Does feed or diet affect blood tests? SOMETIMES!©

Dr Jennifer Stewart BVSc BSc PhD

Equine veterinarian, CEO Jenquine and consultant nutritionist

Equine Clinical Nutrition

Blood is the basic internal transport system for oxygen from the air and nutrients from the diet. Once in the body, each nutrient in the diet has multiple roles. The role of blood is to deliver nutrients to the tissues. Because blood levels of nutrients are kept within tight limits, a blood test doesn’t necessarily reflect the total amount of the nutrient in the whole body.

The body’s regulation of blood levels of nutrients may preclude finding simple relationships between blood test results and diet. Deficiencies of protein, sodium, potassium, magnesium, phosphorus, copper, manganese, selenium, zinc and vitamins A, B1 and E may cause a decrease in their blood concentration. However, imbalances may be present without changes in the blood levels and for many nutrients, an excess or a deficiency has to be quite prolonged before it is reflected by changes in the blood.

The major electrolytes (sodium, potassium, chloride, calcium and magnesium) are involved in so many processes – including processes that are vital for life, such as the regulation of breathing, heart rate, blood pressure, nerve transmission, muscle contraction and digestion. Because they are essential for life itself, blood levels are precisely controlled. Other mineral and trace elements are high in specific tissues where they are stored or have major roles. Calcium and phosphorus are highest in bone; potassium in the brain, muscle and red blood cells; magnesium in muscle and bone; copper in the liver and kidneys; manganese and zinc in the liver. Most nutrients are present in every tissue. The relationships between them and their various functions are complex and an excess or deficiency can affect several body systems and hence have several symptoms (Tables 1 and 2). So what are the odds of a blood test being useful for detecting imbalances in your horse’s diet? It depends on the nutrient!

Blood can be tested for many different substances and there are a number of reasons your horse may have a blood sample taken: illness, monitoring response to treatment and general health or fitness checks.

Table 1. Blood test results that can occur with nutrient imbalances.

BLOOD TEST RESULT DEFICIENCY EXCESS

Low albumin Energy, protein, amino acids, vitamin A

Vitamin A

Low protein Energy or protein -

Low BUN Protein

Vitamin K

High bilirubin Energy -

High triglycerides Energy -

High sodium Water Salt

Low sodium Salt

Vitamin K

High potassium Salt -

Low potassium Potassium -

Low chloride - Vitamin K

High calcium - Vitamin D

Low phosphorus Phosphorus

Low magnesium

Phosphorus, vitamin D

Magnesium -

High AST Selenium -

Low LDH Zinc -

Low PCV, Hb & RCC Copper, vitamin A

Selenium, vitamin A

Table 2. Symptoms that may occur with some nutrient imbalances

SYMPTOMS DUE TO A DEFICIENCY OF:

Reduced feed intake

Weight loss or poor performance

Poor coat

Fatigue

Weakness

Lameness

Stiffness

Hoof problems

Diarrhoea

Constipation

Colic

Dehydration

Excess licking

Dirt eating

Excess urine

Blood in urine

Muscle tremors

Convulsions

Reduced immunity

Anaemia

Excess tears

Water, phosphorus, electrolytes, zinc, vitamins A,D and B1

Energy, protein, calcium, phosphorus, electrolytes, selenium, zinc, vitamins A. D. E B

Protein, iodine, phosphorus, zinc, vitamins A & D, oil

Potassium, selenium, vitamin A, amino acids

Magnesium, vitamins B1 & E

Calcium, phosphorus

Selenium, vitamins E & B1, calcium

Protein, biotin, essential amino acids, zinc

Fibre, selenium

DUE TO AN EXCESS OF:

Fluorine, vitamins A & D

Fibre, zinc, vitamins A & D

Selenium, iodine, vitamin A

Selenium, vitamins A & K

Sodium, selenium, vitamin A

Selenium, fluorine

Selenium, fluorine, vitamin D

Selenium, excess grain in diet

Grain, selenium, sodium

Sodium -

Fibre, magnesium

Water, sodium

Grain, sodium, vitmain K

Sodium, potassuim, chloride -

Sodium, chloride, phosphorus, protein, zinc, copper

Magnesium, vitamin B1

Vitamin A

Sodium, vitamins D & K

Vitamin K

Magnesium, vitamin D

Selenium, vitamin A Iron

Vitamin A, amino acids, copper, zinc

Vitamin A, selenium, zinc

Vitamin A -

Pitted teeth - Fluorine

Is a blood test a good indicator of diet imbalances?

The answer here depends on the mineral (Table 3). Hydration, time of day, time of feeding, diet, exercise and general health can all affect blood levels. With some minerals and electrolytes, a horse can have a normal blood level but be severely deficient or be excessively over-supplemented. Selenium level is a good measure of extremes of intakes but measurement of the selenium-containing enzyme (glutathionine peroxidase) is more accurate; blood magnesium is a good measure of nutritional status; iron levels in the blood are not indicative of whole body status. Copper and zinc are also a little complicated because blood levels can be in the normal range even when the diet is excessive and very low or very high levels can occur for reasons other than diet. In addition, high zinc intake can damage the gut biome without affecting the blood test.

