Here's another signpost that may be less familiar: Make sure you consume enough zinc.
The study, conducted by scientists at Tufts University and Boston University and published last month in the American Journal of Clinical Nutrition, also found that those who began the study with normal zinc levels had a 39% lower mortality rate -- from any cause -- than those who were deficient.
In a second small study, led by Dr. Ananda Prasad at the Wayne State University School of Medicine, 50 free-living, healthy elderly subjects were recruited from a senior center to receive either a daily zinc supplement or placebo for a year to determine if zinc supplementation offered protection against colds and flu.
By the end of the yearlong study, zinc-takers had significantly higher zinc levels in the bloodstream and had suffered significantly fewer infections -- seven cases versus 35 cases in those taking a placebo.
The study also found lower serum levels of a chemical called malondialdehyde and other signs of oxidative stress (the production of cell-damaging free radicals) in the group taking supplements, confirming earlier evidence that zinc can function as an antioxidant.
The study was published in the American Journal of Clinical Nutrition in March.
Adequate zinc is critical for the production of lymphocytes -- the army of specialized white cells in the immune system that help defend against foreign invaders.
Zinc deficiency leads to impaired immunity and thus decreased resistance to a host of bacterial, viral, fungal and parasitic invaders. Recovery from illness takes longer when zinc levels are low than when zinc status is normal.
Men require 11 milligrams of zinc a day, and women require 8 milligrams.
For the most part, this amount can usually be obtained from the diet through eating foods such as protein-rich meats, fish and poultry.
But the authors of the two recent reports suggest that dietary intake of zinc in the elderly may be marginal because some older people may find these foods too expensive or difficult to chew.
Still, even with these restrictions, it's still possible to get adequate zinc. Dairy products, beans and nuts provide zinc, as do some fortified, ready-to-eat cereals. Multiple vitamin and mineral supplements can help meet needs, particularly if the quality of the diet is good to start with.
A recent analysis of food and supplement use from a large survey of Americans taken by the U.S. Department of Agriculture (known as the USDA's Continuing Survey of Food Intakes by Individuals) found that supplement users had better dietary intake of zinc than nonusers.
Among people age 71 and older, 43% of those who shunned supplements consumed inadequate intake of zinc from foods compared with 29% of supplement users. And when the amount of zinc supplied from supplements was factored in, only 5% of supplement users failed to meet the estimated average requirement.
Poor zinc intake is not just a problem among the elderly. Data from another large government food intake survey known as NHANES III indicated that after seniors, teenage females had the lowest zinc intake, with 61% failing to meet recommendations. Fewer than half of women at any age took in an adequate amount.
Men, who tend to consume more protein foods, fared better.
If you decide to use supplements, note that zinc and copper compete with one another for absorption in the digestive tract, so it's wise to take in about 2 to 3 milligrams of copper per day along with the recommended 8 or 11 milligrams of zinc.
Rather than fussing with individual nutrients, you can take a balanced vitamin and mineral supplement, which should contain the proper ratio of zinc to copper.
Susan Bowerman is a registered dietitian and assistant director of the UCLA Center for Human Nutrition.