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What Is Metabolic Inflexibility? How To Overcome It and Lose Weight Over 50

Carb cycling and restricted eating work in tandem.

By now, we’ve been on so many diets, we know exactly what to eat to lose weight, so we find healthy foods we like, eat them daily (hello, protein smoothies, salads, and grilled chicken!) and wait for the needle on the scale to start moving. Despite our virtuous choices, though, the pounds don’t budge. What gives?

“It’s extremely common as we get older to become metabolically inflexible,” says Harvard-educated nutrition guru Ian K. Smith, MD. When our metabolism is healthy and nimble, it can easily burn any fuel we give it: fat or carbs. We’re like a hybrid car, capable of running on battery power and gasoline, whichever is available that day. But when we eat restrictive diets, our metabolism gets inflexible and forgets how to burn whatever macronutrient it has been deprived of. Dr. Ian says, “We lose the ability to shift back and forth between burning carbs or fat.” So, how can we overcome metabolic inflexibility? Dr. Ian recommends a few techniques in tandem: carb cycling and time-restricted eating. Keep reading to get the details.

How Carb Cycling and Time-Restricted Eating Work Together

Metabolic inflexibility is common in women over 50 and makes weight loss all the more difficult, per the National Institutes of Health. And since this glitch can lead to wildly fluctuating blood-sugar levels, it can trigger cravings, weight gain, low energy, brain fog, and depression.

Luckily, it’s easy to restore your metabolism to its youthful, ready-for-anything baseline, says Dr. Ian. He created a plan combining cyclical keto and intermittent fasting — approaches that may wake a stalled metabolism. He says, “I have carefully combined strategies to yield maximal results.” How it works:

1: Carb cycles may prevent deprivation.

Each week starts with two high-carb days, followed by five low-carb days. “It’s not like straight keto, which says if you don’t stay in ketosis [a metabolic state in which the body doesn’t have enough carbs to burn for energy and burns fat instead], you’re going to mess up. Here, you’re cycling in and out of keto,” explains Dr. Ian. You prime the body to get energy from carbs, then fat, and repeat. By quickly alternating your food sources, the body becomes reacquainted with processing previously deprived food groups. “The simple idea of switching acts as a rocket booster for metabolism.”

Frequent fuel switching is proven to disrupt plateaus. And the carb days may prevent fat-packing stress-hormone flares that occur in older women after long-term carb restrictions.

2: Fasting may power off belly fat.

Some 90 percent of participants with type 2 diabetes who followed intermittent fasting reduced their need for blood sugar–lowering meds, including insulin, according to research in The Journal of Clinical Endocrinology & Metabolism. Even better: 55 percent of participants achieved total diabetes remission. “Insulin responds exceptionally well to intermittent fasting,” says nurse practitioner Cynthia Thurlow, author of Intermittent Fasting Transformation. “Food is the impetus for insulin secretion. But when you fast, your body naturally needs lower levels.” Another benefit, points out lead researcher Dongbo Liu, PhD, “Diabetes medications are costly— our study saw medication costs decrease by 77 percent for people after fasting.”

Dr. Ian says, “This is the first time in my career I’ve touted an eating strategy with reducing belly fat. But there is a lot of scientific data to back it up.” Fasting expert Jason Fung, MD, also hears raves from women over 50: “Unlike other interventions that require work, you get the benefits of fasting by not doing something.”

Why Eliminating Carbs Completely Is Bad News

Women who have spent years avoiding certain macronutrients — shunning either carbs or fats — in the name of weight loss are at risk of a damaged metabolism, says Ian K. Smith, MD, author of The Met Flex Diet. But you can restore your metabolism so it burns whatever fuel you give your body simply by switching up your food intake and making some meal-timing tweaks. The result: You’ll break plateaus and power off pounds like a kid again. Dr. Ian says, “People are eating some of the right things already. They’re just not eating them in the right pattern, at the right time. This is a more strategic approach.” The two-part plan, created exclusively for FIRST dieters, “attacks weight loss from different angles.”