Table 3. Assessment of nutritional status from blood tests

Calcium Blood levels do not reflect intake or bone levels

Phosphorus Blood levels reflect diet excess or deficiency

Magnesium Blood levels reflect diet excess or deficiency

Sodium Blood levels reflect diet excess

Chloride Blood levels reflect diet excess

Potassium Blood levels reflect diet excess or deficiency

Copper Blood levels not indicative of diet levels

Zinc Blood levels not indicative of diet levels

Selenium Blood levels useful for excess intake

Manganese Insufficient knowledge

Iron Blood levels not sensitive to intake

Iodine Blood levels not indicative of diet levels

Vitamin A

Blood levels low if diet deficient, excess doesn’t show until liver storage capacity exceeded

Vitamin D Blood calcidiol level reflects intake

Vitamin E Blood levels reflect diet excess or deficiency

Vitamin B1 Blood reflects diet intake but not tissue levels

Anaemia

One widespread belief is that anaemia is common in horses. A diagnosis of anaemia is based on blood tests and low red cell parameters are often attributed to iron deficiency. But although a dietary iron-deficiency anaemia occurs in humans, it has not been reported in horses.

Anaemia is not diagnostic of an iron deficiency because there are many other, more likely causes of anaemia in horses.

Nutrient deficiencies in horses that can result in anaemia include vitamins E, B6, B12, folic acid, copper, niacin. Diet is just one factor in the multistep system of red cell production, which is also influenced by breed, sex, temperament, training and diseases.

Biochemical tests

Hundreds of tests can be performed on a blood sample, ranging from a simple count of red blood cells to a biochemical analysis of hormone and enzyme levels. These substances are used to assess specific organs, diseases, clinical conditions (such as insulin-resistance and PPID) or metabolic pathways and processes - but because they can come from more than one tissue, results must be interpreted. Muscle and liver enzymes are synthesized from dietary nutrients. Because of this, diet indirectly affects muscle function and elevated SGOT can be due to imbalances in B vitamin, calcium, essential amino acids, magnesium, potassium, vitamin E and selenium deficiencies. There are many varied reasons for elevated GGT including insufficient dietary selenium, zinc and vitamins E and C all of which result in oxidant damage and reduced immune function – which can cause changes in blood electrolyte, protein and white blood cell levels. Because of the complex relationships between nutrients, hormones and blood levels, stress can lead to high sodium and low potassium – both may appear in the normal range on a blood test, but the pattern of upper range sodium combined with lower range potassium is characteristic of stress, cortisol and ACTH changes – not dietary intake.

Selenium deficiency can be involved in thyroid problems, because selenium is required to convert thyroid hormones into active forms.

Can a blood test be useful in diagnosing food allergies?

Some dietary nutrients can cause an allergic reaction – usually seen as lumps on the skin or gut problems.

Allergies to oats and bran have been reported to cause widespread hives on the head, neck, shoulder, ribs and flanks, and to corn, spectacular hives within 15 minutes.

Blood tests that measure the immune response to a feedstuff (IgE) are available. It’s important to be aware that a horse cannot be allergic to a food to which they have never been exposed and that they need to actually be on the feedstuff when they show symptoms. If neither of these apply when you have the test done, the results are likely to be unreliable. A recent study in which 17 healthy ponies were tested 3 times found 10 tested positive to one or more food allergies in the first test, only three re-tested positive on the second test and only one tested positive twice for the same allergy.

In conclusion

Dietary mineral and vitamin imbalances may occur individually or in various combinations.

Symptoms of diet imbalances can affect several body systems and may lead to poor growth and development; muscle and bone conditions; abortion and retained placenta; foetal abnormalities; reduced immunity and increased susceptibility to disease; infertility; poor skin, hair and hoof quality.

Evaluation of the diet is the easiest and best way to diagnose and correct many nutrient imbalances. Reliance on blood tests can be misleading and subject to errors. Using the results of a single blood test in the absence of a full history and comprehensive veterinary clinical examination is unlikely to shed much light on whether the diet or feed is contributing to any clinical or performance problems.

The best protection against deficiencies is a diet analysis that goes beyond minimum recommended intakes and standard feeding guides. Mathematical calculations and generic spreadsheets don’t reflect the complexity of requirements for a wide range of veterinary clinical conditions.

All content provided in this article is for general use and information only and does not constitute advice or a veterinary opinion. It is not intended as specific medical advice or opinion and should not be relied on in place of consultation with your equine veterinarian.

©Dr Jennifer Stewart May 2024

ABOUT THE AUTHOR

Dr Jennifer Stewart

CEO BVSc BSc PhD Dip BEP Equine Veterinarian and Consultant Nutritionist

Dr Jen Stewart has been an equine veterinarian for more than 40 years and an equine nutritionist for more than 10 years. Jen has been developing premium formulas for studs, trainers and feed companies in Australia and around the world and regularly consults to leading international studs and trainers in various countries.

Jen has spent a fair bit of time researching and being involved in nutritional management of developmental orthopaedic diseases, colic, tying-up, laminitis, performance problems, post-surgery and other conditions. And is currently the only practicing equine veterinarian and clinical nutritionist in Australia. Jen’s promise is to continue to BRING SCIENCE TO YOUR FEED BIN www.jenquine.com