In the crowded landscape of weight loss strategies, the Protein-Sparing Modified Fast (PSMF) diet emerges as a short-term option for individuals seeking significant weight reduction. Typically recommended by doctors for individuals with obesity, the PSMF diet necessitates medical supervision by a doctor or registered dietitian. This article delves into the intricacies of the PSMF diet, providing guidelines, benefits, risks, and practical advice for those considering this approach.
Understanding the PSMF Diet
The Protein-Sparing Modified Fast (PSMF) is a very low-calorie diet designed for quick, short-term weight loss. The PSMF diet was originally designed by physicians to help their patients lose weight quickly. It is a very effective method for those who wish to lose a large amount of weight in a relatively short amount of time. First created in the 1970s, it was made to help morbidly obese patients (those with a BMI much greater than 30) lose weight that was affecting their health in an immediate way. It has been studied exclusively in the adult population and has been proven to be a safe way to achieve rapid weight loss. It’s typically unsustainable and likely to lead to weight regain. However, within the last few decades, it has gained widespread popularity among dieters looking for a quick and easy way to drop extra pounds.
The diet involves consuming very low amounts of calories and carbohydrates and high amounts of protein, the idea is that weight loss happens quickly. The term now refers to many varieties of very low-calorie diets, many undertaken without medical supervision.
How PSMF Works
The PSMF diet operates on the principle of creating a large calorie deficit while prioritizing protein intake to preserve muscle mass. It severely restricts calorie consumption while increasing the intake of protein-rich foods. Additionally, the intake of carbohydrates and fats is very limited in this diet.
Registered dietitian Beth Czerwony, RD, describes it as “a modified keto,” where the majority of calories come from protein, with minimal fat and limited vegetables.
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Ketosis and Protein-Sparing
Apart from the very low calories, this diet follows a very important principle that many might be familiar with in our day and time. That little magic trick is called Ketosis and it’s what Protein-Sparing is all about. Ketosis is a state that occurs when there are not enough carbohydrates available to the body and in order to provide enough energy for its every day functions, the liver starts to oxidize fatty acids. During that state, ketones are produced. Ketosis has been linked with a suppression of appetite and rapid weight loss. This happens because while regular weight loss is taking place, the body starts to produce some hormones that try to promote weight gain in order to resupply the fat that was lost. In Ketosis though, fatty acid oxidation seems to protect against the production of those hormones and therefore makes it a lot easier to stay on track and continue with the weight loss plan.
Protein-sparing allows the body to lose weight without losing any muscle tissue. That happens because the proteins you consume are not what your body uses as a main source of energy. This way, you make sure that your body uses the protein you eat in order to build and repair tissues and muscles as well as create enzymes and antibodies. Therefore, your body uses bigger amounts of your stored fat in order to have enough energy for its daily activities and you end up losing more fat while maintaining muscle.
Macronutrient Breakdown
In a clinically supervised PSMF, patients receive 1.2-1.5 grams of protein per kilogram of their goal body weight per day. The diet allows only 20-50 grams of carbohydrates per day. Fat is limited to whatever comes along with protein sources. Added fats of any kind - oils, dressings, spreads, etc.
Two Phases of PSMF
A typical PSMF is divided into two phases: the intensive phase and the refeeding phase.
Intensive Phase: During the intensive phase, carbs, fat, and calories are severely restricted. In the first phase of the diet, the Intensive phase where the proteins are high, it is essential to consume conventional foods which contain high biological value protein such as:
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- Lean Meats
- Egg whites
- Fish and seafood
- Poultry
During this phase, it is also important to consume some vegetables that are low in carbohydrates such as:
- Asparagus
- Broccoli
- Cauliflower
- Lettuce
- Carrots
- Brussels Sprouts
- Mushrooms
- Zucchini
As far as condiments are concerned you can choose to use very small amounts of the following:
- Butter sprays (sparingly)
- Mustard (max. 1 tbsp. per day)
- Tabasco sauce
- Vinegar
- Bouillon cubes
- Fresh garlic, onion, parsley and celery
As far as dairy is concerned, milk, yogurt and fatty cheeses are definitely out of the question. On the other hand though, you can consume some nonfat variations of cheese and cottage cheese but in smaller amounts too.
You can also drink decaffeinated coffee or herbal tea along with your water. You should try staying away from diet sodas or any other drinks that contain caffeine as it can slow down your weight loss process.
Refeeding Phase: In the refeeding phase, foods are gradually added back into the diet. Carbohydrate levels are increased up to 45 grams during the first month, and up to 90 grams during the second month. Daily protein is reduced by about 40 grams per month. Once the person reaches the refeeding phase, during the first month of it carbs are increased up to 45 grams and during the second up to 90. On the other hand, protein is reduced by about 40 grams per month. This part of the diet is progressively getting higher in calories as more carbohydrates and fats are being added to the person’s every day diet.
