Blood Testing for Vitamin & Mineral Levels in Horses – Can it Detect Nutritional Deficiencies?

Assessing a horse’s nutritional status is critical for maintaining health and performance. Identifying and correcting vitamin and mineral deficiencies can help prevent a range of health issues, from poor coat condition and hoof quality to more serious problems like immune dysfunction or muscle weakness.

Many horse owners and veterinarians use blood vitamin and mineral analysis to detect these deficiencies and ensure their horses are receiving the necessary nutrients.

However, while these tests can provide some valuable insights, they may not always present a complete picture of a horse’s overall nutritional status. Factors such as exercise, feeding time, medical conditions, and stress can temporarily alter blood nutrient levels.

For certain nutrients, such as calcium, copper, zinc, and iron, blood levels don’t correlate well with dietary intake. In other cases, such as sodium, chloride, and selenium, blood tests are primarily useful for identifying extreme levels, revealing toxicity or long-term deficiencies.

Continue reading to learn more about the applications and limitations of blood vitamin and mineral analysis in equine nutrition. We’ll also discuss the most reliable ways to assess nutritional status in equines and how a nutritionist can use this information to develop a balanced diet for your horse.

Vitamins & Minerals in the Equine Diet

Vitamins and minerals make up a small portion of the equine diet, but are critically important for supporting many processes in the body, including:

• Hormone synthesis
• Energy metabolism
• Electrolyte balance
• Immune function
• Antioxidant protection

In horses, prolonged deficiencies in vitamins and minerals can lead to many health issues such as: ^[1]

• Poor hoof health
• Suboptimal exercise performance
• Skin disorders
• Skeletal disorders
• Neuromuscular dysfunction
• Reproductive failure

Vitamins

Vitamins are essential organic compounds that function as catalysts, accelerating chemical reactions within the body to support various physiological processes.

Important vitamins for horses include:

• Vitamin A: Essential for vision, immune function, and skin health.
• Vitamin D: Supports calcium absorption and bone health.
• Vitamin E: Acts as an antioxidant, supporting muscle and immune function.
• Vitamin K: Important for blood clotting and bone health.
• Vitamin B1 (Thiamine): Helps convert carbohydrates into energy and supports nerve function.
• Vitamin B7 (Biotin): Critical for hoof health, skin, and coat condition.
• Vitamin C (Ascorbic Acid): An antioxidant that supports immune function and collagen production.

Most vitamins must be supplied through the diet because the horse’s body cannot produce them in sufficient quantities. ^[1]

Vitamins are categorized into two main groups: fat-soluble and water-soluble.

• Fat-soluble vitamins — such as vitamins A, D, E, and K — are stored in the body’s fat tissues and liver, meaning they don’t need to be consumed daily. However, because they are stored, there’s a risk of toxicity if too much is provided. Vitamins A and E must be supplied by the diet, while vitamin D can be synthesized through exposure to sunlight, and vitamin K is partially synthesized by microbes in the horse’s gut. ^[1]
• Water-soluble vitamins — like the B-vitamins and vitamin C — are not stored in the body and are excreted through urine, so they need to be consumed regularly. B-vitamins are synthesized by microbes in the horse’s hindgut, although the extent of their absorption is still uncertain. Vitamin C can be produced from glucose within the horse’s body and is also provided through the diet. ^[1]

Minerals

Minerals are inorganic substances that play crucial roles in various physiological processes. They must be obtained through the diet because the body cannot synthesize them. ^[2]

Some minerals act as cofactors in enzymatic reactions and are involved in energy production, while others are vital for structural functions — supporting strong bones, hooves, and teeth. Additionally, certain minerals function as electrolytes, helping to regulate fluid balance, nerve signaling, and muscle contractions.

Minerals are divided into two categories based on the amounts required: ^[2]

• Macrominerals are needed in larger quantities, typically measured in grams per day.
• Microminerals, or trace minerals, are required in much smaller amounts, measured in micrograms per day.

