Most calcium in your small intestine is absorbed in the lower regions called the ileum and jejunum. These are the main portions of your intestine that absorb nutrients and minerals. Calcium here will move passively into the blood stream. This means that it can easily slip through little spaces in the skin layer (called the epithelium) and into the blood. If your diet provides your body with enough calcium, this is the main type of transport you'll use.
If calcium levels are low, your body has to work a little harder to get the amount it needs. This is where vitamin D comes in. Vitamin D operates channels that collect calcium in the upper third of the small intestine, called the duodenum. These channels are activated when vitamin D binds to proteins in the epithelium (skin) layer. These proteins work with active transport, where the body uses energy to pump the calcium into the blood stream, increasing its levels in the blood. This is why, if you increase the vitamin D in your diet, you end up increasing the calcium in your blood stream.
This vitamin D metabolism is one of three theories I could find that try to explain infantile hypercalcemia in WS. Researchers have found that when children with hypercalcemia are managing their calcium levels and only slightly rise the vitamin D in their diet, their calcium levels increase dramatically. They found that by only making small increases in vitamin D, children with WS absorb 2-3 times more calcium than what would be expected in a typical child.
Another factor that influences calcium absorption is the type of food you eat. If you are a milk drinker, you're going to have more passive calcium absorption- the easy kind in the lower intestine. Milk contains sugars called lactose and an enzyme called lactase both of which help the body collect the calcium and absorb it into the blood stream. Other foods also contain calcium, such as spinach. Foods that are high in fiber and contain calcium tend to be harder for your body to digest. The fiber, called oxalate, binds to the calcium and holds on to it as it passes through the gut. Therefore, if your primary calcium sources are in high fiber foods, you will essentially absorb less of them and excrete more due to the food's chemical nature.
Oxalate has other affects on your body, too. If the level of calcium is low, your body will start to absorb more oxalate instead of excreting it. Essentially, if you are in a pinch for calcium, you'll take what you can get even if it's in a form that you don't really prefer. The increase in oxalate signals the kidneys to work harder to get rid of it. Calcium oxalate then builds up in the kidneys and can cause stones, or nephrocalcinosis (see the growth and diet page on this blog). This is why some doctors will place a child with hypercalcemia on a low oxalate diet- to prevent the uptake of calcium and reduce dangers of developing kidney stones.
It's important to note that although many WS infants with hypercalcemia have higher than normal levels of vitamin D, there are exceptions to the rule. There is a significant population of individuals who have high levels of calcium and LOW levels of vitamin D. Since most foods contain both, this can create quite the dilemma to try and maintain proper levels of each. Stay tuned for future blog posts discussing this topic and other theories of why our little ones have hypercalcemia!