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In the refeeding phase, it is important to start incorporating more complex carbohydrates, foods rich in fiber including fruits and vegetables and also fats, once the goal weight is achieved. If you wish to take a closer look at this diet, you can use our Meal Planner to help you automatically create a PSMF weight loss diet plan.Just remember that this is not a conventional ketogenic diet. This means that fats should also be kept at a low amount along with the carbohydrates.
Duration of the PSMF Diet
The Protein-Sparing Modified Fast is not recommended for long term use. In fact, most studies have shown that it is safe when it lasts around 12 weeks but it is usually administered for 6-16 weeks depending on the severity of the obesity. For people with class 3 obesity (morbidly obese), the PSMF diet can be continued beyond week 16 as long as that person is closely monitored and the fast is well tolerated.
The Protein-Sparing Modified Fast consists of two phases. This phase is very restrictive and it is not recommended that you follow it for more than six months. The second part of the diet consists of the Refeeding phase. In this stage, the person can slowly start to add back carbohydrates and fats into their diet. The amounts of protein consumed are finally reduced. This is the stage of the diet that helps a person learn how to manage the weight that they lost so they are able to keep it off for good. For this part, experts encourage those who wish to follow it, to stay on the increased carbohydrate load for a minimum of 12 weeks.
The PSMF diet is not a lifestyle and should only be used on a short term basis, lasting for a few weeks for initial motivation and rapid weight loss.
Nitrogen Balance
Nitrogen is an essential component of protein. Its urinary secretion can be measured in a lab. Inadequate nutrition and an unbalanced diet result in the production of a negative nitrogen balance in the body. As a result, the body starts to lose lean body mass. Lean body mass is very important when it comes to weight loss as it plays a very important part to the body’s metabolic rate. The PSMF diet has shown in clinical trials that it has helped increase the nitrogen balance in the bodies of the participants. As a result, people who follow this diet to the letter will be able to have a higher metabolic rate and therefore lose weight faster.
Foods to Eat and Avoid on the PSMF Diet
Foods You Can Eat
While on the PSMF diet, you’ll eat mostly high-protein foods. As long as the foods you consume are high in protein but low in fats and carbohydrates, you will be able to stay in ketosis and benefit from this fast. You should always try and read labels of the products you buy so you make sure you don’t accidentally consume anything you shouldn’t.
Most foods included on a PSMF are lean-protein foods, such as poultry, eggs, tofu, fish, and lean cuts of red meat. Here are some of the foods you can eat as part of a PSMF:
- Poultry: skinless chicken, turkey, goose, duck
- Meat: lean cuts of beef, pork, lamb
- Seafood: flounder, sole, cod, catfish, halibut
- Non-starchy vegetables: leafy greens, broccoli, cabbage, cauliflower, Brussels sprouts, celery, tomatoes, onions
- Low fat dairy: cottage cheese, cheese, skim milk
- Eggs and egg whites
- Tofu
Foods to Avoid
A PSMF restricts the intake of most foods high in carbs or fat. Here are some of the foods you should avoid as part of a PSMF:
- Fruits: apples, berries, oranges, grapes, melons, pears, peaches
- Starchy vegetables: potatoes, corn, peas, parsnips
- Grains: wheat, quinoa, oats, barley, buckwheat, millet
- Legumes: black beans, lentils, chickpeas, kidney beans, peanuts
- Processed foods: convenience meals, baked goods, potato chips, fast food, candy bars
- Sweetened beverages: juice, sweet tea, sports drinks, soda
- Sugars and sweeteners: honey, maple syrup, table sugar, molasses, brown sugar, high-fructose corn syrup
- Fats and oils: olive oil, coconut oil, vegetable oils, salad dressings, butter, margarine
- Full-fat dairy: yogurt, cheese, milk
Sample Meal Plan
This 5-day sample meal plan demonstrates what a typical PSMF might look like.
Monday
- Breakfast: eggs with spinach and tomatoes
- Lunch: grilled cod with steamed broccoli
- Dinner: herbed turkey breast with roasted Brussels sprouts
Tuesday
- Breakfast: tofu scramble with onions, garlic, and bell peppers
- Lunch: oven-baked chicken with a side salad (no dressing)
- Dinner: pork chops with roasted asparagus
Wednesday
- Breakfast: egg-white omelet with zucchini, tomatoes, and garlic
- Lunch: baked catfish with boiled cabbage
- Dinner: lettuce wrap with lean ground beef, mushrooms, garlic, ginger, and scallions
Thursday
- Breakfast: low-fat cottage cheese with cinnamon
- Lunch: low-carb turkey meatballs with zucchini noodles and tomatoes
- Dinner: lemon garlic roasted chicken with side salad (no dressing)
Friday
- Breakfast: hard-boiled eggs with salt and pepper
- Lunch: baked tofu with steamed green beans
- Dinner: grilled sirloin steak with oven-roasted eggplant
Benefits of the PSMF Diet
The primary benefit of PSMF is fast weight loss. Studies show that a PSMF can be effective for rapid weight loss when performed under proper medical supervision.
