains a good idea. But widening the appropriate time window means you don’t necessarily have to rely on specially formulated protein powders and sugary sports drinks to maximize your workout gains. The most effective, healthy, convenient, and cost-efficient post-workout nutrition plan may be the one you were planning to eat anyway: dinner.

Alex Hutchinson is the author of Endure: Mind, Body, and the Curiously Elastic Limits of Human Performance. Follow him on Twitter @sweatscience.

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