Protein-sparing modified fast (PSMF): what it is, how IPA fits
By Nashua Nutrition Editorial · Last reviewed September 2026
Quick answer: A protein-sparing modified fast, or PSMF, is a very-low-calorie diet that is mostly protein. It has little carb or fat. Clinical sources describe it as a plan done with a medical team. IPA (Ideal Protein Alternatives) foods follow a similar protein-first, low-carb pattern. Buying them does not make a plan a PSMF, though.
You may have seen the words "protein-sparing modified fast" on a clinic handout or in a keto group. They also show up on boxes of ProtiDiet foods. The name sounds technical, but the idea is simple. This guide covers what a PSMF is, where it came from, and what people eat on one. It also covers safety, how it compares with keto, and how IPA foods fit in. It describes the approach. It is not a plan to follow on your own.
What is a protein-sparing modified fast?
A Cleveland Clinic Journal of Medicine review sums it up. It calls the PSMF "a very-low-calorie diet containing mostly protein and little carbohydrate." Fat is kept low, too. The review calls it a medically supervised diet.
The name points to the goal. The review says the plan is designed to help people lose fat while preserving lean body mass. It describes lean body mass as muscle. One of its figures links the diet's high protein intake to less muscle loss.
Cleveland Clinic adds a note of care. Its article says some muscle loss is likely during weight loss. The hope is that enough protein will help protect muscle.
A PSMF has two parts. First comes an intensive phase. Then comes a refeeding phase, when carbs and fats are added back slowly. The team supervising the plan sets the timing for each part.
Where did the PSMF come from?
The PSMF was developed in the 1970s, according to the Cleveland Clinic Journal of Medicine review. The review says versions of it have since been used in clinics around the world.
The ProtiDiet line traces its roots here, too. ProtiDiet's maker says its protein diet is derived from clinical studies by Dr. George Blackburn. The maker also says the line was created for doctors who treat obesity. Dr. Blackburn is also an author of one of the first PSMF studies, which the review cites.
The same review is open about the early years. It says safety concerns about the PSMF stem from the first very-low-calorie diets of the 1970s. Those diets were linked to fatal heart rhythm problems and sudden death. That history is worth knowing. Today, a PSMF comes with close medical care, as the safety section below explains.
What do people eat on a protein-sparing modified fast?
Most of the plate is lean protein, with some non-starchy vegetables. Cleveland Clinic lists protein foods such as chicken, turkey, lean beef, pork, lamb, fish, eggs, egg whites, and tofu. The table below sorts the rest of its list by food group. A supervising team's own list always comes first.
| Food group | Often included | Often left out |
|---|---|---|
| Dairy | Low-fat dairy, such as cottage cheese, skim milk, and cheese | Full-fat yogurt, milk, and cheese |
| Vegetables | Non-starchy vegetables, such as leafy greens, broccoli, tomatoes, onions, cabbage, and celery | Starchy vegetables, such as corn, peas, and potatoes |
| Grains and beans | None on the list | Wheat, oats, quinoa, peanuts, lentils, chickpeas, and black beans |
| Fruit | None on the list | Grapes, oranges, apples, and berries |
| Fats and oils | Little beyond the fat in lean protein foods | Butter, margarine, salad dressing, and olive oil |
| Sweet foods and drinks | None on the list | Honey, brown sugar, maple syrup, high-fructose corn syrup, soda, sports drinks, and juice |
| Ultra-processed foods | None on the list | Candy bars, potato chips, and fast food |
Cleveland Clinic notes that the carbs on a PSMF come from vegetables. Fluids matter, too. Cleveland Clinic lists dehydration as a possible side effect. It also says your team may suggest mineral supplements. Some teams allow an electrolyte drink mix, such as Legion Hydrate. Ask yours before you add one.
Ready-made protein foods can play a part as well. In Cleveland Clinic's own PSMF program, the review says high-protein shakes or bars can be used now and then. The reasons it gives are convenience and helping people stay with the diet.
Some people find a sweet pudding or a warm soup makes a strict day easier to keep. That works as long as it fits the plan their team gave them. Options include the Proti-thin vanilla pudding and shake mix and the Weight Loss Systems custard toffee pudding. For something savory, there is ProtiDiet chicken noodle protein soup. You can browse more in our Proti-thin, Weight Loss Systems, and ProtiDiet collection. Check each label against your team's list.
Is a protein-sparing modified fast safe?
Clinical sources tie the answer to one thing: a medical team watching over it. The review says not to attempt a PSMF without medical supervision. Cleveland Clinic says the same: a doctor or registered dietitian supervises the diet.
That care is hands-on. In Cleveland Clinic's program, people see nutrition and medical staff on a set schedule. They also get lab tests often. The team keeps an eye on vital signs, lab results, and side effects.
A PSMF is not a fit for everyone. The review lists health situations where it is not advised. Some of them are pregnancy or breastfeeding, a recent heart attack, liver failure, and end-stage kidney disease. These are only some of the situations on its list, and the full list is in the review. Cleveland Clinic names a few more: type 2 diabetes, high blood pressure, gout, and kidney problems. It advises avoiding a PSMF with these unless a medical team supervises it.
The review also notes that some medications for these conditions can cause problems during a PSMF. Your prescriber and care team are the people to ask about that.
Some side effects are common. The review lists headache, tiredness, muscle cramps, feeling cold, and constipation, among others. Cleveland Clinic says dehydration can bring on nausea and headaches, too. Tell your team about side effects like these. Checking on them is part of their job.
