Before Your Operation:
What Nutrition Can and Can’t Do

The grades in this post: Strong means good randomised trials agree. Mixed means the trials are few, small or conflicting. Mechanistic means the biology makes sense but hasn’t been shown to change outcomes in people. None means there’s no usable evidence. Observational studies are labelled as such: they show a link, not proof of cause.

What’s checked before surgery, and what isn’t

• NICE recommends a full blood count before major surgery. It doesn’t list iron stores, vitamin D, zinc, vitamin C or CRP among the routine tests (NICE NG45).
• That’s a choice about safety and cost, not an oversight. Routine tests are there to make the operation safe.
• Whether checking more makes you heal better is a separate question, and mostly an untested one.

Healing needs raw materials

Wounds heal in overlapping stages: clotting, inflammation, rebuilding (new collagen and blood vessels), then months of remodelling.

• Collagen building needs vitamin C and iron.
• Immune cells need zinc.
• Every stage needs protein and enough energy.

That biology is well established. Mechanistic The open question is whether topping up someone who isn’t deficient changes the result.

What the evidence shows, nutrient by nutrient

Anaemia and iron

• In 227,425 people having major non-cardiac surgery, anaemia beforehand, even mild, went with higher death and complication rates at 30 days (odds of death 1.42 times higher) (Musallam 2011). Observational, large. The study was funded by a maker of intravenous iron.
• But treating it isn’t simple. In a UK trial of 487 anaemic patients before major abdominal surgery, intravenous iron didn’t reduce the combined risk of needing a transfusion or dying, compared with placebo (Richards 2020, PREVENTT). Strong For that use.
• Low iron stores can be missed when the full blood count looks fine. In anaemic patients, a ferritin of 30 µg/L (30 ng/mL) or less picked out iron deficiency far more reliably than the old cut-off of 12 µg/L (12 ng/mL) (Mast 1998). The British Society of Gastroenterology reads a ferritin below 30 µg/L (30 ng/mL) as low iron stores (BSG 2021). Finding and treating iron deficiency is ordinary good medicine. It’s worth asking your GP or pre-assessment team if you’re tired or have heavy periods.

Vitamin C

• Blood levels fall sharply after knee replacement: in people given a dummy pill, from about 49 to 12 µmol/L (0.86 to 0.21 mg/dL) a week after the operation. Vitamin C supplements stopped the fall, but movement in the knee at a year was no better. Fewer people taking vitamin C developed a stiff knee, but that difference could have been chance (Behrend 2019).
• In a blinded trial of 88 people with pressure ulcers, higher-dose vitamin C didn’t speed healing (ter Riet 1995).
• Mixed Scurvy stops wounds healing; extra vitamin C in people who aren’t deficient hasn’t been shown to help.

Zinc

• Severe deficiency impairs healing.
• In six small trials (183 people) of leg ulcers, zinc tablets didn’t improve healing (Wilkinson 2014, Cochrane). These are long-standing wounds, not surgical ones, and the trials were small.
• Measuring zinc in one person is hard: blood levels change with meals and the time of day, and fall with inflammation (King 2016).
• Mixed Correct a real deficiency; don’t expect extra to speed healing.

Vitamin D

• In a review of eight cohort studies (12,737 people), vitamin D below 30 ng/mL (75 nmol/L) before surgery went with more surgical-site infections; at the lower cut-off of 20 ng/mL (50 nmol/L) the link wasn’t statistically clear (Farsakoury 2026). In four cohorts of shoulder surgery (over 41,000 people), vitamin D deficiency went with slightly more problems afterwards, such as re-tears and repeat operations (Papadopoulos 2025).
• Whether taking vitamin D before an operation prevents infections hasn’t been tested in trials. Observational.
• UK deficiency cut-off: 25 nmol/L (10 ng/mL) (SACN 2016). My working range: 75–125 nmol/L (30–50 ng/mL), a clinical target, not official guidance.

Protein and energy

• Europe’s clinical nutrition society (ESPEN) treats malnutrition and underfeeding as risk factors for complications, and says any surgical patient at nutritional risk should get nutrition support. It also advises against long periods of fasting before surgery, and eating again as early as possible after (Weimann 2025). Guideline
• This matters most for older people, people who’ve lost weight without trying, and anyone eating very little.
• For amounts, see the “How much?” table in my injury nutrition article.

Inflammation (CRP)

• In 52 studies (121,849 people), a higher CRP before non-cardiac surgery went with higher death rates, and in two studies with more major heart complications (Baskaran 2024). Observational.
• It doesn’t show that lowering CRP first changes the outcome.

Supplements around surgery overall

• Trials are few. A 2025 review of 11 trials in gut surgery found too few to draw strong conclusions (Salman 2025).

Fish oil and other supplements before surgery

• You’ll often hear “stop fish oil two weeks before”. In a trial of 1,516 people having heart surgery, fish oil taken around the operation didn’t increase major bleeding, and fewer units of blood were transfused (Akintoye 2018). Strong For heart surgery. One author works for a company that sells omega-3 testing.
• That’s one trial in one kind of surgery. The rule that applies to everyone: tell your surgical team about every supplement you take, including herbal ones, and follow their instructions on what to stop and when.
• If you take a blood thinner, see Blood Clots: What Raises Your Risk, What Lowers It.

