Nutrient Sinks: Scenarios Leading to Reduced Nutrient Absorption

By Kathy Zawadzki
Oct 21, 2005, 21:09

People tend to look at nutrients independently when they consider their nutrition needs. We’re concerned about our bones, so we look at how much calcium we’re consuming. We’re worried about anemia, so we find sources of iron. What we rarely consider, however, is how different minerals and other micronutrients interact with each other when they’re present in the same meal, and how that can impact what compounds make it into our bodies.

Many different minerals and other nutrients interact in ways that affect their availability or absorption in the body.  Some minerals even compete with each other. Calcium and iron are two minerals that are very important to the active individual.  Since the absorption of these minerals is affected by the other foods we eat and drink on a regular basis, it is important to take a look at some of these complex interrelationships.

Calcium

We have been hearing for years how ingesting more calcium may help to prevent osteoporosis or fragile bones as we age.  Active individuals need to be aware of their calcium intake since we lose more in sweat and use more for muscle contractions than the average adult.  There is new evidence that suggests the key it is not only the availability of calcium in our bodies, but also the interaction between calcium and phosphorus.  The amount of calcium in our bones is very carefully regulated by hormones and increasing calcium intake does not fool these hormones into building more bone any more than delivering an extra load of bricks will make a construction crew build a larger building. 

The high intake of phosphorus found in protein-rich foods such as meat, poultry, fish, eggs, nuts, and legumes as well as carbonated beverages, may be contributing to much of our bone loss. Although its exact role is far from clear, scientists believe that diets in which phosphorus and calcium intake are roughly equal (1:1 ratio) help keep calcium in the body, while diets in which the two are unbalanced are thought to harm calcium balance.  Unfortunately, the typical Western diet is imbalanced in these two minerals. Most people consume roughly two to four times more phosphorus than calcium. For example, meat and poultry contain 10 to 20 times as much phosphorus as calcium, and carbonated sodas have as much as 500 mg of phosphorus, and no calcium, per serving.

When there is more phosphorus than calcium in the system, the body draws on calcium stored in bones to balance out the ratio. This can lead to reduced bone mass (namely, osteopenia or osteoporosis) that makes bones brittle and fragile. For the vast majority of people, the answer is not only in boosting calcium intake but, rather, limiting calcium loss by reducing the amount of phosphorus they ingest.  

Sodas do contain phosphorus, but there's no need to be concerned about drinking the occasional can. However, drinking too much soda (approximately five cans/day according to a USDA research study) has been shown to upset the body's calcium/phosphorus ratio. Under these circumstances, the body attempts to maintain balance by drawing calcium from bone.

To help improve calcium availability, be sure to wait until after your morning coffee to consume your calcium-rich foods (low-fat dairy products) such as skim milk and low fat yogurts.  There is some evidence that suggests caffeine may also decrease availability of calcium when the two are consumed together.  Other good calcium sources include green leafy vegetables such as kale and collard greens, broccoli, and calcium-fortified items such as fortified orange juices and soy milks.

Iron

Iron is another very important mineral for active individuals.  When iron combines with protein to form hemoglobin, they act together as an oxygen carrier in our bodies. Hemoglobin is a major component of red blood cells and a deficiency in iron can cause anemia. The body has to absorb enough iron each day to compensate for the loss of old blood cells and meet the body’s needs to remain healthy.

The body absorbs various forms of iron at different rates. Heme iron comes primarily from animal products, such as meat, fish, poultry and seafood. It is more readily absorbed than the non-heme form of iron, which is mostly found in plant sources such as green leafy vegetables, nuts, whole grain breads, and iron-fortified cereals. Generally, non-heme iron is more difficult for the body to absorb.  This is because the absorption of non-heme iron can be inhibited by other foods, even though it usually takes large amounts of the offending foods to interfere.  Some of the basic foods that will do this are: oxalic acid found in spinach and phosphates found primarily in milk, dairy products and egg whites. Also many sodas and tannins in tea and coffee can interfere with iron absorption.

Caffeine blocks iron absorption when the two are consumed at the same time. When caffeine is consumed one hour before eating, iron absorption is not affected.  So, if you must have tea or coffee, avoid having them with or just after your meals containing non-heme iron sources, such as iron-fortified cereals.  This means waiting about 30 minutes after you finish your bowl of raisin bran before the morning coffee.  However, combining plant sources of iron with foods high in vitamin C will help to increase your absorption of plant iron, so replace that coffee with orange juice or other citrus food and you are ready to roll.

Clearly, the interactions and interrelationships between nutrients are complex and varied. It is nearly impossible to know everything a food item may interfere with for optimal metabolism and absorption of other nutrients.  Just keep in mind that by eating a variety of foods and not overloading on one nutrient, you should be able to keep things in balance.

Kathy Zawadzki is a licensed Sports Nutritionist and cycling coach for Carmichael Training Systems.  She lives and trains in Littleton, CO.

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