- Weight Loss: For example, one 2016 study of 12 adolescents on a PSMF found that participants lost an average of 25 pounds (lb) (11 kg) over a 6-month period. This accounted for about 10% of their total body weight. Another newer 2020 study found that people who followed PSMF lost an average of 3% more weight over five years compared to people who didn’t follow this diet. That said, after five years, there was no more difference in weight loss between the subjects and the control group. A 2017 study also found that a PSMF was more effective than a conventional low calorie diet for short-term weight loss. In this case, after only one year, the weight loss was similar between groups. Obese patients typically lose weight on a PSMF. Average weight loss is about 40 pounds in 12 weeks, with men losing more than women.
Losing body fat through the PSMF diet can make a positive difference in other areas of your health. It can lower inflammation (which drives chronic diseases) and restore high blood glucose to normal levels in those who have diabetes or are at risk. Furthermore, it also helps reduce some of the risks of carrying excess weight, such as high triglycerides, sleep apnea, and decreased mobility, according to the CDC.
In addition to helping you lose weight quickly, following a PSMF has been associated with several other health benefits.These include:
- Decreased cholesterol levels: A short-term PSMF may help reduce levels of total and LDL (bad) cholesterol, but not necessarily by a significant amount.
- Improved blood sugar control: Some research has found that PSMF may help lower blood sugar levels in those with type 2 diabetes and in people with obesity. In a study conducted by Bistrian, Blackburn et al, the PSMF diet had helped reduce the insulin and glucose concentration in the participants and had allowed for the withdrawal of exogenous insulin after just 6.5 days.
- Reduced blood pressure: Research shows that calorie restriction may help reduce blood pressure levels to improve heart health. A 2020 study found that patients who started the PSMF diet achieved more weight loss by 3% compared to those on the control diet. Weight loss was already obvious during the first year of doing PSMF. It’s also interesting to note that the patients on PSMF experienced a decrease in their systolic and diastolic blood pressure, especially those who lost at least 5% of their weight.
- May help protect against metabolic syndrome: Like other types of calorie-restricting diets, this diet could improve several components of metabolic syndrome, which may help reduce your risk of heart disease, stroke, and type 2 diabetes. Along with that, studies have shown that those who sustained the weight loss experienced an improvement in many comorbid diseases associated with obesity. Even 5% weight loss has great health benefits for people suffering from diabetes, hypertension, hyperlipidemia and cardiovascular disease. The important thing here though is to only use this diet for a very short amount of time and then continue with a weight loss plan that allows for consistent weight loss, as that is the best way to ward off these diseases.
Risks and Considerations
Following a PSMF may be a safe and effective option for weight loss when performed under the guidance and supervision of a healthcare professional. However, the diet is highly restrictive, low in calories, and eliminates many important nutrients. This can increase your risk of nutritional deficiencies if you’re not being closely monitored.
Like other diets, the PSMF diet isn’t perfect. Because it’s very low in calories, it can lead to nutrient deficiencies. This is the reason why you will likely need to take a supplement. Another thing to watch out for is the keto flu. Since protein-sparing modified fasts induce ketosis during the intensive phase, you may feel temporary symptoms like fatigue, headache, brain fog, reduced exercise performance, and insomnia.
Side Effects of PSMF
There have been many reported side effects in subjects who have participated in trials of this diet. The side-effects that a person will experience during the PSMF are to a great extend the results of your body going into ketosis. This happens because of the lower insulin levels that are achieved due to the low amounts of carbohydrates that are consumed. The lower insulin causes the kidneys to excrete more water which leads to the body losing not only water weight but also important electrolytes.
Some of the most common are:
- Cold intolerance
- Constipation
- Dizziness
- Dry skin
- Fatigue
- Brain fog and headaches
- Hunger
Along with that though, the fact that this diet is so low in calories will definitely aid to the appearance of these side effects that were mentioned earlier.
Who Should Avoid the PSMF Diet?
A PSMF is not suitable for older adults, those with a history of disordered eating, women who are pregnant or nursing, or those with a body mass index lower than 27 kg per meter squared (m2). It’s also not advisable for those who have a history of gallstones or have had their gallbladders removed. Very low calorie diets may increase the chance of adverse side effects in those with these conditions.
