Vitamins

Vitamin D for athlete health and performance

It is quite likely that you may have low Vitamin D levels.  Recent estimates indicate that over 75% of the general population may be Vitamin D deficient.  If you are an athlete, you may be at even higher risk of having low levels, and this is a problem because Vitamin D is important for health and potentially performance.  In recent weeks I have found myself talking to many athletes about Vitamin D.  The end of winter is approaching in Australia, a time of year where Vitamin D levels can be on the downward slide.  I have also read a number of journal articles of late that highlight the important role of Vitamin D for athletes.

I am prone to low Vitamin D levels.  I am not a great one for regularity in taking supplements, so I am probably Vitamin D deficient right now if I am honest.  Particularly as we are continuing to endure a pretty cold winter here in Melbourne, and most of us in the southern states will find that our Vitamin D levels decline by the end of winter when we haven’t seen much sun for a while.  So what is all the carry-on about Vitamin D?  Why do we need it, who is at risk of deficiency and how can you improve your Vitamin D levels?

Why are so many people Vitamin D deficient?

Vitamin D is a pretty clever little vitamin and plays an important role in many of our body systems.  The big problem with Vitamin D is that we generally don’t know that our levels are low until something major happens eg. bone issues.  Unlike iron, where our body will often let us know via various symptoms that our levels are on the decline, Vitamin D isn’t quite as helpful and we can go for a pretty long time without being alerted to low levels.  The only reason I found out that my levels needed a boost was through a routine blood test when I was pregnant, so goodness knows how long my levels had been low for.

Unfortunately sometimes when you address one issue it can create another.  Sunscreen is essential for protecting our skin from the sun’s rays.  If you block the sun, you help to reduce the risk of skin cancer.  But you also block the sun’s amazing ultraviolet (UV) radiation which is required for the production of Vitamin D in the skin.  So all of our slip, slop, slapping, which is absolutely important to avoid burning our skin, doesn’t do much for our Vitamin D. 

Who is at risk?

Lack of sunlight is the number one risk factor for low Vitamin D.  So if you spend a lot of your daylight hours inside, like a number of athletes I work with who train predominantly indoors, your levels may be low.  Athletes may also have increased physiological demands for Vitamin D, compared to the general population.

Other individuals at higher risk include:

  • indoor lifestyle eg. work, study.

  • if you cover your skin for religious reasons.

  • if you have naturally very dark skin.

  • if you avoid the sun for cosmetic reasons or skin protection.

  • specific medical conditions.

There is some debate over the cut-off values for Vitamin D levels in the blood, and whether the set levels are in fact too high, meaning that more people are being diagnosed as being low in Vitamin D.  It is important to speak with your GP or medical professional to determine the best way to address your Vitamin D status and needs.

Why is Vitamin D important?

Bones - Vitamin D controls calcium levels in the blood and is required for the absorption of calcium from the gut, which in turn is important for bones.  Low Vitamin D can increase the risk of musculoskeletal problems, including bone conditions such as osteopenia and osteoporosis.  For athletes, an increase in bone turnover with low Vitamin D can increase the risk of bone injuries such as stress fractures.  Sufficient Vitamin D may help to prevent this.

Immune System - Vitamin D is thought to have a role in maintaining a healthy immune system, and some studies suggest that Vitamin D deficiency may increase the risk of viral respiratory tract infections. Winter is often when Vitamin D levels decrease, so if you are prone to getting sick in the cooler months make sure your Vitamin D levels are kept up throughout.

Mental health – There seems to be a link between Vitamin D and mental health, including moods and even depression.

Muscle strength – Vitamin D may have a particularly important role for improving muscle strength in athletes.  There is a potential for increased size and number of type II fast twitch muscle fibres and a study in athletes showed a positive impact on muscle function with Vitamin D supplementation if levels are low.

Injury prevention - Low Vitamin D may increase the risk for inflammatory-related injuries.

Performance - Few studies have looked at Vitamin D and its direct effect on performance in young adults, however multiple performance studies in older adults have related low vitamin D levels to decreased reaction time and poor balance.  There may also be a potential impact on VO2 max.

Strategies to increase Vitamin D levels

  • Spend some time out in the sun without sunscreen on  

    • Find a balance between sun exposure for Vitamin D and protecting your skin against skin cancer.  The amount of time required for exposure will vary depending on where you live.  Check the SunSmart website for more details on exposure times in Australia.    

