Harvard Medical School has provided us with this article on"Nutrients that work together—and that you should eat together"

Nutrition guidelines can make things look very cut and dry. They tell us to get this amount of that vitamin and that amount of this mineral. Separating out nutrients this way makes the guidelines relatively easy to understand. And this kind of thinking probably helps us avoid diseases of nutritional deficiency, such as scurvy (not enough vitamin C) or pellagra (not enough niacin).

But most nutrients don't fly solo. They interact—sometimes they join forces, other times they cancel each other out. You have probably heard before that eating vitamin-rich foods is better for you than taking a vitamin supplement. One reason why this is true is that food contains a mixture of nutrients that interact with one another in each mouthful.

The following is a list of nutrients that work in pairs. It's just a sampler, and far from a complete catalog. But hopefully it will help you when you're choosing what to eat.

Vitamin D and calcium

Like most nutrients, calcium is mostly absorbed in the small intestine. Calcium is important because it strengthens bones, but the body often needs vitamin D's assistance to absorb the nutrient. Vitamin D also has many other benefits throughout the body. 

There's debate these days about whether to raise the daily intake goal for vitamin D. Right now, the official nutrition guidelines recommend that adults get 1,000 milligrams (mg) of calcium and 400 international units (IU) of vitamin D daily. For older adults, the recommended daily allowance is a bit higher: 1,200 mg of calcium starting in your 50s, and 600 IU of vitamin D starting in your 70s.

To give you an idea of how much that is, an 8-ounce glass of milk contains 300 mg of calcium and, because of fortification, 100 IU of vitamin D.

Sodium and potassium

Sodium is one essential nutrient that most Americans consume more of each day than they need (mostly in the form of salt).

Excess sodium interferes with the natural ability of blood vessels to relax and expand, increasing blood pressure—and increasing the chances of having a stroke or heart attack.

But potassium encourages the kidneys to excrete sodium. Many studies have shown a connection between high potassium intake and lower, healthier blood pressure. According to the current guidelines, adults are supposed to get 4,700 mg of potassium and 1,200 mg to 1,500 mg of sodium daily. 

To meet these criteria, you need to follow general healthy eating guidelines. To increase potassium intake, load up on fruits and vegetables. To decrease sodium intake, cut back on cookies, salty snacks, fast foods, and ready-made lunches and dinners.

Vitamin B₁₂ and folate

Vitamin B₁₂ and folate (also one of the eight B vitamins) form one of nutrition's best couples. B₁₂ helps the body absorb folate, and the two work together to support cell division and replication, which allow the body to replace cells that die. This process is important during times of growth in childhood, and throughout the body of adults as well. Cells that line the stomach and the cells of the hair follicle, for example, divide and replicate often.

Good food sources of vitamin B₁₂ include:

•	meat
•	eggs
•	milk

Natural sources of folate include:

•	leafy green vegetables
•	beans
•	other legumes

Nutrition guidelines recommend 2.4 micrograms of B₁₂ and 400 micrograms of folate daily. This can usually be achieved easily by eating a reasonably well-balanced diet.

However, vegans—people who don't eat meat and other animal-based products—may have B₁₂deficiencies. And people who eat poorly or drink too much alcohol may have folate deficiencies.

Folate deficiencies can be corrected with multivitamins or folic acid pills. For a B₁₂ deficiency, you can get injections every few months or take a pill daily.

Deficiency in either or both vitamins may cause a form of anemia called macrocytic anemia. B₁₂ deficiencies can also cause mild tingling sensations and memory loss.

Zinc and copper

Copper and zinc don't work together—they actually compete for places to be absorbed in the small intestine. If there's a lot of zinc around, copper tends to lose out and a copper deficiency may develop.

One way the knowledge of the copper-zinc interaction has been put into practice is in treating people with an eye condition called macular degeneration. Some people with the condition are prescribed a special vitamin-mineral combination, called AREDS. The combination has been shown to slow down progression of the disease, which can cause blindness. The AREDS pills include 80 mg of zinc, enough to cause a copper deficiency, so 2 mg of copper were added to the pills.

Niacin and tryptophan

Niacin is one of the B vitamins, although it rarely goes by its B-vitamin moniker, B3. The daily niacin requirement is 16 mg for men and 14 mg for women. Niacin deficiency causes pellagra, a disease that causes a bad rash, diarrhea, and dementia. Tryptophan, an amino acid, is a source of niacin. So one way to avoid niacin shortfalls is to eat foods that contain a lot of tryptophan, including chicken and turkey. 

For more on nutrients to keep you healthy, buy The Truth about Vitamins and Minerals from Harvard Medical School.