Vitamins & Minerals

25-OH vitamin D is the storage form of vitamin D measured in your blood, and it is widely considered the best indicator of your overall vitamin D status. Your body produces vitamin D when sunlight hits your skin, and you can also obtain smaller amounts from certain foods and supplements. This marker reflects how much vitamin D your body has accumulated and has available to draw upon. Maintaining a healthy level matters because vitamin D helps keep your bones strong, supports your immune system, and contributes to normal muscle function. In the UK, where sunlight is limited for much of the year, many people have lower levels than they might expect. Think of vitamin D as a key that unlocks your body's ability to absorb calcium from food. Without enough of it, calcium passes through your digestive system without being properly taken up, no matter how much dairy or other calcium-rich food you eat. Beyond its well known role in bone health, vitamin D is also involved in regulating your immune defences, helping your body mount an appropriate response when needed and keeping things balanced the rest of the time. It may also play a role in mood and energy, although the evidence here is less clear. The 25-OH form measured in this test is produced in your liver and then converted into its active form by your kidneys when your body needs it. Several factors affect your vitamin D level. The most significant in the UK is sun exposure: between October and March the sun sits too low in the sky for your skin to produce meaningful amounts, and even in summer, sunscreen use and indoor lifestyles can limit production. People with darker skin tones produce vitamin D more slowly, and older adults tend to synthesise it less efficiently. Dietary sources such as oily fish, egg yolks, and fortified foods contribute relatively small amounts, which is why the NHS advises everyone to consider a vitamin D supplement during autumn and winter. Your GP can help you decide whether supplementation or further investigation is appropriate based on your individual result and circumstances.

Active B12, also known as holotranscobalamin, measures the portion of vitamin B12 in your blood that is actually available for your cells to use. Unlike a total B12 test, which includes both usable and inactive forms, this marker focuses specifically on the biologically active fraction, the B12 that is bound to a transport protein called transcobalamin and ready to be delivered to the tissues that need it. This makes active B12 a more sensitive and reliable early indicator of your true B12 status. Vitamin B12 is essential for healthy nerve function, the production of red blood cells, and the process of copying your DNA every time a cell divides, so knowing whether your cells have enough of it is genuinely important for your overall wellbeing. Imagine your B12 supply as a delivery service. Total B12 counts every parcel in the warehouse, whether it is labelled and ready for dispatch or sitting in a corner gathering dust. Active B12 counts only the parcels that are on the delivery van, addressed, and on their way to customers. Because it measures the fraction that is immediately usable, active B12 can flag a developing deficiency earlier than a standard total B12 test, sometimes months before symptoms appear. Your body cannot make B12 on its own, so you rely entirely on dietary intake and absorption through the gut. Once absorbed, B12 is stored in the liver, but those stores can be depleted over time if intake or absorption is inadequate. Several factors influence your active B12 level. A diet low in animal products, particularly a vegan or strict vegetarian diet, is one of the most common reasons for lower levels, since B12 is found naturally in meat, fish, eggs, and dairy. Absorption can also be affected by digestive conditions, certain medications such as proton pump inhibitors, and age-related changes in stomach acid production. Regular supplementation or fortified foods can help maintain healthy levels, especially for those at higher risk of deficiency. If your active B12 result is outside the expected range, your GP can advise on appropriate supplementation or further tests to understand the cause.

Plasma zinc measures the level of zinc circulating in your blood. Zinc is an essential trace mineral, meaning your body needs it in small but steady amounts and cannot produce it on its own, so it must come from food or supplements. Despite being required in only tiny quantities, zinc is involved in a remarkably wide range of bodily processes, from keeping your immune system functioning properly to supporting wound healing, maintaining your sense of taste and smell, and helping your body build new proteins. A zinc level within the expected range suggests these systems have the raw material they need. Think of zinc as a versatile assistant that shows up in hundreds of different departments across your body. It acts as a co-factor for over three hundred enzymes, meaning those enzymes simply cannot do their jobs without it. When you catch a cold, zinc helps your immune cells mount a defence. When you cut yourself, zinc is part of the repair team that helps new tissue form. It also plays a role in maintaining healthy skin, supporting normal testosterone production, and even helping your body process carbohydrates and fats for energy. Because zinc is involved in so many processes, even a modest shortfall can have wide ranging effects that may be subtle at first, such as feeling run down more often or finding that small wounds take longer to heal. Your zinc level can be influenced by a number of dietary and lifestyle factors. The richest food sources include red meat, shellfish (especially oysters), poultry, beans, nuts, and seeds. Vegetarian and vegan diets can sometimes fall short because plant-based sources of zinc are less readily absorbed, owing to compounds called phytates that bind to the mineral. Intense exercise, stress, and certain digestive conditions can also increase your body's zinc requirements or reduce absorption. It is worth noting that plasma zinc levels can fluctuate throughout the day and may be temporarily lowered by recent meals, infection, or inflammation. If your result is outside the normal range, your GP can help determine whether a dietary adjustment or supplement might be beneficial for you.