  • It's difficult to obtain enough Vitamin D just from foods.  Only 5-10% of our Vitamin D may come from food.  Foods that are rich in Vitamin D include– salmon, dark-flesh fish, egg yolks, fortified foods (like milk), UV mushrooms.

  • Vitamin D supplementation may be required for certain individuals.  Speak with your health professional about appropriate dosages if you have been found to have low Vitamin D levels

Summary:  For athletes, there is limited evidence to support vitamin D as a direct performance enhancer, however optimal Vitamin D is important for health, immune function and reduced risk of bone injuries such as stress fractures, and muscle injury. Although Vitamin D is not shown to have a direct performance effect, the indirect impact could make a significant difference to performance and health/injury outcomes.  Further research is required to determine the magnitude of effect of vitamin D on performance, in particular the areas of strength, power, reaction time and balance.

This post presents information of a general nature only.  For individual advice about nutrition and supplementation you should consult with an Accredited Practicing Dietitian or appropriate health professional.
References: 
  • Ogan,D. & Pritchett, K.  Vitamin D and the Athlete: Risks, Recommendations, and Benefits. Nutrients 2013, 5:1856-1868

  • Von Hurst, P.R. & Beck, K.L. Vitamin D and skeletal muscle function in athletes. Curr Opin Clin Nutr Metab Care 2014, Nov;17(6):539-45

  • SDA Fact Sheet – Vitamin D

  • AIS Fact Sheet – Vitamin D

Green leaf goodness: kale vs spinach vs rocket, and the winner is.....

Do you love kale.  Or do you just eat it because it is supposed to be good for you?  I like kale, but I do question the hype.  So I decided to look a little closer and do some nutrient comparisons.  Specifically, I was keen to look at the differences between kale and other common green leafy vegetables that are used in similar ways to kale - spinach and rocket (arugula) and your everyday iceberg lettuce.   

Kale is not a new vegetable.  Kale has been around for centuries, with its popularity as a 'superfood' having only skyrocketed in recent times.  Does kale deserve this reputation as the king of all things green?  Kale is from the cabbage family and related to the highly nutritious cruciferous group of vegetables, which includes broccoli, cauliflower and brussel sprouts.  There are many different types of kale, with either curly or flat leaves, which can be eaten in a variety of ways, from salads to soups and smoothies.

To start with, let's just clarify that ALL dark green vegetables are packed with nutrients. Any small differences in nutrient content can be balanced out by eating a variety of greens for a range of different nutrients and health benefits.  This is easy if you like vegetables!  But if your palate hasn't quite extended to enjoy green vegies and you know you probably don't eat enough, then small differences could have more of an impact, and it may be well worth focusing on those vegetables with a higher nutrient density.

So here are the statistics for kale vs. spinach vs. rocket vs. iceberg lettuce.  Of course there are many other green vegetables, but these represent some that are often used interchangeably with kale.  These figures are from the US and represent a tested sample of each vegetable.  Therefore they are approximate values only, as nutrient content can vary from place to place, often due to growing conditions.  Although approximates, these figures still provide a useful comparison for measurable nutrients.  

What do you think?  First of all, I know some people may have looked straight at thecarbohydrate values and panicked when they saw that kale has over twice the amount of carbs compared to the other greens.  Before you throw your kale out, remember that 8.8g is a tiny amount of carbohydrate and these values are per 100g, which is a lot more than a standard serve.  Leafy greens are pretty light, so a serve of kale with other mixed vegetables is more likely to be around 20g, providing less than 2g of carbohydrate, which is practically nothing.   

My interest is not so much in the kilojoules or carbs, but the vitamin and mineral content and nutrient density.  Looking per 100g (which indicates the % of the nutrient in a food), the nutrient contents for kale look pretty impressive and you can see where the claims arise for 'high calcium' and 'rich in Vitamin C'.  But again, because leaves are so light, you need to divide the nutrient by 4 or 5 to get a better idea of actual nutrient content per serve.  

The firs thing that really stands out to me when I look at this table is the vast difference in nutrient content between the darker green leafy vegetables and iceberg lettuce.  Iceberg just doesn't compare really, right across the board.  This provides a very clear message that for nutrition, stack your salads with darker leaves.  Of course iceberg lettuce provides wonderful texture and can be a great base for other flavours within a salad, but for nutrition it really lags behind.

