Vitamin D deficiency is the most common nutrient deficiency in the world and is particularly common in people with kidney disease. Add to that the fact that vitamin D deficiency is linked to progression of kidney disease and increased risk of mortality and I think we have a few pretty good reasons to explore the role vitamin D and the importance of maintaining optimal vitamin D levels.
Interestingly enough, vitamin D is completely different than any other vitamins. In fact, it’s not really a vitamin at all! Vitamin D is actually a prohormone- a nutrient that converts into a hormone within the body.
Being a fat-soluble vitamin, vitamin D is stored in the body in fat cells which can be accessed as needed.
Where do you get vitamin D from?
The short answer is from food, the sun, or supplements.
Few foods contain sufficient vitamin D so most of it is made in our body after exposure to sunlight.
When our skin is exposed to sunlight, or rather, the ultraviolet B (UV-B) radiation that the sun emits, it makes vitamin D from cholesterol. The amount of vitamin D your skin makes depends on the season, the time of day, where you live, your age and the melanin content of your skin- the darker your skin the more sun exposure you need to make sufficient vitamin D.
Like all other vitamins we do get some vitamin D from food, but they don’t contain large enough amounts to be our primary source.
There are two main dietary forms of vitamin D:
- Vitamin D3 (cholecalciferol)- found in some animal foods, like fatty fish and egg yolks.
- Vitamin D2 (ergocalciferol)- found in some plants, mushrooms and yeasts.
How does the body process vitamin D?
Image from: Vuolo, L., Di Somma, C., Faggiano, A. & Colao, A. (2012). Vitamin D and cancer. Front Endocrinol, 3(58), 1-13.
Vitamin D that comes from our skin, our diet or supplements needs to be switched on or converted into the active form that the body can actually use which is a process that involves both the liver and the kidneys.
The first step occurs in the liver where cholecalciferol (vitamin D3) or ergocalciferol (vitamin D2) is converted to 25-hydroxyvitamin D2 also known as calcidiol.
The second step happens in the kidneys where 25-hydroxyvitamin D2 (calcidiol) is converted into 1,25-dihydroxyvitamin D3 also known as calcitriol which is the physiologically active form of vitamin D which just means that it’s the form that vitamin D has to be in to exert its effects on the body.
This final process largely takes place in the kidneys however some other cells in the body do have the ability to activate vitamin D in smaller amounts.
The many roles of vitamin D
Meehan, M. & Penckofer, S. (2014). The role of vitamin D in the aging adult. Journal of Aging and Gerontology, 2(2), 60-71). doi: 10.12974/2309-6128.2014.02.02.1
Traditionally, vitamin D was associated with bone health but after researchers found vitamin D receptors in the cells of most organs of the body it prompted them to dig a little deeper which has resulted in a greater understanding of the widespread effects vitamin D has throughout the body, including:
- Promoting healthy bones and teeth
- Supporting immune, brain and nervous system health
- Regulating insulin levels and supporting diabetes management
- Supporting lung function and cardiovascular health
- Influencing the expression of genes involved in cancer development
Vitamin D deficiency
Vitamin D deficiency has been linked to various health problems including cognitive decline, depression, osteoporosis, autoimmune diseases, cardiovascular disease, high blood pressure, diabetes and cancer.
Globally, vitamin D deficiency is a major public health problem affecting all ages and ethnic groups. It has even overtaken iron deficiency as the most common deficiency in the world!
Among those most vulnerable to vitamin D deficiency are the elderly, people living in higher altitudes, people with darker skin, obese individuals and people with CKD.
The prevalence of vitamin D deficiency increases with progression of CKD and approaches 90% in people with stage 5 CKD.
Why is that I hear you asking.
Well, it’s likely to do with a number of factors including reduced exposure to sunlight, proteinuria (loss of protein in urine), reduced skin synthesis of vitamin D, decreased intake of vitamin D rich foods and decreased physical activity.
The other main factor linked to lower vitamin D levels in people with kidney disease has to do with a reduction in kidney function. Because the final stage of converting vitamin D into the active form (calcitriol) happens in the kidneys, as kidney function declines there is also a progressive decline in the activity of 1-alpha hydroxylase, the enzyme needed to convert vitamin D into its active form.
Low levels of vitamin D in people with kidney disease have been associated with:
- Secondary hyperparathyroidism
- Low bone mineral density
- Low calcium levels
- Muscle weakness and risk of falls
- Metabolic syndrome, insulin resistance and obesity
- Enlargement of the left ventricle in the heart
- Hardening of blood vessels
- Cognitive impairment
- Higher levels of protein in urine
- Progression of kidney disease
- Higher mortality risk
There are a couple of actions of vitamin D that I want to make particular mention of, its role in immune function and its role in maintaining the balance of calcium and phosphorus within the body.
Vitamin D and the immune system
Vitamin D is essential to the health and functioning of your immune system and can reduce the risk of infections. Vitamin D enhances the pathogen fighting effects of monocytes and macrophages- white blood cells that are important parts of your immune defence- and decreases inflammation in the body which helps promote immune responses.