Folate, also known as vitamin B9, is a water-soluble B-vitamin that your body needs for DNA synthesis, healthy cell division, and the production of red blood cells. This blood test measures the level of folate circulating in your system. Adequate folate is important for everyone, but it takes on particular significance for women who are pregnant or planning to become pregnant, because it is critical to the very earliest stages of foetal development. Beyond reproduction, folate supports your overall energy levels and helps keep your blood cells healthy and functioning as they should. To understand what folate does, picture your body as a vast construction site where new cells are being built and repaired around the clock. Folate is one of the essential materials on that site. Without it, the process of copying DNA (your body's instruction manual) before a cell divides cannot happen properly. This is why rapidly dividing tissues, such as bone marrow where blood cells are made, are among the first to be affected when folate runs low. Folate also works closely with vitamin B12 to keep levels of an amino acid called homocysteine in check. When both vitamins are present in adequate amounts, homocysteine is efficiently recycled; when either is lacking, homocysteine can build up, which is something your GP may want to monitor. Your folate level is influenced primarily by your diet. Rich sources include dark leafy greens such as spinach and kale, broccoli, asparagus, chickpeas, lentils, and fortified breakfast cereals. Because folate is water-soluble and sensitive to heat, cooking methods matter: steaming and microwaving preserve more folate than boiling. Alcohol consumption can interfere with folate absorption and increase the rate at which your body uses it up, while certain medications and digestive conditions may also lower levels. Unlike some vitamins, your body does not store large amounts of folate, so regular dietary intake is important to maintain healthy levels. If your result is outside the normal range, your GP can advise on whether dietary changes or supplementation would be helpful and check related markers such as B12.

Magnesium is one of the hardest working minerals in your body, involved in over 300 enzymatic reactions. It supports muscle and nerve function, energy production, bone health, heart rhythm, and blood sugar regulation. Despite its importance, low magnesium can be easy to overlook. Your body stores most of its magnesium inside cells and in bone. Only about 1% circulates in the blood. This means a blood test can sometimes appear normal even when your total body stores are running low. Still, serum magnesium remains the standard screening test and is useful for detecting significant depletion. Low magnesium is common in people who eat a highly processed diet, drink alcohol regularly, take certain medications (such as proton pump inhibitors or diuretics), or experience chronic stress. Symptoms of deficiency can be subtle at first (muscle cramps, poor sleep, irritability and fatigue), but prolonged depletion can affect heart rhythm and bone density. Magnesium works in close partnership with calcium and vitamin D. When magnesium is low, your body may struggle to regulate calcium properly, and vitamin D activation can be impaired. Athletes and those who exercise intensely may have higher magnesium requirements due to losses through sweat and increased metabolic demand. High magnesium from diet alone is extremely rare because healthy kidneys excrete the excess efficiently. Elevated levels are most commonly seen with kidney impairment or excessive supplementation. If your magnesium level is outside the expected range, your GP can help identify the cause and advise on whether dietary adjustments or supplementation would be beneficial.

Total calcium measures the overall amount of calcium circulating in your blood, including both the portion bound to proteins like albumin and the free, active fraction. Calcium is essential for building strong bones and teeth, muscle contraction, nerve signalling, blood clotting, and maintaining a steady heartbeat. Your body regulates calcium very tightly, and larger deviations outside the normal range can produce noticeable symptoms. A raw total calcium reading is useful as a quick check, but it is influenced by your albumin level. About half the calcium in your blood travels bound to albumin, so when albumin is abnormal, the total calcium value may under- or over-state your true calcium status. This is why labs typically report both total calcium and corrected calcium (adjusted for albumin). The two are read together. For most people with normal albumin, total calcium and corrected calcium are effectively interchangeable. Low total calcium can cause tingling in the fingers, muscle cramps, and occasionally muscle spasms. Common causes include vitamin D deficiency, low albumin, underactive parathyroid glands, kidney impairment, and magnesium deficiency. High total calcium can produce fatigue, excessive thirst, frequent urination, constipation, and confusion; the most frequent causes are hyperparathyroidism, excessive vitamin D supplementation, prolonged immobility, and dehydration (which can falsely elevate the reading). Your GP will interpret total calcium alongside corrected calcium, albumin, vitamin D, parathyroid function, and kidney function to build an accurate picture. If your total calcium falls outside the expected range, your GP can arrange further investigation to identify the cause and advise on appropriate next steps.

Corrected calcium measures the total calcium in your blood, adjusted for your albumin level. About half the calcium in your blood travels bound to albumin, so if your albumin is unusually high or low, the raw calcium reading can be misleading. The correction ensures you get an accurate picture of your true calcium status. Calcium is best known for building strong bones and teeth, but it plays equally important roles in muscle contraction, nerve signalling, blood clotting, and maintaining a steady heartbeat. Your body regulates calcium very tightly, and larger shifts outside the normal range can cause noticeable symptoms. Low corrected calcium can cause tingling in the fingers, muscle cramps, and in more significant cases, muscle spasms. It is most commonly related to low vitamin D (which is needed to absorb calcium from food), underactive parathyroid glands, or kidney problems. Magnesium deficiency can also impair calcium regulation. High corrected calcium is less common and can cause fatigue, excessive thirst, frequent urination, constipation, and confusion. The most frequent causes are overactive parathyroid glands and excessive vitamin D supplementation. Prolonged elevation can affect kidney function and bone density. Because calcium balance depends on a network of hormones and minerals working together, an abnormal result is rarely interpreted in isolation. Your GP will typically look at your vitamin D, albumin, kidney function, and alkaline phosphatase alongside calcium. If your corrected calcium falls outside the expected range, your GP can arrange further investigation to identify the underlying cause and recommend appropriate management.