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What To Know About the Protein-Sparing Modified Fast Diet

This short-term diet is for those who want to lose a significant amount of weight quickly

Chicken and beef skewers.

In the world of diets, it can be overwhelming with the choices available — keto, paleo, flexitarian, just to name a few.

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But for those trying to lose a lot of weight, the protein-sparing modified fast diet, or the PSMF diet, may be a good short-term option.

Doctors typically recommend this diet for a person who has obesity. And you should be supervised by a doctor or registered dietitian while on the PSMF diet.

So, how does the PSMF diet work? By consuming low amounts of calories and carbohydrates and high amounts of protein, the idea is that weight loss happens quickly.

“It’s a modified keto,” says registered dietitian Beth Czerwony, RD. “The majority of your calories are going to come from protein. There’s very minimal fat and your options include lean protein with limited vegetables. You won’t eat fruits, starchy vegetables, breads or pastas. And this will help your body go into ketosis, which is a fat-burning mode.”

Czerwony explains how the PSMF diet works and its pros and cons.

How it works

The PSMF diet is divided into two phases: The intensive phase and the refeeding phase.

During the intensive phase, which typically lasts about six months, you’ll eat about 800 calories per day. The goal is to consume about 0.7 grams of protein per pound of your body weight.

When it comes to carbs, you’ll want to eat about 20 grams or less a day — with those carbs coming from vegetables. You’ll also need to avoid any added fats that come from foods like oils and salad dressings.

“Most of the time, people stay on it on average of about six months because they get bored, they get tired or they want some variety,” says Czerwony.

Your doctor or dietitian will monitor any nutritional deficiencies and may recommend that you take magnesium, potassium and/or sodium supplements.

“We asked people to salt their foods to make sure that their blood pressure doesn’t get too low,” says Czerwony.

Once you’ve reached your weight loss goal, then you’ll begin the maintenance part of the diet known as the refeeding phase.

“This is over several weeks of slowly adding carbohydrates back into your diet,” explains Czerwony. “And the idea is to get a more balanced intake, to stop that ketosis and to help you maintain the weight that you’ve lost. Ideally, you’ll stay on the maintenance phase indefinitely.”

Foods you can eat on the PSMF diet

While on the PSMF diet, you’ll eat mostly high-protein foods like:

  • Chicken and other types of poultry like turkey, goose and duck.
  • Lean meats like beef, pork and lamb.
  • Seafood like halibut, cod and catfish.
  • Eggs and egg whites.
  • Low-fat dairy like cottage cheese, skim milk and cheese.
  • Tofu.

You can also eat non-starchy vegetables like leafy greens, broccoli, tomatoes, onions, cabbage and celery.

There are foods you’ll need to avoid like:

  • Fruits like grapes, oranges, apples and berries.
  • Starchy vegetables like corn, peas and potatoes.
  • Grains like wheat, oats and quinoa.
  • Legumes like peanuts, chickpeas, lentils and black beans.
  • Ultra-processed foods like candy bars, potato chips and fast food.
  • Sweetened beverages like soda, sports drinks and juice.
  • Fats and oils like butter, margarine, salad dressing and olive oil.
  • Full-fat dairy like yogurt, milk and cheese.
  • Sweeteners like honey, brown sugar, maple syrup and high-fructose corn syrup.

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Pros and cons

Here are some benefits and drawbacks of the PSMF diet:

Pro: It can help you lose weight

For a person who has obesity, the PSMF diet could be a good option to lose a certain amount of weight.

But a reminder that it’s best to undergo this diet while under the supervision of your doctor or dietitian.

Pro: It can improve your blood sugar levels

Studies show that a low-calorie diet like the PSMF diet may lower blood sugar levels in those with Type 2 diabetes.

“We see improvements in blood sugar control because of the limited carbohydrates,” says Czerwony. “So, if you have high glucose numbers you may see an improvement.”

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Pro: It can improve hypertension

Studies also show that by consuming fewer calories, your blood pressure may be reduced.

“We see improvement in hypertension a lot of times because it’s directly related to weight,” notes Czerwony.

Con: It’s difficult to do long-term

The PSMF diet is very restrictive when it comes to what you can eat, what you can’t and how many calories you can consume. It can be difficult for people to maintain long-term.

“What we find is that most people just get bored with it. Or they go on a vacation or to a party and they eat something that has carbs in it and it takes them out of ketosis, so then they have increased hunger,” says Czerwony. “Then, it’s hard for them to get back into ketosis, so they kind of fall off the wagon.”

Con: It’s not for everyone

Those with medical conditions like Type 2 diabetes, high blood pressure, gout and kidney problems should avoid the PSMF diet, unless it’s medically supervised.

“If you already have a history of gout, since it’s such a high protein diet, you might be more likely to have a flare-up. And because it’s such a high protein diet, we don’t want to overtax the kidneys,” states Czerwony.

Con: You’ll lose some muscle mass

Part of the process when it comes to weight loss means you’re likely to lose some of your muscle mass.

“That’s because as you’re losing weight there’s less of you, so you’re not burning as many calories even walking because you don’t have that resistance,” explains Czerwony. “Hopefully, by consuming enough protein, it will protect your muscles. Physical activity is something that is part of that program to maintain your muscles and healthy fat metabolism.”

Con: You can get dehydrated

It’s easy to become dehydrated while on the PSMF diet. That’s why Czerwony says it’s key to drink at least 64 ounces of fluids daily and that you salt your foods.

“We call it the keto flu — it’s really just a shift in fluid status,” says Czerwony. “You may become nauseous or experience headaches and dizziness.”

That dehydration can also lead to constipation — and high-protein diets are known to cause it as well.

“That’s because you’re eating limited vegetables and that can lead to muscle cramping, which happens if your electrolytes are off,” Czerwony explains. “We want to make sure that you’re taking all your supplements as well.”

Con: It can be expensive

You may have to adjust your budget while on the PSMF diet. As most of the diet is high-protein foods like lean meats, your grocery bill can add up quickly.

“Traditional protein alternatives like beans and lentils aren’t allowed and those options tend to be a bit more budget-friendly,” says Czerwony.

Should you try the PSMF diet?

So, is the PSMF diet worth a try? It’s up to each person, says Czerwony.

You may want to talk to your doctor about other options like anti-obesity medication or even bariatric surgery.

“Your doctor should consider your long-term goals,” says Czerwony. “The problem a lot of times is that people will do this diet and they’re very successful the first time. Then fast-forward a couple of months or years and they want to do it again. They’re not as successful the second time.”

And this isn’t the right diet if you just want to lose a few pounds.

“It’s not necessary to try something as extreme,” says Czerwony. “You should cut out excessive calories, whether that’s from sugary beverages or fried foods. Also, look at your physical activity and increase that as well.”

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