A deficiency in vitamin D is associated with immune dysfunction in people with CKD, increased susceptibility to infection and increased prevalence of cancer.
Maintaining balance of calcium and phosphorus
Vitamin D helps to balance calcium and phosphorus in the body by controlling absorption of these minerals from the food you eat as well as regulating parathyroid hormone (PTH).
PTH is produced by the parathyroid glands and its main role is to help regulate calcium and phosphate levels. It does this through its actions on the kidneys, bones and intestines.
What happens to PTH in kidney disease?
People with more advanced kidney disease may have elevated PTH. This excess of PTH tells the body to pull calcium out of the bones and put it in the bloodstream which can result in bone pain and weak bones that fracture easily as well as calcium build up in tissues and organs such as the heart and blood vessels.
Vitamin D deficiency is one of the causes of secondary hyperparathyroidism.
If I haven’t already impressed on you the importance of maintaining optimal vitamin D levels here are a few more of the reported benefits of vitamin D supplementation for people with kidney disease:
- Serum parathyroid hormone levels decrease
- Reduced proteinuria
- Improvement in endothelial cardiovascular markers
- Reduced inflammatory markers and reduced oxidative stress
- Better control of blood sugar levels and diabetes
- Lower blood pressure
- Increased red blood cell production and iron levels
- Decreased need for erythropoiesis stimulating agents
- Slower progression of kidney disease and improved survival
So, do you need to supplement with vitamin D?
Remember all those reasons why people with kidney disease are more likely to be deficient in vitamin D? Well, in order to maintain optimal vitamin D levels in people with kidney disease, supplementation is frequently needed.
Vitamin D supplements can be either vitamin D3 (cholecalciferol) or vitamin D2 (ergocalciferol). If possible, we recommend supplementing with vitamin D3 (cholecalciferol) because that’s the same form we make in our body from sunlight exposure and has been shown to be more effective at raising vitamin D levels in the body.
Being a fat-soluble vitamin, vitamin D is better absorbed when taken with a meal containing some fat.
How do you know if you’ve got low vitamin D levels?
Great question! Depending on how low your levels are, you might notice any of the following signs: frequent infections, fatigue, bone and back pain, low mood, impaired wound healing, hair loss and muscle pain.
The thing is, many of these signs could be related to other conditions so the best way to figure out if you are deficient in vitamin D is to have a blood test checking your vitamin D levels.
Optimal vitamin D levels
Defining optimal vitamin D levels isn’t easy as there are no universally agreed on guidelines. Most clinical studies support the view that serum vitamin D levels of less than 20ng/mL (50 nmol/L) indicate vitamin D deficiency. Serum vitamin D levels below 30 ng/mL (75nmol/L) indicate insufficiency with levels between 30 and 60 ng/mL (75 and 150 nmol/L) representing normal values.
Can you have too much vitamin D?
The short answer is yes but let me explain.
Our body has certain checks and balances so it won’t allow vitamin D from the sun to elevate our vitamin D levels too high, but vitamin D levels can end up too high with excessive supplementation.
Remember, vitamin D is a fat-soluble vitamin, so it’s stored in the body unlike water soluble vitamins that are excreted if we take too much.
High vitamin D levels can increase calcium levels in the blood so side-effects of too much vitamin D are mostly attributed to high calcium levels which manifest as
loss of appetite, nausea, vomiting, muscle pain, tiredness, dry mouth and persistent headache. Over extended periods of time, this state of vitamin D toxicity can result in calcification of soft tissues in the body including kidneys, blood vessels, heart and lungs.
Everyone with kidney disease should get their vitamin D levels checked to determine if supplementation is needed AND to guide what dose of vitamin D they need to take.
Ongoing monitoring is needed to assess the effectiveness of vitamin D supplementation and to assist with continuing to titrate the dose of vitamin D to avoid over or under supplementation. As well as monitoring vitamin D levels, calcium, phosphate and parathyroid hormone levels should also be monitored because vitamin D can alter these levels. If any of these levels are too high or too low, blood tests should be monitored more frequently.
The therapeutic dose of vitamin D typically sits between 400IU and 5000IU per day. Quite a range, I know! This will depend on your vitamin D levels and will likely need to be altered over time to maintain optimal vitamin D levels (not too low and not too high).
Before starting to supplement with vitamin D make sure you get your level checked first and discuss the appropriate dose with your health practitioner.
Cautions and Contraindications
Vitamin D should not be taken by people with hypercalcaemia (high calcium levels) because it increases the absorption of calcium and may further elevate levels.
Vitamin D may potentially interact with some medications including atorvastatin, calcium channel blockers, thiazide diuretics and allopurinol so supplementation should only occur alongside these medications under medical supervision.
There you have it folks. I hope I’ve been able to impress on you the importance of maintaining optimal vitamin D levels. If you haven’t had your vitamin D levels checked recently then I encourage you to speak with your doctor about getting this done, then supplement if necessary.
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Until next time, take care.