Below is a list of some important minerals that horses require in their diet:

• Calcium (Ca) – Essential for bone structure and muscle function.
• Phosphorus (P) – Works with calcium to support healthy bones and energy metabolism.
• Magnesium (Mg) – Important for muscle function, nerve transmission, and enzyme activity.
• Potassium (K) – Crucial for electrolyte balance, muscle contractions, and nerve function.
• Sodium (Na) – Helps regulate fluid balance and is important for nerve and muscle function.
• Chloride (Cl) – Works with sodium to maintain proper fluid and electrolyte balance.
• Selenium (Se) – Acts as an antioxidant and is important for muscle function.
• Iron (Fe) – Essential for oxygen transport in the blood.
• Copper (Cu) – Supports the formation of connective tissue and red blood cells.
• Zinc (Zn) – Important for immune function, skin, and hoof health.
• Iodine (I) – Necessary for thyroid function and regulating metabolism.

Blood Analysis for Vitamins & Minerals in Horses

Horse owners are increasingly interested in using blood analysis as a tool to evaluate their horse’s nutritional status. Some services even claim to use these results to create personalized feeding programs. But how does blood testing for vitamins and minerals work and is it reliable for all nutrients?

Blood tests can measure specific nutrient levels in the serum, the clear fluid in blood that contains important substances like electrolytes, proteins, vitamins, and minerals. These results can then be compared against reference ranges to determine if a horse’s blood is high or low in a particular nutrient.

Veterinary clinics commonly offer blood tests that measure levels of minerals and vitamin E in a horse’s bloodstream, along with other metrics to assess organ function, immune status, and hydration. Below are examples of nutrients commonly tested in the blood analysis of horses, along with their typical reference ranges in serum:

Table 1: Example reference ranges for vitamins and minerals in equine. ^[3]

Note that the trace mineral molybdenum is often included in blood mineral analysis. Molybdenum plays a role in DNA and RNA synthesis, but there is no established minimum intake requirement for horses. It is generally assumed that horses get enough of this mineral from forages, but high intake may interfere with copper balance in horses. ^[4]

Some laboratories require whole blood for mineral testing, so it’s important for your veterinarian to confirm what type of blood sample is appropriate for the test you are requesting. For example, selenium is routinely run off of a whole blood sample. Reference ranges for serum samples may be different than those for whole blood samples.

Reliability of Blood Testing

How reliable is blood analysis in determining whether your horse has too much or too little of a specific vitamin or mineral?

Even though these nutrients can be detected and quantified in a blood sample, this does not guarantee that blood testing accurately reflects the horse’s nutritional status (i.e. whether the nutrient levels are deficient, adequate, or toxic) within the body.

This is because blood doesn’t simply store nutrients as they are absorbed from the diet. Instead, the body tightly regulates the composition of blood through a process known as homeostasis.

This system ensures that essential nutrients and minerals in the blood remain within specific ranges, regardless of dietary intake.

As a result, blood levels for certain nutrients are not correlated to dietary intake. Furthermore, blood levels can be influenced by various factors such as timing of the test, recent meals, exercise, stress, and underlying health conditions.

The following is an overview of whether dietary intake of specific nutrients is reflected in blood vitamin & mineral analysis of horses.

Table 2. Relationship between blood vitamin & mineral levels and dietary intake in horses. ^[5]

Note that there is significant individual variability in how horses respond to dietary intake, resulting in varying blood levels of vitamins and minerals even when horses consume the same amounts.

This variability is evident even for nutrients where blood levels are typically more indicative of dietary intake. For instance, in a study of selenium-deficient horses supplemented with sodium selenite, only 12% reached adequate blood selenium levels, while 63% remained in the marginal range, and 25% still showed deficient levels after 30 days of supplementation. ^[6]

Due to this variability, it is essential to conduct repeated blood testing after any changes to supplementation practices in order to accurately evaluate each horse’s individual physiological response.

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Limitations of Blood Analysis

Although blood analysis is a convenient way to investigate vitamin and mineral levels in the horse’s blood at a given time, it does not necessarily reflect nutrient absorption from the diet or overall nutritional status.

Blood tests only show the levels of vitamins and minerals present in the blood at the time of sampling. These levels are influenced not only by the absorption of the nutrient from the gastrointestinal tract but also by the efficiency of nutrient utilization, redistribution, and removal from the bloodstream.