Potential for Weight Regain
Very low calorie diets often lead to rapid weight loss, which carries a high risk of weight regain once a normal diet is resumed. Many people who lose weight on a PSMF regain it after ending the diet. Slow, consistent weight loss is usually a much better option for maintaining long-term results long term.
The Importance of Medical Supervision
The Protein-Sparing Modified Fast is one of these diets that definitely need medical supervision if the person following it is carries a lot of extra weight. Not only will that person have to be working with a licensed dietician when deciding to follow this plan, but they should also let their doctor know so they can run the tests that are needed to help see if they are in a good enough condition to follow this plan.
The biggest problem for these people is that this is a very low calorie diet that completely changes a person’s eating habits when followed for a long period of time. Most people are definitely not used to consuming such low amount of carbohydrates and fats and that can become a great issue once they reach the Refeeding stage.
On the other hand, if you wish to find a diet plan that is very efficient and safe and that will help you kick start your weight loss, you can give the PSMF a try yourself and benefit from the amazing results. This applies to those who are not intending to follow the meal plan for more than two weeks though. Therefore, if you have some weight to lose but you aren’t very overweight, you should definitely consider giving this a try at the beginning of your dieting journey as it will boost your confidence and make you want to try harder to achieve your goals.
As long as after the two weeks have passed you return to a normal meal plan and proceed with your weight loss in a way that is more sustainable and easy to follow, you will be healthy even if you don’t work with any particular medical practitioner.
Supplementation
The answer here is yes. A PSMF diet is not nutritionally complete and is usually low in some essential compounds in a person’s every day diet and therefore supplements are the only way to get them without breaking the Macronutrient plan. People on a PSMF are usually advised to take supplements to prevent electrolyte imbalances.
Here are some of the most important ones:
- Potassium
- Calcium
- Multivitamins
- Magnesium
- Sodium
Not only will those nutrients help you stay hydrated and healthy, but they can also help ease some of the symptoms of the diet that are naturally occurring when the body is going into ketosis.
Hydration
In most studies, participants were instructed to consume a minimum of 64 ounces (around 2 liters) of fluids on a daily basis, so you will need to be drinking about that much water and liquids too. Drinking a lot of water is very important in this diet as the more water you drink the faster you will be able to eliminate ketones from your body. This also plays an important role in preventing renal injuries. The liquids you consume will have to be sugarless, so the best options here are either plain water or some enhanced drinks with electrolytes and vitamins. Be very careful of sport drinks though, as they might be hiding added sugar or glucose in the ingredients.
Exercise During PSMF
In most studies, exercising is encouraged in order to intensify the effects of the weight loss. In a study conducted by Blackburn, Whatley et al, they found that the group that incorporated walking and resistance training into their diet that was very low in carbohydrates had achieved additional loss of fat mass and at the same time had achieved the preservation of fat free mass. In any case, exercising while being on a PSMF diet can help you not only see results much faster but it can also help you feel a lot healthier in a short amount of time. It is especially needed when the person moves to the sustainable diet where they will have to maintain their weight. People who lose weight without exercising are the ones that usually put the weight back on and that is very sad as it makes all their hard work and effort go to waste. However, it is strongly advisable that exercise is mainly limited to just fast walking and resistance training as opposed to vigorous cardiovascular exercise.
PSMF vs. Keto Diet
Although a protein-sparing modified fast limits carbohydrates, which results in ketosis, it differs in that it focuses on protein as your main source of calories. The keto diet, on the contrary, makes dietary fat your main fuel. While you can stay on the keto diet for years, PSMF diets are only meant to be followed short-term to minimize the risks of eating too few calories and fat.
Is PSMF Right for You?
This diet is suitable for those who are very overweight and need to kick start their weight loss. This quick fix, as some might say, can act as a great means of motivation to those who need to lose a great amount of weight and wish to see some immediate results. People who have a smaller amount of weight to lose and want get their diet off to a good start can also benefit from this fast as long as they don’t follow it for an extended amount of time. The PSMF diet can also act as a treatment prior to bariatric surgery with the consumption of either foods or liquid meal replacement shakes. This lasts for a period of around two weeks and results in quite considerable weight loss given the short length of time. Positive effects have also been noted with many people who suffer from type 2 Diabetes. This is a result of the glycemic benefits that a PSMF diet provides.
The PSMF should only be used by overweight individuals who have failed to lose weight with other methods. At the Cleveland Clinic for example, PSMF outpatients meet with a dietitian twice during the first month on the diet, and at least monthly for the remainder of the diet.