Short answer: your thigh muscle shrinks fast after knee replacement, and most of that loss happens in the first 2 weeks. In one trial, people on a placebo lost about 14% of their quadriceps by 2 weeks and 18% by 6 weeks. People taking essential amino acids from 1 week before surgery lost about 3% and 6% [1]. Steady protein plus physical therapy is the best-supported plan.

This guide explains why the muscle goes so quickly, what the trials found (including where they came up short), how much protein to aim for, and what each week of recovery looks like. If you want the full surgery plan, see our knee replacement nutrition guide or our hip replacement nutrition guide.

Key points

  • In the first trial below, about three quarters of the placebo group's 6-week quad loss happened within 2 weeks of surgery, so starting protein about 1 week before surgery makes sense.
  • Aim for about 1.2 to 1.5 g of protein per kg of body weight a day, unless your doctor has told you to limit protein.
  • Amino acids protected muscle size in the knee trials below. Their effect on strength and day-to-day function was mixed.
  • Protein works alongside movement, not instead of it. Your physical therapy plan still does the heavy lifting.

Understanding the Role of Protein and Essential Amino Acids

Total knee arthroplasty (TKA) and total hip arthroplasty (THA) are done to ease pain and get you moving again. But the surgery itself puts your body under short-term stress.

In the first 1 to 2 weeks after joint replacement:

  • Muscle protein breakdown increases
  • Inflammation rises
  • Your body responds less well to insulin. In one small hip replacement study, insulin sensitivity dropped 43% right after surgery in the placebo group [2]
  • Activity drops sharply
  • Appetite is often low

The result is fast muscle loss, especially in the quadriceps (the big muscle on the front of your thigh) after knee replacement. That muscle straightens your knee and gets you up from a chair, so losing it early is not a small thing.

How Much Muscle Do You Lose After Knee Replacement?

In a randomized trial of 28 older adults, the placebo group lost 14.3% of their quadriceps volume by 2 weeks after surgery and 18.4% by 6 weeks, measured by MRI [1]. Do the math and about three quarters of the 6-week loss (14.3 ÷ 18.4 = 0.78) had already happened in the first 2 weeks, before rehab really ramps up.

Two more details from that research surprise most people:

  • Your other leg loses muscle too. In a follow-up trial, the non-operated quadriceps shrank 7.2% in 6 weeks in the placebo group, just from moving less [3].
  • Muscle loss tracked with mobility. People who lost more quad muscle did worse on mobility tests [1].

Eating enough is harder than it sounds in this window. In a study of 101 older adults in hospital after elective hip or knee replacement, patients ate under 0.6 g of protein per kg of body weight a day, and about 32% of the food they were served went uneaten [4]. For an 80 kg (176 lb) person, that is 80 x 0.6 = 48 g a day, well short of the targets below.

What the Trials Found

High-Quality Trial #1: Essential Amino Acids Cut Early Quad Loss

In a randomized, double-blind, placebo-controlled study, 28 older adults having knee replacement took:

  • 20 grams of essential amino acids twice daily, between meals
  • Beginning 1 week before surgery
  • Continuing through 2 weeks after surgery

Quadriceps muscle volume was measured using MRI [1].

Quadriceps volume change Placebo group Amino acid group
2 weeks after surgery Lost 14.3% (± 3.6%) Lost 3.4% (± 3.1%)
6 weeks after surgery Lost 18.4% (± 2.3%) Lost 6.2% (± 2.2%)

At 2 weeks, the amino acid group lost about a quarter as much muscle (3.4 ÷ 14.3 = 0.24), roughly a fourfold difference. They also did better on mobility tests at 2 and 6 weeks [1].

High-Quality Trial #2: Faster Return of Daily Function

A second double-blind randomized trial in 60 knee replacement patients gave 9 grams a day of essential amino acids (or a lactose placebo) from 1 week before surgery through 2 weeks after [5]. It measured one thigh muscle (the rectus femoris) by ultrasound.

At 4 weeks:

  • Rectus femoris muscle area: 116% of baseline in the amino acid group vs 97% in the placebo group
  • Knee pain score: 39% of baseline vs 56%
  • Time to get back to activities of daily living (ADLs) was shorter in the amino acid group (p = 0.005)

In other words, on average the amino acid group did not just hold on to that muscle. It measured slightly larger than before surgery, although results varied a lot from person to person (71% to 206%). Getting back to everyday tasks sooner matters for your confidence and independence at home.

