Guest blog by Lillian Craggs-Dino, DHA, RDN, LDN, CLT, FASMBS-IH Colds, the flu, COVID-19, strep throat, and foodborne illness. Germs are everywhere, and none of us live in a sterile bubble. We travel, gather with friends and family, and share meals, which means our immune systems are working every single day. Here's something important to know: Excess body fat is linked to weaker immune defenses and ongoing, low-grade inflammation in the body.1 The encouraging part is that weight loss, including weight loss after metabolic and bariatric surgery, has been shown to improve markers of immune function.1 Good nutrition plays a big role in keeping that progress going. What Does the Immune System Do? Your immune system is your body's security team. It constantly watches for germs like viruses, bacteria, and parasites, and changes in your own cells that could lead to illness. It works in two main layers: Innate immunity is your first line of defense. It includes physical barriers like your skin and the moist linings of your nose, mouth, and gut, which help keep germs out. Adaptive immunity is your targeted defense. It relies on specialized white blood cells, such as macrophages, T cells, and B cells, that learn to recognize specific germs and make antibodies to fight them.2 Why Nutrition Matters More After Bariatric Surgery Bariatric surgery changes how much you eat and, depending on the procedure, how well your body absorbs certain nutrients. Many people also start their weight loss journey already low in at least one vitamin or mineral.3 That's why lifelong vitamin and mineral supplements, along with regular lab checks, are a standard part of care after surgery. After gastric bypass, sleeve gastrectomy, or duodenal switch, current guidelines recommend a daily bariatric multivitamin with minerals equal to two standard adult multivitamins, plus separate calcium and vitamin D.3 Vitamins and Minerals That Support Your Immune System Several nutrients work together to keep your immune system running well:2 Vitamin A helps keep your skin and the linings of your airways and gut healthy, so they can block germs. Vitamin D helps regulate immune cells and supports your body's natural germ-fighting responses. Vitamin E acts as an antioxidant that helps protect immune cells from damage. Vitamin C supports your skin barrier and helps white blood cells do their job. Vitamin B12 and folate are needed to make new immune cells, which your body produces constantly. Iron is needed for immune cells to grow and function. Zinc supports the development and activity of immune cells and helps maintain healthy skin and linings. Selenium supports antioxidant defenses and healthy immune responses. How Much Do You Need The table below shows the typical daily recommendations after bariatric surgery alongside the general recommended dietary allowance (RDA) for adults. Your bariatric team may adjust your doses based on your procedure and lab results. Nutrient After Bariatric Surgery General Adult RDA Food Sources Vitamin A Gastric bypass and sleeve: 5,000 to 10,000IU daily Duodenal switch: 10,000IU daily3 Women: 700mcg Men: 900 mcg4 Eggs, dairy, liver, and colorful produce like spinach, sweet potatoes, pumpkin, carrots, kale, and red peppers Vitamin D 3,000IU of vitamin D3 daily, adjusted by your care team to keep your blood level above 30ng/mL; Higher doses are used short-term only to correct a deficiency3 Ages 19 to 70: 600IU (15mcg) Over 70: 800IU (20mcg)4 Salmon, tuna, eggs, fortified milk and cereals, and UV-exposed mushrooms Vitamin E 15mg daily for all procedures3 15mg for all adults4 Vegetable oils, nuts, seeds, leafy greens, and wheat germ Vitamin C No set dose after surgery; Upper limit is 2,000mg daily4 Women: 75 mg Men: 90mg4 Citrus, strawberries, tomatoes, bell peppers, broccoli, Brussels sprouts, and potatoes Vitamin B12 350 to 1,000mcg daily by mouth (tablet or dissolvable), or 1,000mcg by monthly injection, or nasal spray as directed3 2.4mcg for all adults4 Clams, salmon, tuna, beef, poultry, eggs, and dairy Folate (folic acid) 400 to 800mcg daily from your multivitamin; Women who could become pregnant: 800 to 1,000mcg daily3 400mcg DFE for all adults4 Leafy greens, beans, lentils, oranges, and fortified cereals Iron Men and people without a history of anemia: 18mg daily Menstruating women and those who had bypass, sleeve, or duodenal switch: 45 to 60mg daily from all supplements combined; Take iron separately from calcium when dose-dependent ratio isn't present in a multivitamin3,4 Women 19 to 50: 18mg Women 51+ and men: 8mg5 Beef, poultry, seafood, beans, lentils, spinach, and pumpkin seeds Zinc Duodenal switch: 16 to 22mg daily Gastric bypass: 8 to 22mg daily Sleeve: 8 to 11mg daily; Should come with copper in a ratio of 8 to 15mg zinc per 1mg copper3 Women: 8mg Men: 11mg5 Oysters, beef, crab, poultry, pumpkin seeds, and sesame seeds Selenium No set dose after surgery; Upper limit is 400mcg daily5 55mcg for all adults5 Brazil nuts, tuna, salmon, poultry, eggs, and pork A note on food safety and balance: More is not always better. Liver is very high in vitamin A, and a single Brazil nut can contain more than a full day's worth of selenium, so enjoy both in small amounts.5 Always check with your bariatric team before adding extra supplements on top of your daily regimen. Other Ways to Support Your Immune System Nutrition is one piece of the puzzle. These everyday habits help too: Prioritize sleep. Regular, quality sleep supports healthy immune responses, while ongoing sleep loss can weaken them.6 Stay active. Regular moderate exercise is linked to better immune function and fewer upper respiratory infections.7 Limit alcohol. Alcohol can interfere with immune function, and after bariatric surgery it is absorbed faster and affects you more strongly. Hydrate well. Sip calorie-free fluids throughout the day to meet your hydration goals after surgery. Keep up with your labs. Routine blood work helps your care team catch and correct low vitamin or mineral levels early.3 Frequently Asked Questions Does Bariatric Surgery Weaken Your Immune System? Research suggests the opposite. Weight loss after bariatric surgery is linked to reduced inflammation and improved immune markers.1 However, nutrient deficiencies after surgery can affect immune health, which is why daily supplements matter. What Is the Most Important Vitamin for Immunity After Bariatric Surgery? No single nutrient does it all. Vitamins A, C, D, E, B12, and folate, along with iron, zinc, and selenium, all work together.2 A complete bariatric multivitamin with minerals, taken consistently, is the foundation. Can I Get Enough Nutrients From Food Alone After Surgery? Food is important, but after surgery your portions are smaller, and absorption may change. Lifelong supplements are needed to meet your needs.3 References de Frel DL et al. Front Nutr. 2020;7:597600. Gombart AF et al. Nutrients. 2020;12(1):236. Mechanick JI et al. Surg Obes Relat Dis. 2020;16(2):175-247. Gaitán D et al. J Nutr. 2011;141(9):1652-1656. National Institutes of Health, Office of Dietary Supplements. Vitamin and Mineral Fact Sheets. Accessed September 2026. Accessed from https://ods.od.nih.gov/factsheets/list-VitaminsMinerals/. Besedovsky L et al. Physiol Rev. 2019;99(3):1325-1380. Nieman DC et al. J Sport Health Sci. 2019;8(3):201-217.