Injury rather than surgery?

The biology is the same. My injury nutrition article grades protein, creatine, omega-3 and collagen for muscle, tendon and joint injuries, with the amounts the trials used. Hurt? Who to See covers what to do first.

What I’d do before planned surgery

1. Ask about anaemia and iron at your pre-assessment, especially if you’re tired, have heavy periods or eat little red meat.
2. Don’t diet in the run-up. Eat enough, with protein at each meal.
3. If you’ve lost weight without trying or are eating poorly, say so. You may need nutrition support.
4. Tell the team about every supplement. Follow their stop-and-restart instructions.
5. If you smoke, ask your GP or pre-assessment team for help stopping, ideally starting at least four weeks before. Smokers have more wound problems after surgery: in 140 studies (479,150 people) they had about twice the odds of slow healing or a wound reopening (Sørensen 2012). Observational, large. In trials, intensive support starting four to eight weeks before surgery cut complications, including wound complications; brief advice didn’t (Thomsen 2014, Cochrane). Mixed Two small trials for the intensive approach.
6. A blood test before surgery can find iron deficiency, low vitamin D or poor blood sugar control. It can’t predict which supplement will make you heal faster.

How strong is the evidence?

Sources

1. Akintoye E et al. Fish oil and perioperative bleeding. Circ Cardiovasc Qual Outcomes 2018;11:e004584. PMID 30571332. doi:10.1161/CIRCOUTCOMES.118.004584
2. Baskaran G et al. Associations of inflammatory biomarkers with morbidity and mortality after noncardiac surgery: a systematic review and meta-analysis. J Clin Anesth 2024;97:111540. PMID 38959697. doi:10.1016/j.jclinane.2024.111540
3. Behrend H et al. Vitamin C demand is increased after total knee arthroplasty: a double-blind placebo-controlled randomized study. Knee Surg Sports Traumatol Arthrosc 2019;27:1182–8. PMID 29955932. doi:10.1007/s00167-018-5030-3
4. Farsakoury R et al. Vitamin D deficiency and risk of surgical site infections: a systematic review and meta-analysis. Surg Infect 2026;27:78–84. PMID 41108122. doi:10.1177/10962964251389152
5. King JC et al. Biomarkers of Nutrition for Development (BOND): zinc review. J Nutr 2016;146:858S–885S (online 2015). PMID 26962190. doi:10.3945/jn.115.220079
6. Snook J et al. British Society of Gastroenterology guidelines for the management of iron deficiency anaemia in adults. Gut 2021;70(11):2030–51. PMID 34497146. doi:10.1136/gutjnl-2021-325210. Below 15 µg/L: absent iron stores; below 30 µg/L: low iron stores; up to 100 µg/L can still mean iron deficiency in active inflammatory bowel disease.
7. Mast AE et al. Clinical utility of the soluble transferrin receptor and comparison with serum ferritin in several populations. Clin Chem 1998;44:45–51. PMID 9550557.
8. Musallam KM et al. Preoperative anaemia and postoperative outcomes in non-cardiac surgery: a retrospective cohort study. Lancet 2011;378:1396–407. PMID 21982521. doi:10.1016/S0140-6736(11)61381-0
9. NICE. Routine preoperative tests for elective surgery (NG45). nice.org.uk, read 14 October 2026.
10. Papadopoulos DV et al. Association between vitamin D deficiency and adverse events following shoulder surgeries: a systematic review and meta-analysis. Indian J Orthop 2025;59:1255–64. PMID 40852540. doi:10.1007/s43465-025-01424-3
11. Richards T et al. Preoperative intravenous iron to treat anaemia before major abdominal surgery (PREVENTT): a randomised, double-blind, controlled trial. Lancet 2020;396:1353–61. PMID 32896294. doi:10.1016/S0140-6736(20)31539-7
12. SACN. Vitamin D and health. 2016.
13. Salman D et al. Does micronutrient supplementation improve outcomes in adult gastrointestinal surgery patients? A systematic review. Front Nutr 2025;12:1719617. PMID 41459083. doi:10.3389/fnut.2025.1719617
14. Sørensen LT. Wound healing and infection in surgery. The clinical impact of smoking and smoking cessation: a systematic review and meta-analysis. Arch Surg 2012;147:373–83. PMID 22508785. doi:10.1001/archsurg.2012.5
15. ter Riet G et al. Randomized clinical trial of ascorbic acid in the treatment of pressure ulcers. J Clin Epidemiol 1995;48:1453–60. PMID 8543959. doi:10.1016/0895-4356(95)00053-4
16. Thomsen T et al. Interventions for preoperative smoking cessation. Cochrane Database Syst Rev 2014;(3):CD002294. PMID 24671929. doi:10.1002/14651858.CD002294.pub4
17. Weimann A et al. ESPEN guideline on clinical nutrition in surgery: update 2025. Clin Nutr 2025;53:222–61. PMID 40957230. doi:10.1016/j.clnu.2025.08.029
18. Wilkinson EAJ. Oral zinc for arterial and venous leg ulcers. Cochrane Database Syst Rev 2014;(9):CD001273. PMID 25202988. doi:10.1002/14651858.CD001273.pub3