High-Quality Trial #3: Strength Recovery at 2 Years

The same research group followed 52 of those patients for 2 years [6]. Taking the starting point as 100%:

  • Rectus femoris muscle area: 134% ± 31% in the amino acid group vs 114% ± 27% with placebo
  • Quadriceps strength: 159% ± 54% vs 125% ± 40%

That is a real long-term strength difference. But pain, range of motion, mobility and knee scores were about the same in both groups. So the muscle results favored amino acids, while the outcomes patients feel day to day did not differ.

Trial #4: Less Muscle Loss, Same Strength

A larger follow-up trial gave 39 adults 20 g of essential amino acids twice daily for 7 days before and 6 weeks after knee replacement [3]. The operated quadriceps shrank 8.5% with amino acids vs 13.4% with placebo. Strength and functional tests did not differ between the groups. Kidney and liver blood tests were also similar between groups.

What the Research Does Not Show

Being honest about the limits helps you set the right expectations:

  • Results are mixed across all orthopedic trials. A 2025 review of 10 randomized trials found better muscle size in 3 of the 7 studies that measured it, and better mobility in 3. Doses ranged from 3.4 to 20 g, and researchers have not agreed on the best dose or timing [7].
  • Protein alone does not stop muscle loss from inactivity. In older men with one leg in a cast for 5 days, extra protein twice a day did not reduce the muscle or strength they lost [8]. Moving is what lets protein do its job.
  • Most of the trials are in knee replacement. Hip replacement patients face the same drop in activity and appetite, but there is less trial data for them.
  • None of these trials tested a Thrive product. They used pure essential amino acid formulas at 9 g to 40 g a day, not whey protein packets.

Why Essential Amino Acids Work in This Setting

Joint replacement patients face:

  • Rapid unloading of muscle (less walking and standing)
  • Stress and inflammation that favor muscle breakdown
  • Reduced food intake

Essential amino acids are the protein building blocks your body cannot make, so they have to come from food or supplements. They provide:

  1. Raw material for building muscle protein
  2. Leucine, which acts as a signal that switches on muscle building
  3. A way to get protein in when your appetite is low, because a drink is easier than a full meal

Importantly, these trials used complete essential amino acid mixtures, not isolated BCAAs or leucine. Leucine on its own has not been shown to build muscle: in healthy older men, 2.5 g of leucine with each meal for 3 months did not change muscle mass or strength [9]. It works best as part of enough total protein.

Why This Is Especially Important After 40

As we age, muscle responds less strongly to the protein we eat. That is one reason an international expert group recommends that adults over 65 get more protein than younger adults: at least 1.0 to 1.2 g per kg of body weight a day when healthy, and 1.2 to 1.5 g per kg for most people with an acute or chronic illness [10].

That makes protein around surgery especially worth planning if you:

  • Are over 50
  • Have lower muscle mass to start with
  • Are at risk of slower rehabilitation

The first 2 to 6 weeks after surgery are the window to focus on. Protecting muscle during this period gives your rehab more to work with.

How Much Protein Do You Need After Knee or Hip Replacement?

European clinical nutrition experts (ESPEN) estimate that surgical patients need about 1.5 g of protein per kg of ideal body weight a day [11]. The PROT-AGE group suggests 1.2 to 1.5 g per kg for older adults who are recovering from illness [10]. Here is what that looks like:

Body weight At 1.2 g per kg At 1.5 g per kg
60 kg (132 lb) 60 x 1.2 = 72 g a day 60 x 1.5 = 90 g a day
70 kg (154 lb) 70 x 1.2 = 84 g a day 70 x 1.5 = 105 g a day
80 kg (176 lb) 80 x 1.2 = 96 g a day 80 x 1.5 = 120 g a day
90 kg (198 lb) 90 x 1.2 = 108 g a day 90 x 1.5 = 135 g a day

To convert pounds to kilograms, divide by 2.2. If you carry extra weight, ask your care team which weight to use, since ESPEN bases its number on ideal body weight.

Important: if you have kidney disease, your doctor may want you to limit protein. Ask before raising your intake [10].

Practical Strategy After Joint Replacement

Based on the evidence from these randomized trials:

1. Start about 1 week before surgery. The knee trials began amino acids 1 week before the operation [1] [5], so you go into surgery well fed instead of playing catch-up.

2. Prioritize early protein adequacy. Use the table above to find your daily target, unless your doctor has told you to limit protein.

3. Distribute intake. A small lab study in 8 healthy adults found that eating about 30 g of protein at each of 3 meals switched on muscle building about 25% more over a day than eating most of it at dinner [12]. When your appetite is low, small, frequent servings and drinks are easier.

4. Combine with physical therapy. Nutrition supports rehabilitation. It does not replace it [8].

Your surgical team's instructions come first. Check with them before starting any supplement, especially if you have kidney disease, diabetes or a special diet, or take a blood thinner.