Homeostasis is the body’s mechanism for maintaining biochemical balance within specific set points, allowing it to function optimally despite environmental changes. As a result, the concentrations of many substances in blood, including vitamins and minerals, do not increase proportionally with higher dietary intake.

Intake of certain nutrients can cause temporary spikes in blood levels, but the process of homeostasis works quickly to restore blood chemistry to its set points. This is why factors like the timing of sample collection in relation to feeding or exercise can impact test results, as these activities can transiently alter nutrient levels before the body restores balance.

Additionally, while some nutrient deficiencies may eventually be detected through blood analysis, it can take a considerable amount of time for significant changes to appear in the horse’s blood. This delay occurs because the body often compensates for deficiencies for an extended period before levels drop enough to be reflected in blood tests.

In some cases, other biomarkers may provide a more accurate assessment of nutrient status than measuring the nutrient itself. For example, a study of horses in Chile, a selenium-deficient region, found that 78% of the horses had blood selenium levels within the reference range, despite chronic deficient intake. ^[6]

However, when researchers measured glutathione peroxidase, an antioxidant enzyme that depends on selenium, they found that 44% of the horses had sub-adequate enzyme levels in the blood, reflecting poor antioxidant protection despite normal selenium levels in the blood. ^[6]

Internal Factors Impacting Blood Test Results

Another limitation of blood analysis is that various factors can temporarily alter the horse’s physiological state before homeostasis restores balance. These internal factors, which may be unknown or unaccounted for, include: ^[7][8][9]

• Time of sampling relative to exercise: Some vitamins and minerals are removed from the blood during exercise. For example, electrolyte minerals (sodium, potassium and chloride) are lost through sweat and must be replenished in the diet.
• Time of sampling relative to food intake: Intakes of vitamins and minerals can cause a temporary spike in blood concentration before internal regulation (homeostasis) brings them back down.
• Underlying medical conditions: Some diseases can impair nutrient absorption. Chronic diseases increase the demand for certain nutrients or impair their metabolism. Some forms of acute illness lead to shifts in trace mineral concentrations in the blood.
• Hydration status: Dehydration or excess hydration impact the concentration of some blood constituents.
• Stress: Physical and mental stress affects hormone levels, which can affect nutrient metabolism. Stress also increases demand for antioxidants such as vitamin E and selenium.
• Age of the horse: Some mineral concentrations decrease as the horse ages.

In addition to the above, other factors can influence the level of vitamins and minerals detected in a blood sample and how those values are interpreted.

External Factors Impacting Blood Test Results

Sometimes blood test results for horses are inaccurate due to issues with sampling and processing methodology. It is estimated that 40 – 65% of errors in laboratory procedures happen before the analysis actually occurs. ^[10]

Examples of how sampling and analytical methods can influence results include: ^[5][11][12]

• Sampling procedure: The type of vacutainer tube that the blood is collected into can impact results. Some anticoagulants such as EDTA can interfere with the laboratory methods.
• Sample characteristics: If hemolysis (breakdown of red blood cells) occurs during sampling, levels of minerals within red blood cells such as iron and potassium will be elevated.
• Sample handling: Exposure to light after sample collection can influence levels of some vitamins, particularly vitamin C and folate.
• Sample storage: The time between sampling and analysis and how samples are stored (frozen vs. refrigeration) before analysis can also influence test results.

Another factor to consider is the reference ranges provided by the laboratory. Each laboratory provides reference ranges for different vitamins and minerals based on their own procedure. These are usually based on a specific type of blood sample (i.e., plasma collected with a certain anticoagulant, or serum), and may not be accurate for other types of samples.

Reference ranges also differ between labs, which can change the interpretation of the result.

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How to Reliably Assess Nutritional Status in Horses

To accurately assess a horse’s vitamin and mineral status, evaluating the diet is the most reliable approach. By determining the amount of each nutrient provided through the diet, potential deficiencies can be effectively predicted using the minimum dietary intake requirements outlined in the National Research Council’s (NRC) Nutrient Requirements of Horses, often eliminating the need for blood testing.

