One of the things that is easy to overlook in the excitement of losing weight after gastric sleeve surgery is the ongoing medical monitoring that needs to happen for the rest of your life. Blood tests are not optional. They are not something you do once in the first year and then forget about. They are a permanent feature of life with a gastric sleeve, and we say that from direct experience of what happens when you let them slip.
At month eleven, James hit a wall. Crushing fatigue, brain fog, feeling cold all the time, struggling to function. It turned out his iron and vitamin D levels had dropped significantly. The symptoms had crept up gradually and were easy to attribute to other things. A thorough blood panel caught it and we were able to address it through adjusted supplementation. But it was a clear lesson in why the testing matters.
Why blood tests matter after gastric sleeve surgery
Your digestive system is fundamentally different after gastric sleeve surgery. The stomach is smaller, digestion moves faster, and your ability to absorb certain nutrients is reduced. Some vitamins and minerals depend on slow, extended contact with the stomach lining to be absorbed properly. When that contact time is cut short, deficiencies can develop even when you are eating well and taking your supplements.
The problem is that deficiencies often develop slowly and silently. By the time you feel noticeably unwell, your levels may have been low for months. Regular blood tests are the only way to catch problems early, before they become serious.
This is not a minor concern. Iron deficiency anaemia, B12 deficiency causing neurological symptoms, low vitamin D leading to bone density loss, and severe calcium deficiency causing osteoporosis are all real and documented risks of bariatric surgery when nutritional monitoring is neglected. These are not things that happen to other people. They can happen to anyone who does not stay on top of their bloods.
What tests you should be getting
A standard post-bariatric blood panel should include a full blood count, which checks for anaemia and gives a picture of your overall blood health. Beyond that, the key markers to track are iron and ferritin, which measures your iron stores rather than just circulating iron, serum B12, folate, vitamin D, calcium, and parathyroid hormone. Parathyroid hormone is a useful indirect indicator of calcium and vitamin D status because it rises when your body is struggling to maintain calcium levels.
Depending on your individual history and risk profile, your team may also want to check zinc, magnesium, and selenium. James had zinc and magnesium tested alongside the standard panel from the beginning, which we would recommend. Both are commonly depleted after gastric sleeve and both affect a surprising range of bodily functions including immune response, wound healing, sleep, and mood.
If you had any metabolic conditions before surgery such as type 2 diabetes, high blood pressure, or fatty liver disease, your HbA1c, liver function, and blood pressure should also be monitored. In our case, James had non-alcoholic fatty liver disease for over ten years before surgery. His liver function returned to normal within three weeks of surgery, but it still gets checked regularly.
When to get tested
BOMSS guidelines recommend blood tests at three months post-surgery, six months, twelve months, and then annually after that. If you are experiencing symptoms of a potential deficiency at any point, do not wait for your next scheduled test. Go and get checked.
In practice, a lot of people struggle to get their GP to run the full bariatric panel. Some GPs are unfamiliar with what post-bariatric monitoring involves and may only run a basic set of markers. It is worth knowing what you need and being specific when you request it. If your GP surgery cannot or will not run the full panel, private blood tests are relatively affordable and widely available. We have used both routes at different points.
Understanding your results
When you get your results, be aware that standard reference ranges are not always appropriate for post-bariatric patients. A vitamin D result that falls within the normal reference range for the general population may still be suboptimal for someone who has had weight loss surgery. The same applies to B12, where some bariatric specialists recommend maintaining levels at the higher end of normal rather than just avoiding deficiency.
If anything comes back flagged, do not panic but do act on it promptly. Adjusting supplementation, switching to a more absorbable form of a particular vitamin, or in more severe cases having an infusion or injection, are all options depending on what is low and by how much. Your bariatric team or GP should be able to advise, and a dietitian with bariatric experience can be particularly helpful in working out the best supplementation approach.
Staying on top of it
Two years in, we have blood tests built into our routine. We book them in advance, treat them as non-negotiable, and review the results properly. We have both adjusted our supplement regimens based on test findings more than once. It requires a level of proactive engagement with your own health that not everyone is used to, but it genuinely matters.
If you are in the early months after surgery, our single biggest piece of advice beyond the obvious dietary rules is this: do not skip the blood tests. The consequences of unchecked deficiencies are significant and largely avoidable. Stay on top of it from the beginning and it becomes a normal part of life rather than a source of anxiety.
Sources
British Obesity and Metabolic Surgery Society (BOMSS): Guidelines on peri-operative and postoperative biochemical monitoring and micronutrient replacement for patients undergoing bariatric surgery. NHS: Vitamins and minerals. National Institute for Health and Care Excellence (NICE): Bariatric surgery for obesity.
About this content
This blog is written by James and Kirsten, a couple from the UK who had gastric sleeve surgery together in March 2024.
We started this blog because we couldn't find any sources of content that details before surgery, the surgery and then life post surgery - so we decided to write one ourselves.
Everything on this site is based on our own experience and the research we have done along the way. It is not medical advice. Gastric sleeve surgery is a serious procedure and every patient's journey is different. Please always consult your own bariatric team or GP before making any decisions about your health or treatment.
Some posts on this site may contain featured or sponsored content, or affiliate links. Where this is the case, it will always be clearly stated at the top of the article. Our opinions are always our own.
Publish Date: 20 March 2026 | Last Reviewed: 7 June 2026 | Next Planned Review: 7 December 2027