Vitamins: Understanding the basics

The first step to understanding your vitamin consumption is to be able to identify different types of vitamins. Vitamins can be split into 2 main groups according to how the are absorbed: fat soluble (vitamins A, D, E and K) or water soluble (vitamins B and C).

SolbuleVitaminsThere are some key differences between water soluble and fat soluble vitamins and I will outline them below:


Fat soluble vitamins are usually absorbed in fat globules that travel through the lymphatic system of the small intestines, and into the body where they are stored in fat tissues and organs such as the liver, and absorbed as needed.
Toxicity & Deficiency:
Because they are stored they are not needed daily and toxicity can develop if a person takes too much of a fat soluble vitamin. Eating a normal, well-balanced diet will not lead to toxicity in otherwise healthy individuals. However, taking vitamin supplements that contain megadoses of vitamins A, D, E and K could lead to toxicity.
People can be also become deficient in fat soluble vitamins if their fat intake is too low or if their fat absorption is compromised, for example by certain drugs or diseases that interfere with the absorption of fat from the intestine.
Fat-soluble vitamins are more stable than water soluble ones. Cooking will not deplete a fat-soluble vitamin. Fat-soluble vitamins are light sensitive and foods or supplements containing them should be stored in a dark environment.
Recommended Intake:
Since the body stores these vitamins it is not imperative that the amount be obtained every day, rather it is acceptable to do a weekly average on the fat-soluble vitamins to ensure they align with the RDAs


Water soluble vitamins dissolve in water and are not stored by the body. Since they are eliminated in urine, we require a continuous daily supply in our diet.
Toxicity & Deficiency:
Some people falsely believe it’s safe to take megadoses of water-soluble vitamins, assuming their bodies will flush out the excess, but taking too much of them can still cause problems. For example, too much vitamin C may cause kidney stones.
Deficiency can occur quite easily if water soluble vitamins are not taken on a regular basis.
Water-soluble vitamins are sensitive and can easily be lost when over-cooked, over-washed, or when exposed to bright light for too long. It is best to keep water soluble vitamin containing foods and supplements in a cool dry place and minimally process them.
Recommended Intake:
Since the body doesn’t store these vitamins, getting the recommended amount daily is important to body function.


There is a difference between vitamins that are naturally water soluble and a “water solubilized” form of a vitamin: for example, vitamin E – which is naturally a fat soluble vitamin – can be made “water dispersible” by the addition of certain compounds during a specific manufacturing process.
This type of vitamin can be useful for those with certain difficulties absorbing fats through the intestinal wall (for example bile or lipase production issues), but it’s important to understand that what is affected is the transportation of the vitamin through the digestive system, not the eventual storage within fat tissues and ongoing absorption into the body, which still happens in the same way.

Recommended Intake:
It is also often difficult to define when precisely someone is seriously short of a particular nutrient, since there may be no signs or only vague and generalised indicators, such as tiredness or poor skin condition. So instead, nutritionists use values much higher than the physiological requirement, and these are called Reference Intakes (RIs). In 2014, RIs replaced Guideline Daily Amounts (GDAs). One of the reasons for replacing the GDAs was that there were several different versions of the guidelines for different groups in the population – men, women and children. Now, there is only one set of RIs and they are based on the maximum amounts needed by an average woman.

Nutrients of concern:

Most people should fulfill their RIs and get all the nutrients they need by eating a varied and balanced diet, however there are some factors that may cause the requirement of extra supplementation of vitamins above what is consumed in your diet, the Institute of Medicine calls these “nutrients of concern”.
-Vitamin D is the only vitamin recommended for supplementation within the general population of the UK at this time. This is due to the low prevalence of it in natural food sources, and high rate of deficiency caused by lack of direct sunlight.
Certain populations also have specific “nutrients of concern”:

    • Pregnant and lactating women need higher amounts of vitamins because of the increased nutritional needs of their bodies. Vitamin C and the B vitamins are especially important because these will deplete quickly. One of the B vitamins: Folate, is also considered very important in fetal development. A pregnant or lactating body puts the baby’s needs first so women who don’t consume enough vitamins will not fulfill their own body’s nutritional needs, and supplements can be helpful in maintaining adequate vitamin levels.
    • Those consuming a vegan diet require extra B12 as it is only available in trace amounts from very few vegetables so consumption of fortified foods or supplements is highly recommended
    • Some health conditions may cause absorption problems and would need to be addressed properly. For instance, gastric bypass surgery greatly reduces the absorption of vitamins and minerals and supplementation is recommended.