To evaluate whether your horse’s diet meets these minimum nutrient requirements, a qualified equine nutritionist can conduct a comprehensive diet assessment tailored to your horse’s individual needs.

A diet evaluation involves analyzing the nutrient composition of all dietary components, including forages, concentrates, supplements, and other feedstuffs that the horse consumes daily. This process not only quantifies the intake of individual vitamins and minerals but also considers the interactions and ratios between key minerals, such as calcium to phosphorus or copper to zinc, which can influence nutrient absorption and utilization.

For accurate results, the nutritionist compares the horse’s nutrient intake to the established requirements for horses at various life stages or activity levels.

To complete this analysis, the nutritionist follows a systematic approach:

1. Assess vitamin and mineral supply from forages

Your nutritionist will start the diet evaluation by assessing the nutrient contribution of forage in the horse’s diet. Forages, such as hay and pasture, form the foundation of most equine diets and are the primary source of essential amino acids, vitamins, minerals, and other nutrients.

• The vitamin and mineral content of hay is influenced by multiple factors, including plant species, the maturity stage at harvest, and the soil composition where it was grown. ^[13][14]
• A forage analysis is recommended to determine the exact levels of macro- and micro-minerals in the sample. Some laboratories also offer vitamin analysis for an additional cost.
• If a forage analysis is not available, the mineral composition can be estimated based on factors such as plant species and the geographic region in which the forage was produced.
• Vitamin degradation occurs rapidly post-harvest as grasses are cut and cured, particularly for fat-soluble vitamins like vitamin A and vitamin E, making it unlikely that hay alone will provide sufficient levels of these nutrients. ^[13][14]

2. Determine the vitamin and mineral supply of feeds and concentrates

The next step in a diet evaluation is to assess the nutritional contributions from any additional feeds, concentrates, or supplements that the horse is consuming. Commercial feeds and concentrates are typically formulated to provide specific amounts of vitamins and minerals, but the actual contribution depends on the type and quantity fed.

• To accurately calculate nutrient supply, it’s important to know the exact amount of each feed or supplement being provided. Nutritionists recommend measuring feed by weight (e.g., grams, pounds) rather than volume (e.g., scoops, quarts, litres) to ensure precise calculations.
• The guaranteed analysis on a feed label provides the concentrations of vitamins and minerals, indicating whether these values represent minimum, maximum, or exact levels. It is important to note that these values are typically reported within an allowable range and may not reflect the precise nutrient content of each bag. To obtain precise nutrient data, a laboratory analysis—similar to a forage analysis—would be required.
• Mad Barn’s FeedBank compiles nutritional information from thousands of feeds, concentrates and supplements, which can be used to determine total dietary supply based on the amount fed. Your nutritionist may use this database to assess your horse’s daily intake.

3. Provide updated vitamin and mineral requirements

In the next step, your nutritionist will calculate your horse’s specific vitamin and mineral requirements based on guidelines from the National Research Council (NRC). These requirements vary depending on factors such as the horse’s age, body weight, activity level, physiological status (e.g., growth, pregnancy, lactation), and overall health.

By using the NRC’s established nutrient requirements, the nutritionist can ensure that the horse’s dietary intake meets or exceeds the minimum levels needed for avoiding a nutritional deficiency.

The following factors influence your horse’s NRC nutritional requirements: ^[1]

• Age: Growing horses (less than 5 years of age) have proportionally higher requirements for most vitamins and minerals to support their rapid development. It’s also crucial to maintain the correct mineral ratios during this growth phase, such as the calcium-to-phosphorus ratio and the balance of manganese, zinc, and copper, to ensure proper bone and tissue development.
• Exercise level: Horses that are in heavy or very heavy work such as racehorses, endurance horses, or elite three-day eventers require higher intake of electrolytes to make up for losses in sweat. They also require higher intake of antioxidants such as vitamin E and selenium to support healthy muscle function.
• Breeding status: Mares that are in late gestation or lactating have higher requirements for some vitamins and minerals to support development of the foal and milk production. Breeding stallions also need higher levels of certain nutrients, such as selenium and vitamin E, to support robust sperm production.