Warning signs are different. MedlinePlus lists signs of severe dehydration. Some of them are dizziness or lightheadedness, a rapid heartbeat, and very dark urine or not urinating. These are only some of the signs, and the full MedlinePlus list is worth reading. If any of them happen, contact your care team the same day.
MedlinePlus lists five reasons to call 911 or your local emergency number:
- The person loses consciousness at any time.
- There is any other change in how alert the person is, such as confusion or seizures.
- The person has a fever over 102°F.
- There are signs of heatstroke, such as a rapid pulse or rapid breathing.
- The person does not get better, or gets worse, even with treatment.
Is a PSMF the same as keto?
The two are related, but they are not the same. Both keep carbs very low, and both aim for ketosis. Cleveland Clinic explains ketosis as a process where your body uses fat as its main fuel source. A Cleveland Clinic article even calls the PSMF "a modified keto."
The biggest difference is fat. Harvard's Nutrition Source describes the classic keto diet as very high in fat, with only a moderate amount of protein. On a PSMF, most calories come from protein, as Cleveland Clinic puts it. Fat stays low. The review adds that a PSMF also limits calories and fat more than many keto diets do.
What about hunger? The review says some studies suggest ketosis may also curb appetite. Harvard notes that ideas like this have not been shown consistently in research. Experiences vary.
If you follow keto on your own, many everyday low-carb foods cross over between the two. Our keto collection gathers low-carb options, such as Proti-thin barbecue protein chips.
How does IPA relate to a PSMF?
IPA stands for Ideal Protein Alternatives. IPA foods are sold as an alternative to the Ideal Protein protocol. Nashua Nutrition does not sell Ideal Protein products.
That protocol's first phase, like phase 1 of an IPA plan, shares part of a PSMF's pattern. As our IPA guide puts it, carbs are limited and protein comes first. Packet foods sit at the center of the day. The IPA calculator on that page can help you count packets for your phase. We call it a similar pattern on purpose. We have not seen a source that says one grew out of the other.
Here is the key point. Buying IPA foods does not turn a plan into a PSMF. A PSMF is a medical diet that a supervising clinic sets up and watches over. The foods are simply foods. They can make a protein-first plan easier to keep. They will not change what your plan is or how it works for you.
Many people lean on IPA unrestricted foods in phase 1. These packets are lower in carbs, fats, and calories. For how each phase works, read our guide to what to eat on an IPA plan. To check a label yourself, see IPA restricted vs unrestricted. If you've had bariatric surgery, IPA after bariatric surgery covers how IPA foods fit your team's plan.
Key takeaway: know the approach, and plan with your team
A PSMF is a very-low-calorie, protein-first diet. Clinical sources pair it with close medical care. It shares a protein-first, low-carb pattern with IPA and with parts of keto. Knowing where these ideas come from can make labels and phases easier to understand.
Your provider is the right person to decide whether a structured plan like this fits you. Still deciding which kind of plan feels right? Our start here page is a gentle first step. Once you know your plan, our Proti-thin, Weight Loss Systems, and ProtiDiet foods can help make each day simpler.
Frequently asked questions
Does protein help with cravings?
It may help some. Cleveland Clinic says protein helps control your hunger. Cleveland Clinic also notes that adding protein and a heart-healthy fat can help lessen cravings. Even so, cravings can still come up. Having one does not mean you did anything wrong. Some people find a savory snack or a protein pudding between meals helps. That works as long as it fits the plan their team gave them.
Can I try a protein-sparing modified fast on my own for a few days?
The clinical sources we read say not to attempt a PSMF without medical supervision. They make no exception for a short stretch. If the idea appeals to you, bring it to your provider. They know your health and your medications, and they can tell you whether it fits. They may also suggest a structured, protein-first plan that is not a PSMF. Asking first is a smart, careful step, not a setback.
What happens when a protein-sparing modified fast ends?
The plan does not simply stop. It moves into refeeding. Cleveland Clinic says carbs are added back slowly over several weeks. The aim is a more balanced way of eating. Eating more after a strict stretch can feel strange at first. Feeling unsure about it is normal. Your team sets the pace, so you are not working it out alone.
Are ProtiDiet foods made for a protein-sparing modified fast?
Its history points that way. The maker says the line comes from Dr. George Blackburn's clinical studies and was made for doctors treating obesity. Once it's in your pantry, though, a ProtiDiet soup is a protein food, not a medical plan. If you are in a supervised program, your clinic decides which packets fit. If you follow an IPA plan, the same label check applies to every brand. Our guide to IPA restricted vs unrestricted walks through it. You can also browse the full ProtiDiet range.
Sources and further reading
- Cleveland Clinic Journal of Medicine: the protein-sparing modified fast, what to expect (Chang and Kashyap, 2014)
- Cleveland Clinic: what to know about the protein-sparing modified fast diet
- Cleveland Clinic: ketosis
- Harvard T.H. Chan School of Public Health: diet review, ketogenic diet
- Cleveland Clinic: healthy, filling foods
- Cleveland Clinic Newsroom: why our bodies crave carbs
- MedlinePlus: dehydration, including when to contact a medical professional
This content is for general information, education, and encouragement only. It is not medical advice and is not intended to diagnose, treat, cure, or prevent any disease or condition. Nutrition, movement, medication, and lifestyle choices should always be discussed with your healthcare provider, surgeon, or registered dietitian. Individual needs vary, especially for those using GLP-1 medications, living with diabetes, or recovering from bariatric surgery. Always follow the guidance of your own care team. Nashua Nutrition does not employ medical professionals and does not provide clinical care.