What is the difference between water-soluble and fat-soluble vitamins?

DEAR DOCTOR K:

When I asked my doctor about supplements, she said something about water-soluble and fat-soluble vitamins. What is the difference? And why does it matter?

DEAR READER:

A vitamin is a natural substance that is required in small amounts for human life. As your doctor said, there are water-soluble vitamins and fat-soluble vitamins.

Water-soluble vitamins dissolve in water; they are packed into the watery portions of the foods you eat. Vitamin C and the B vitamins are water-soluble vitamins, for example.

Water-soluble vitamins have many tasks in the body. They are involved in energy production; they help cells multiply; and they make collagen, which knits together wounds, supports blood vessel walls and forms a base for teeth and bones.

These vitamins are absorbed directly from the digestive tract into the bloodstream. They travel to all parts of your body that contain water — and that means all parts of your body. Every one of the 13 trillion cells in our body contains water. And the space between those cells contains water, too — as, of course, does the blood.

Your kidneys continuously regulate levels of water-soluble vitamins. They eliminate the excess out of the body in your urine. For this reason, the risk of harm from consuming large amounts of supplements is relatively small.

The fat-soluble vitamins are vitamins A, D, E and K. They keep many parts of your body in good repair. They are essential for bone formation; they protect your vision; and they protect your cells and DNA against damage from unstable molecules.

Fat-soluble vitamins enter the bloodstream through channels in the wall of the intestine. However, because they dissolve in fat solvents and oil, but not water, they need special ways to move around the body. Most travel through the body under the escort of special fat-binding proteins. These proteins act as carriers to allow fat-soluble vitamins to interact with water-rich blood and cells. (I’ve put an illustration of this process, below.)

Absorption of fat-soluble vitamins

fat soluble

  1. Food containing fat-soluble vitamins is ingested.
  2. The food is broken down by stomach acid and then travels to the small intestine, where it is digested further. Bile is needed for the absorption of fat-soluble vitamins. This substance, which is produced in the liver, flows into the small intestine, where it breaks down fats. Nutrients are then absorbed through the wall of the small intestine.
  3. Upon absorption, the fat-soluble vitamins enter the lymph vessels before making their way into the bloodstream. In most cases, fat-soluble vitamins must be coupled with a protein in order to travel through the body.
  4. These vitamins are used throughout the body, but excesses are stored in the liver and fat tissues.
  5. As additional amounts of these vitamins are needed, your body taps into the reserves, and the liver releases them into the bloodstream.

 

Fat-soluble vitamins get stored in fat tissues and the liver. Your body squirrels away any excess you consume and doles it out gradually to meet your needs.

Because these vitamins are stored for long periods, however, dangerous levels can build up. As a result, excessive intake of fat-soluble vitamins is more likely to cause you harm than taking too many water-soluble vitamin supplements.

A dangerous overdose is most likely to happen if you take high doses of a vitamin supplement over a prolonged period of time. It’s very rare to get too much of any vitamin just from food, but it can happen.

As a medical student, I saw a young woman who had an obsessive desire to eat carrots. Lots of carrots! Carrots contain chemicals called carotenes that are transformed into vitamin A in the body. Her skin and the whites of her eyes were slightly but definitely orange. With several months off her unusual diet, the orange hue disappeared. Fortunately, she suffered no health problems.