If we then look back to compare kale with spinach and rocket lettuce, for most nutrients there are only small differences.  When you compare the differences to the huge gap to the iceberg lettuce, then you realize that kale, spinach and rocket are jostling for first line position, with iceberg a distant last, rather than being spaced out well across the field.   Kale and rocket have more calcium than spinach but spinach has more iron (although not terribly well absorbed).  Most differences are minimal and although I could analyse every nutrient, when we consider the overall impact on health there is really no point.  There are, however, a handful of nutrients for which kale is a standout.

Kale is a far greater source of Vitamin C and Vitamin K compared to spinach and rocket.  Kale contains 120mg/100g Vitamin C, so per serve may contain around 25-30mg Vitamin C, making it a useful source when eaten raw.  Vitamin C can be damaged with heat/cooking so cooked kale may not provide the same benefits as raw. 

Kale is high in Vitamin K, which is particularly important for blood clotting, but not a nutrient that is at a high risk of being low or deficient for most people.  So strong is the blood clotting effect, that people need to monitor their intake of Vitamin K if they are taking blood-thinning medication such as warfarin.

Kale and spinach are both able to supply plenty of Vitamin A, an important nutrient for the health of our skin and eyes. The beta-carotene in kale and spinach can also act as an antioxidant.

All other differences in nutrients are either minor, or insignificant or not all that important for overall health.  One thing that is missing from the table above is baby spinach leaves.  I have found it difficult to find nutrient breakdown info for baby spinach to compare to regular spinach, but have read that the baby leaves may be higher in some nutrients and lower in others.  Current data on baby spinach would be welcomed. I have a sneaking suspicion that the baby spinach, such a popular option in salads, may in fact not be quite as nutrient dense as regular spinach, but I would love to compare the figures to be sure.

It is important to remember that the nutrients presented in the table above are those that we can measure readily in food, but this analyss neglects those other phytonutrients which are not routinely tested for.

Nitrate is the other one that is not listed above, but is present in dark green leafy vegetables.  Nitrate may have an important role for athletes by reducing the energy cost of exercise - nitrate is taken in concentrated form via beetroot juice for performance effects, but there is potential for green vegetables to contribute to nitrate intake also.  For more on nitrate visit Sports Dietitians Australia or read this article by Alan McCubbin Beetroot Juice: Good Science or Great Marketing Hype.  

Kale is reported to contain important antioxidants, including flavonoids and polyphenols.  Kale contains the flavonoids quercetin and kaempferol.  Quercetin has received some attention for a potential influence on endurance exercise capacity, however published research findings have been mixed (http://www.ncbi.nlm.nih.gov/pubmed/21606866).  Antioxidants provide a range of health-related benefits and dark green vegetables, including kale, contain plenty.

So it seems that kale really is good for us, but is the nutrition value worth making the effort for?  I recall the first time I tried kale and it was definitely a case of take it or leave it!  I continued to revert back to the trusty spinach leaves as my tried and true salad base.  But lately I have been experimenting a bit, and recently ordered a kale salad with prawns and haloumi at a local restaurant.  When it came out I was actually very concerned about how I was going to manage to eat the amazingly large bowl full of green curly raw leaves, that to be honest looked terribly unappetizing.  But here is the thing with kale, and in fact most green vegetables.  How you prepare it and what you add to it can make or break your eating experience.  On this occasion, the chilli and lemon on the prawns, with the salty haloumi, pinenuts and a yoghurt based spicy dressing made the seemingly throat scratching unchewable bowl full of kale totally edible. In fact it was delicious.

The benefit of adding some healthy fats, like olive oil, avocado and nuts, is that the fat helps with the absorption of fat-soluble vitamins, such as Vitamin A, from the leaves.  So dressing your kale with some oils for flavour has the added benefit of boosting the nutrient availability.

The other great thing about kale is that, unlike many other so-called superfoods, kale is not ridiculously expensive when you consider other similar alternatives.

So what's the verdict, does kale win out over all other green vegetables as the senior member of the superfood brigade?  Kale is just one of the wonderful foods that is no doubt super, but certainly not that much more super than spinach, and for some nutrients less super.  But well worth including as one of your green vegetable options within different coloured vegetables every week. 

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