4. Tailor recommendations to the individual horse

An equine nutritionist may also offer additional recommendations tailored to your horse’s specific needs. For instance, horses prone to tying up (recurrent exertional rhabdomyolysis) may require more electrolytes and/or increased levels of selenium and vitamin E to support muscle function and prevent oxidative damage. ^[15]

Horses that are prone to frequent illnesses may benefit from increased antioxidant support in the diet. Horses with hoof issues may need higher levels of biotin, zinc, copper and methionine in their diet. Horses that seem excitable or anxious may benefit from additional magnesium.

Whether your horse needs joint support, respiratory support, improved topline, or better athletic performance, an equine nutritionist can help develop feeding and management strategies tailored to your horse’s specific needs and your specific goals.

After considering all of these factors, your nutritionist will design a customized feeding program that optimizes your horse’s vitamin and mineral intake, balances energy requirements, and addresses any specific health or performance goals.

Equine nutritionists recommend forage-based diets, which maximize the nutrient intake from hay and pasture, providing fiber, vitamins, and minerals while supporting overall digestive health.

Feeding a balanced, forage-based diet promotes overall well-being in horses by offering:

• A diet that closely approximates the horse’s natural grazing behavior
• Support for gut health by keeping ingesta moving continuously through the digestive tract and supporting the hindgut microbiome
• Mental stimulation through regular feeding, which mimics the horse’s natural foraging habits

This management strategy helps ensure that the horse’s physical and mental health are maintained in alignment with its natural dietary needs. However, forage alone will not meet all of a horse’s vitamin and mineral requirements, making supplementation necessary to support your horse’s health.

Mad Barn’s Omneity is a balanced vitamin and mineral supplement that can be added to a forage-based diet to prevent common nutritional deficiencies. Omneity is formulated to balance the majority of hay-based diets, providing 100% organic trace minerals for optimal bioavailability, 20 mg of biotin per day to support hoof health, and a full profile of B-vitamins.

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Using Blood Analysis in Diet Evaluations

In some cases, it may be relevant to take into account your horse’s blood test results for certain vitamins and minerals when evaluating the diet.

If your horse is showing signs of a vitamin and mineral deficiency or toxicity, it is critical to discuss potential causes with your veterinarian and to pursue diagnostic tests. A blood vitamin & mineral analysis alone might not be sufficient to reach a diagnosis.

After a clinical diagnosis from your veterinarian, dietary adjustment of certain vitamins and minerals may be recommended. For example, horses with Hyperkalemic Periodic Paralysis (HYPP) require controlled intake of the electrolyte mineral potassium. ^[16]

Horses with neuromuscular disorders such as Equine Motor Neuron Disease (EMND) or White Muscle Disease require higher levels of vitamin E and/or selenium in the diet. ^[17]

An equine nutritionist can help you formulate an appropriate diet for your horse that supplies these nutrients in formats that are best utilized by the body. Your veterinarian and nutritionist can recommend timelines for when to re-test your horse’s blood levels to monitor the effectiveness of any dietary adjustments.

Frequently Asked Questions

Here are some frequently asked questions about blood testing for vitamin and mineral levels in horses:

Summary

Although blood analysis is convenient and non-invasive, its accuracy in reflecting a horse's vitamin and mineral levels depends on the specific nutrient being tested. For some nutrients, such as vitamin E or potassium, blood levels can provide useful insights, while for others, like calcium or copper, blood tests may not accurately reflect dietary intake or overall body status due to the body's tight regulation of these minerals.

• Prolonged deficiencies in vitamins and minerals can lead to serious health issues in horses, including suboptimal performance, weak hooves and a dull coat.
• Blood vitamin & mineral analysis has limitations for assessing nutritional status in horses, as blood may not accurately reflect nutrient intake. Factors such as sampling conditions, temporary changes in blood chemistry, and errors in processing can affect the results.
• A diet evaluation by a qualified equine nutritionist is the most effective and reliable method to ensure your horse is receiving optimal levels of all essential nutrients.