Science 101

How Does Diet Affect Metabolism?

It’s a little like dynamite, but not as dramatic.

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Your metabolism is not something you’re born with and never changes. It fluctuates, constantly — from time of day, feeding inputs, exercise and body weight.

“It's debated how much a fast versus slow metabolism is real,” says Joseph A. Baur, PhD, associate professor of physiology at the University of Pennsylvania Perelman School of Medicine Institute for Diabetes, Obesity and Metabolism. “Basically most people have similar metabolism for a given amount of tissue that's present on their bodies.” So you do not have a comparably slow or fast metabolism. You have both.

This is a vitally important mental switch. Paolo Sassone-Corsi, PhD, director of the Center for Epigenetics and Metabolism at the University of California, Irvine, adds: “The biggest misconception about metabolism is that it is something that is fixed — that is a huge misconception.”

Maximize Your Metabolism

The speed of your metabolism is descriptive, not prescriptive. It’s a result of your lifestyle choices. It reacts to your inputs and the timing of those inputs. And when we talk about diet and metabolism, what we are really talking about is: how do we optimize our constantly fluctuating metabolism? How do we help it run smoothly? And how do we maximize our body’s own ability to metabolize food into energy without storing unnecessary fat?

“Depending on what you have as a food, you are reprogramming the circadian clock in different ways,” Sassone-Corsi says. “If you think about evolution, there’s a very strong logic — we weren’t used to having food every day all the time and every place we went.”

The availability of massive amounts of calories can be tricky to navigate. And, consuming a proverbial gut bomb — the grande burrito, cheeseburger and fries, that carton of take-out, or half the pizza pie — is, to use Sassone-Corsi’s term, like “putting a stick of dynamite in your body.” And that affects the body’s natural rhythms and therefore the metabolism.

A Calorie Is Never Just a Calorie

But not all foods are dynamite. Proteins, whole grains, fibrous foods and plenty of water are ideal for the metabolism. Non-ideal for the metabolism are sugars, refined grains and alcohol.

And while the types of calories you are consuming matters, it also matters when you consume them.

The body operates on two main cycles: the active phase and the resting phase. When we play nice with these circadian rhythms, the body hums along in good health. When we alter them, things can go awry. 

If you think about evolution, there’s a very strong logic — we weren’t used to having food every day all the time and every place we went.

Time-restricted feeding — often incorrectly termed intermittent fasting — is one way to stay on track with your body’s rhythms, which is earning more attention not just in scientific circles but also in popular media. Each person’s chronotype is different, so the morning larks and the night owls may follow different rhythms. But, generally, the goal of time-restricted feeding is to reduce food intake to an eight to 10-hour window in the day. For some, that may be 8:00 am to 6:00 pm. Others, 10:00 am to 8:00 pm.  

 “You have to improve the quality of your circadian life,” Sassone-Corsi says. “That is as good as caloric restriction.” And if you do optimize the food you eat and the time you eat it, you may see health benefits. Baur referenced a study by John O. Holloszy and Luigi Fontana of Washington University where researchers analyzed the health of two mice groups that were both the same weight — one achieved by exercise and the other by restricted feeding. “The ones where they just take away the food actually live longer,” Baur summarized. “But the point is that it doesn’t matter how you lost the calories. And, he adds, exercise is a key component of human health. “I'd be very hesitant to recommend that humans not exercise,” Baur says. “I think we're probably a lot better off that way.”

That was 2004 and in mice. The Holloszy and Fontana research has, in the interim years, added more perspective on how this relates to humans. Numerous results have been positive for long-term health when calorie restriction is followed by humans.

Plus, as Sassone-Corsi says: “I'd like to make this point very, very clear: humans are different than mice.” So for now, what we do know is to continue on track with the healthy lifestyle choices — what you eat, when you eat, how you exercise, when you exercise — that keep metabolic functions running properly. From there, it’s a hurry-up-and-wait game by keeping an eye on the research, in mice, sure, but in how this could potentially translate to humans, too.

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