What to Expect: A Week by Week Timeline

Here is what recovery usually looks like and where nutrition fits. The research column describes what studies found, not what will happen to you.

When What is usually happening Your nutrition step What the research found
About 1 week before surgery You are still eating normally and getting your home ready. Start a daily protein drink between meals and get protein at every meal. The knee trials that protected muscle started amino acids 1 week before surgery [1] [5].
The night before Fasting rules usually start. If your team approves, have a carbohydrate drink at bedtime instead of a long fast. Then follow your fasting rules. People who had a carb drink the evening before and the morning of surgery felt less hungry and less anxious than people who fasted [13]. For hip and knee replacement, the ERAS Society says it may improve well-being but is not an essential step [14].
Hospital and days 1 to 3 Most people walk with a walker, crutches or a cane soon after surgery, and knee exercises can start within hours [15]. Sip protein through the day. Restart your daily protein drink once you are eating again. Older joint replacement patients ate under 0.6 g of protein per kg a day in hospital [4].
Weeks 1 to 2 at home Pain, swelling and short walks. Activity is still low. Hit your daily target (table above). Spread protein across meals and snacks. In the placebo group, about three quarters of the 6-week quad loss had already happened by week 2 [1].
Weeks 2 to 6 Physical therapy picks up. Most people resume most normal daily activities within 3 to 6 weeks [15] [16]. Most knee patients resume driving at about 4 to 6 weeks, once their surgeon clears them [15]. Keep protein steady. Take your protein drink on PT days too. The amino acid group got back to daily activities sooner [5]. At 6 weeks, another trial saw less muscle loss but similar strength [3].
After 6 weeks Rebuilding strength with exercise. Some pain with activity and at night is common for several weeks [15]. Keep your protein target while you train. Exercise rebuilds the muscle; protein supplies the raw material. At 2 years, the amino acid group had more thigh muscle and strength, but similar pain and knee scores [6].

What a Normal Day Looks Like

  • Breakfast: a protein-rich meal, such as eggs or Greek yogurt.
  • Mid-morning: one OptiFuel packet mixed with 12 to 16 oz of water, milk or non-dairy milk. It is thin enough to drink through a straw on days you feel queasy.
  • Lunch and dinner: a palm-sized portion of protein, such as fish, chicken, beans or tofu.
  • Mid-afternoon (optional): the Recovery and Strength Mix between lunch and dinner, if your surgeon agrees.
  • Every day: your home exercises and PT sessions. This is the part that turns protein into muscle.

Where Thrive Protocol Fits

Thrive Protocol is a surgeon-led recovery nutrition brand. Our kits are built to make the daily protein habit above easy to keep. Each OptiFuel packet gives 25 g of protein (whey plus collagen) with 3 g of leucine, plus vitamin C, vitamin D, omega-3s and curcumin. It is made in the USA and Informed Sport tested. One packet will not cover your whole daily target on its own: at 70 kg ideal weight, ESPEN's number is 70 x 1.5 = 105 g, so a packet covers about a quarter (25 ÷ 105 = 0.24) [11].

  • Surgery still ahead: the Surgical Protocol Kit ($299.99 for 4 weeks) includes daily OptiFuel, 2 OptiCharge carbohydrate packets for the night before surgery (each mixed in 12 to 16 oz of water at bedtime) and a shaker. Start OptiFuel about 1 week before surgery.
  • Surgery already done: the Recovery Protocol Kit ($289 for 4 weeks) is the same OptiFuel without OptiCharge.
  • Add to either kit: the Recovery and Strength Mix ($49.99) is the only Thrive product with creatine (5 g) and arginine (2 g).

Thrive's guidance is that you do not need to stop OptiFuel before surgery because of its fish oil or turmeric, since the amounts are low. Still, show the label to your surgeon, whose instructions come first. If you are insulin dependent, get your physician's approval before using OptiCharge.

Thrive Protocol kits are HSA/FSA eligible, so you can use pre-tax health dollars.

These statements have not been evaluated by the Food and Drug Administration. This product is not intended to diagnose, treat, cure, or prevent any disease.

Comparing options? Our guide to the best supplements after knee replacement surgery looks at vitamins, collagen and more, and our guide to what to eat after surgery covers meals for every procedure.

Thrive Protocol Surgical Protocol Kit with OptiFuel and OptiCharge packets

Bottom Line

Joint replacement surgery predictably causes fast quadriceps loss, and most of it happens in the first 2 weeks.

