Nutrition After Bariatric Surgery: Supplements and Deficiencies

After bariatric surgery you will need a daily multivitamin for life, regular blood tests and, for many people, extra supplements such as iron, vitamin B12, calcium or vitamin D. Smaller meals reduce how much of each nutrient you take in, and bypass operations also reduce how much you absorb. Deficiencies are common, often develop without obvious symptoms, and are much easier to prevent than to correct.

This topic is also in our free guide, Bariatric Surgery: Before and After. Get it emailed to you

Why deficiencies happen

Every operation reduces the amount you can eat. After a gastric bypass, food also skips the first part of the small intestine, where many vitamins and minerals are normally absorbed, which is why supplements are needed for life (healthdirect). The one-anastomosis gastric bypass bypasses a longer length of intestine and has been associated with more nutritional complications, such as malnutrition and anaemia, than the Roux-en-Y bypass (Li et al., Obesity Surgery, 2023). A sleeve gastrectomy does not reroute the intestine, but deficiencies are still common after it.

The lifelong multivitamin

The Australian Journal of General Practice describes a daily multivitamin as essential for life after bariatric surgery (AJGP, 2025). Not every multivitamin is formulated for people who have had bariatric surgery, so ask your bariatric team or dietitian which product and dose suit your operation, rather than choosing one off the shelf. Additional supplements are added on top according to your blood results.

The common deficiencies

According to the AJGP review, deficiencies affecting more than 10% of people after sleeve gastrectomy or Roux-en-Y gastric bypass include vitamins B1 (thiamine), B3, B6, B9 (folate), B12, A, C, D and E, along with iron, magnesium, zinc and selenium (AJGP, 2025). The ones you are most likely to hear about are:

  • Iron. Low iron leads to anaemia, tiredness and poor exercise tolerance. US bariatric guidelines advise extra iron for people who menstruate and for those who have had a bypass or sleeve (ASMBS, 2017).
  • Vitamin B12. Needed for blood and nerve health. It may be taken as a tablet that dissolves under the tongue, as a liquid or by injection, depending on your levels and your operation (ASMBS, 2017).
  • Vitamin D and calcium. Important for bone health. Calcium supplementation is recommended after all the common operations, and vitamin D is adjusted to blood levels (ASMBS, 2017).
  • Folate. Needed for healthy blood cells, and particularly important for anyone who may become pregnant.
  • Thiamine (vitamin B1). Included in bariatric supplement recommendations (ASMBS, 2017). Tell your bariatric team promptly if you are vomiting repeatedly or unable to eat.
  • Zinc. Involved in wound healing, skin and hair.

Deficiencies can persist well after the early recovery period. In a Brazilian study of 32 people seeking plastic surgery at least two years after bariatric surgery, 56% had anaemia, 31.2% were iron deficient, 25% were vitamin B12 deficient and 18.7% were zinc deficient (Revista Brasileira de Cirurgia Plástica).

Protein

Protein is the other nutrient that most often falls short, because the smaller stomach fills quickly. The same Brazilian study found mild deficiencies in its participants’ protein profile (Revista Brasileira de Cirurgia Plástica). Low protein affects how much muscle you keep and, later, how well you heal from any further surgery. Your bariatric program will give you a protein goal; protein and muscle while losing weight explains the principles.

Blood tests

Annual nutritional screening bloods are recommended for life (AJGP, 2025). US guidelines suggest checks every three to six months in the first year and yearly after that (ASMBS, 2017). Your bariatric team or GP will decide the exact panel, which commonly includes:

  • Full blood count, to look for anaemia.
  • Iron studies, including ferritin. You may be asked to fast and to stop iron supplements before the test.
  • Vitamin B12 and folate.
  • Vitamin D.
  • Other markers, such as zinc, thiamine, calcium-related tests, kidney and liver function, as your doctor advises.

Keep copies of your results. They are useful for any future doctor, and if you later consider body contouring surgery, recent bloods are part of the preparation (see preparing your nutrition before body contouring surgery and health checks and investigations).

Staying on track long term

Supplements and blood tests are easy to let slip once life returns to normal. Simple habits help: take supplements at the same time each day, set an annual reminder for bloods, and keep your GP informed about your operation. A dietitian can review your intake if results start to drift. Dr Kumar’s patients work with Amy Kellner APD, an Accredited Practising Dietitian who consults by telehealth anywhere in Australia, and ANZMOSS lists dietitians with bariatric experience.

Meal-prep containers of salmon, rice, salad and pasta

Where to next

If your weight has settled after bariatric surgery and loose skin is now the main issue, take the readiness assessment or book a consultation with Dr Kumar.

Sources