What To Eat

For three weeks, you’ll practice “cycling keto”— two days of high-carb eating, where you enjoy foods like pasta and pizza — followed by five days of low-carb or keto eating. This pattern may wake up a stuck-in-a-rut, over-50 metabolism and restarts slimming as you quickly move in and out of keto. Dr. Ian says, “Your body needs to be continuously surprised.”

On high-carb days, you’ll devote 60 to 70 percent of your calories to carbohydrates, 15 to 20 percent to protein, and 5 to 10 percent to fat. During this time, your body will get energy from the starches you eat. Think: oatmeal for a late breakfast, pizza for lunch, and spaghetti and meatballs for dinner.

On low-carb days, you’ll get 70 to 90 percent of your daily calories from healthy fatty foods. (Choose monounsaturated and polyunsaturated sources like avocados, eggs, olive oil, fish, flaxseed, full-fat dairy, nuts, seeds, and olives). Make protein 10 to 20 percent of your daily calories and the remaining percentage healthy carbs like veggies and quinoa. During this absence of carbs, your body will be compelled to burn its secondary source of fuel: fat. Try an egg burrito for a late breakfast, Cobb salad for lunch, and lamb chops for dinner.

When To Eat

Dr. Ian advises taking a break from eating for 16 hours (many of them while you sleep). You’ll delay breakfast and consume the day’s three meals and two snacks during an eight-hour window. If you want to ease in, Dr. Ian recommends starting with a 10-hour fast and gradually widening the window. Rest assured, he says, “People love the diversity of food on this plan. It does not deprive.”

Extra Tips To Rev Your Results

Supplement with B-12. Dr. Ian cites preliminary research that suggests B-12 enhances metabolism and is associated with a decreased risk of weight gain. Strive for 2.4 micrograms daily of this “metabolism booster.”

Switch up your movement. “Different exercises utilize different fuels,” says Dr. Ian. High-intensity activities like hiking, biking, stair climbing, or hula-hooping burn carbs, so try them on high-carb days, and save low-intensity moves like walking for low-carb days.

Sample Meals To Get You Started

Pancake Lunch: Pour 1 cup of your favorite batter on skillet to form 2 cakes. Flip. Serve with 2 slices bacon and ½ cup fruit.

Pasta Dinner: Form ½ pound ground beef, ⅛ cup keto panko bread crumbs, and 1 teaspoon soy sauce into balls. Bake 10 minutes at 400 degrees Fahrenheit. Enjoy with 2 cups pasta.

Chicken Salad Lunch: Combine 2 cups spring mix with 3 ounces roast chicken. Add tomatoes, avocado, and 1 tablespoon oil-based dressing.

Steak Dinner: Rub 6 ounces steak with minced fresh garlic and butter. Grill and pair with ½ cup low-starch cooked veggies.

Hunger-Taming Sips That May Help

Green Tea: An 8 ounce cup with 1 teaspoon of honey has just 24 calories and contains theanine, an amino acid that promotes calm and reduces appetite. Evidence also suggests green tea improves the function of leptin, an anti-hunger hormone.

Coffee: Enjoy an 8 ounce mug with ½ tablespoon of heavy cream for just 31 calories. In the process, researchers say you’ll reduce the blood-sugar fluctuations that cause cravings.

Bone Broth: The soup’s natural collagen is shown to satisfy hunger so much that dieters unknowingly eat 400 fewer calories per day (compared to high-fat soup). And each ½ cup of the healing broth has just 48 calories.

Lemon Water: An 8 ounce cup of this gold-standard slimmer contains 6 calories and may help you burn fat. The reasoning? Arizona State University researchers theorize that low vitamin C levels make it harder to oxidize fat.

This content is not a substitute for professional medical advice or diagnosis. Always consult your physician before pursuing any treatment plan.

A version of this article originally appeared in our print magazine, First For Women.

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