Randomized trials show that essential amino acid supplements started about 1 week before surgery can:

  • Reduce early muscle loss by a large margin [1] [3]
  • Support a faster return to daily activities in one trial [5]
  • Go along with better quadriceps strength 2 years later in one trial [6]

Strength and function results were not consistent across trials, and none of them tested a retail protein product. Still, this is one of the few areas in surgery nutrition with MRI-measured muscle data and long-term strength follow-up.

If the goal of joint replacement is mobility, protecting muscle in the first few weeks after surgery deserves a place in your plan, not an afterthought. Pair steady protein with your physical therapy, and let your surgical team guide the details.

Frequently Asked Questions

How much muscle do you lose after knee replacement?

In one MRI trial, people on a placebo lost 14.3% of their quadriceps volume in the first 2 weeks and 18.4% by 6 weeks. People taking essential amino acids lost 3.4% and 6.2% [1]. The other leg can lose some muscle too, just from moving less [3].

Why are my legs so weak after knee replacement, and can I get the muscle back?

Weakness comes from surgery stress, pain and much less activity, all at once, which together shrink the quadriceps quickly. Muscle does come back with rehab. In one trial, the front-thigh muscle measured in both groups was larger at 2 years than before surgery, and more so in the amino acid group [6]. Keep up your exercises and your protein. If weakness is not improving, tell your surgeon or physical therapist.

How much protein do I need after knee or hip replacement?

ESPEN estimates that surgical patients need about 1.5 g per kg of ideal body weight a day [11], and the PROT-AGE group suggests 1.2 to 1.5 g per kg for older adults recovering from illness [10]. At 80 kg, that is 80 x 1.2 = 96 g to 80 x 1.5 = 120 g a day. If you have kidney disease, ask your doctor before increasing protein.

When should I start taking protein before joint replacement?

About 1 week before surgery. That matches the knee replacement trials, which started amino acids 1 week before the operation and continued for 2 to 6 weeks after [1] [3]. If your date is closer, start as soon as you can.

Can you take creatine after knee replacement?

Many people can, but do not count on it to change early recovery. In the one trial of creatine around knee replacement, it did not improve strength or function in the first month [17]. In healthy adults whose leg had been in a cast, creatine plus strength training supported faster regain of thigh muscle and power during rehab [18]. Creatine has a long safety record at usual daily amounts in healthy adults [19]. If you have kidney disease or reduced kidney function, check with your doctor first. Read more in our article on whether creatine is safe for your kidneys.

Is a protein shake as good as the amino acid supplements in the studies?

No study has compared them directly after joint replacement. Whey is a complete protein, so it contains the essential amino acids the trials used, but the trials gave pure amino acid formulas at their own doses. What the evidence supports most clearly is reaching your daily protein target and staying active. A shake is one easy way to close the gap when your appetite is low.

This article is for education and does not replace advice from your surgeon or care team.

References

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  2. Soop M, et al. Preoperative oral carbohydrate treatment attenuates immediate postoperative insulin resistance. Am J Physiol Endocrinol Metab. 2001;280(4):E576-83. PubMed 11254464
  3. Dreyer HC, et al. Essential amino acid supplementation mitigates muscle atrophy after total knee arthroplasty: a randomized, double-blind, placebo-controlled trial. JB JS Open Access. 2018;3(2):e0006. PubMed 30280129
  4. Weijzen MEG, et al. Protein intake falls below 0.6 g/kg/d in healthy, older patients admitted for elective hip or knee arthroplasty. J Nutr Health Aging. 2019;23(3):299-305. PubMed 30820520
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  7. Brown E, et al. Amino acid supplementation may help prevent muscle wasting after orthopedic surgery, but additional studies are warranted: a systematic review of randomized clinical trials. HSS J. 2025;21(2):200-210. PubMed 39802328
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  13. Hausel J, et al. A carbohydrate-rich drink reduces preoperative discomfort in elective surgery patients. Anesth Analg. 2001;93(5):1344-50. PubMed 11682427
  14. Wainwright TW, et al. Consensus statement for perioperative care in total hip replacement and total knee replacement surgery: Enhanced Recovery After Surgery (ERAS) Society recommendations. Acta Orthop. 2020;91(1):3-19. PubMed 31663402
  15. American Academy of Orthopaedic Surgeons. Total Knee Replacement. OrthoInfo. OrthoInfo
  16. American Academy of Orthopaedic Surgeons. Total Hip Replacement. OrthoInfo. OrthoInfo
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  18. Hespel P, et al. Oral creatine supplementation facilitates the rehabilitation of disuse atrophy and alters the expression of muscle myogenic factors in humans. J Physiol. 2001;536(Pt 2):625-33. PubMed 11600695
  19. Kreider RB, et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr. 2017;14:18. PubMed 28615996