Here, we are not talking about vitamins absorbed through a healthy balanced diet but rather the supplements that people take, either for specific ailments or ‘nutrients of concern’, or the general self-medicating that occurs regularly. I would like to clarify that supplement does not just mean synthetic tablets, there are natural sources for vitamin supplementation in the forms of plant extracts, herbs, etc.

If vitamin supplements are taken on an empty stomach or in conjunction with certain other nutrients, they may not be absorbed. Usually, when getting vitamins from the food we eat, this is not an issue. However in taking supplements the following provide some general guidelines:

      • Generally the fat-soluble vitamin supplements (A, D, E, and K) are likely to be better absorbed if taken with a meal that contains fats. Fat-soluble vitamins are more bioavailable when taken with fat because it helps dissolve them quicker so the body can access them. For example, one study found that taking vitamin D with a fatty meal increased blood levels of vitamin D by about 50%.
      • Water-soluble vitamin supplements (B group, and C) should be taken with at least half a cup (8oz) of water. Water-soluble vitamins need water to dissolve and be available for use. If a vitamin supplement will not dissolve in water, then it may not be the best choice for obtaining water-soluble vitamins

Vitamins should really come from food sources as much as possible. Unlike from foods, the mega-doses found in supplement tablets can be toxic or can affect the absorption of other nutrients, so should be considered carefully before use. If you do go for a synthetically produced tablet, choose those that use whole food sources like herbs, fruits, and vegetables. Avoid supplements that use words ending in -acid, -ide, and sometimes -ate or that use the “dl” before the name. Synthetic vitamins are isolated or simulated nutrients that do not take into account all the countless phytonutrients that come along with them. Nature is not a select few things isolated from the rest. We are only beginning to understand how many of the lesser known compounds in plants react with one another as we eat them, but we do know humanity has been eating whole foods for a very long time. We have evolved to recognize the whole, not just individual chemicals that have been created to approximate an essential vitamin.

There is only one vitamin-vitamin interaction recognised at this time; if taken in large doses, excess vitamin A can inhibit your body’s absorption of vitamin K, a vitamin essential for normal blood clotting.

There are also a number of vitamin-mineral and mineral-mineral interactions that can cause problems for those who take high-dose supplements. Be aware that vitamins and minerals can also affect the absorption and effectiveness of other medications.


You’ve probably come across these acronyms, here’s what they mean:

  • Reference intake (RI): Not intended as targets, as energy and nutrient requirements are different for all people. But they give a useful indication of how much energy the average person needs and how a particular nutrient fits into your daily diet. Unless the label says otherwise, RI values are based on an average-sized woman doing an average amount of physical activity. This is to reduce the risk of people with lower energy requirements eating too much, as well as to provide clear and consistent information on labels.
  • Guideline Daily Amount (GDA): Guidelines for daily levels of intake sufficient to meet the nutrient requirements of nearly all healthy healthy adults and children. Now replaced with RI.
  • Recommended Dietary Allowance (RDA): see GDA
  • Adequate Intake (AI): established when evidence is insufficient to develop an RDA and is set at a level assumed to ensure nutritional adequacy.
  • Estimated Average Requirement (EAR): average daily level of intake estimated to meet the requirements of 50% of healthy individuals. It is usually used to assess the adequacy of nutrient intakes in population groups but not individuals.
  • Tolerable Upper Intake Level (UL): maximum daily intake unlikely to cause adverse health effects.
  • International Units (IU): a unit commonly used in the measurement of medications, vaccines, and vitamins. The volume or mass that makes up one IU is dependent on the concentration or potency of the substance and varies between substances meaning converting between international units and micrograms or milligrams is not a simple calculation. The exact measure of one IU of a substance is in fact established by international agreement for each substance.


I hope this post has provided a basis for understanding your vitamin needs and consumption, and that you will understand why in a healthy individual, supplements are generally not needed if you are consuming a varied and balanced diet.
If you suspect any deficiency please discuss it with a professional. A full blood workup will show the amounts of vitamins in the blood, this is important to know before determining if supplementation or dietary changes are needed. The amount of supplementation should be advised by a health specialist to ensure toxic amounts are not being consumed.


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