A New Bible for Eating Well

A new book offers one-stop shopping for authoritative nutrition advice—the first time the Institute of Medicine in Washington, D.C., has issued such a tome in nearly 2 decades. It summarizes a wealth of the institute’s eating guidelines that have been trickling out over the past decade.

Since at least the 1950s, people who wanted to know how much of some vitamin or mineral they ought to be getting in their diet would seek out the institute’s recommended daily allowance—or RDA—for that substance. Dieticians, doctors, and plain folk all have used RDAs as the prescription for what a person needs to consume to stay in good health.

It’s turned out, however, that many RDAs don’t really meet that goal. Because of the way they were developed, many often amounted to the minimum intake of some dietary constituent needed to prevent a specific disease associated with nutrient deficiency. For instance, vitamin D’s RDA was initially set largely to prevent rickets in children.

Such guidelines tended to neglect a growing body of data suggesting that optimal health—especially staving off atherosclerosis or other chronic diseases—might require daily intakes of certain substances substantially higher than a deficiency-fighting RDA for them. So, some 20 years ago, the institute set about redoing its RDAs from top to bottom. Where data existed, the government-sponsored agency also identified safe upper-intake limits so that people wouldn’t be left to assume—inappropriately, in many cases—that if a little of something is good for us, megadoses of it must be even better.

Without abandoning RDAs, the institute issued its new nutrient guidelines, which it termed Dietary Reference Intakes, or DRIs, in eight volumes. The first, published in 1997, tackles calcium, phosphorus, magnesium, vitamin D, and fluoride (SN: 4/19/97, p. 237). The latest, released in 2005, covers energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids (macronutrients). Along the way, the institute issued reports on such related issues as general guidelines for nutrition labeling and food fortification.

To mine the collective wisdom in these reports, one would have to read some 5,000 pages of text that’s dense with scientific analyses, notes Linda D. Meyers, director of the institute’s Food and Nutrition Board. The new book, released last week with Meyers’ editing help, brings the count down to 543 more-manageable—and significantly more attractive pages—than were in the original documents.

Those earlier reports offer valuable depth and context for people researching information on nutrients, Meyers says. However, when the institute convened focus groups of dieticians and health professionals, she notes, her Institute confirmed what it had suspected: that even these professionals found the big books tough going and their useful information buried. So, Meyers says, the Institute of Medicine has now issued a book “for most of the rest of us.”

Four types of DRI’S

The new DRIs include RDAs as one of four types of nutritional guidelines. A box on page 8 of the new book works to clarify what Meyers concedes had been a confusing issue for many people: What are DRIs and how do they differ from the old RDAs?

The new book says that RDAs describe the amount of any nutrient or food constituent deemed sufficient to meet the needs of perhaps 98 percent of healthy individuals. A newer category, an “estimated average requirement” (EAR), identifies the somewhat lower daily intake that should satisfy the needs of half of the population. A third category, “adequate intakes” (AIs), suggests the amount most people need of a nutrient that doesn’t have enough data behind it to support a true RDA. An AI is an educated guesstimate. Finally, “tolerable upper intake levels” (UL) identify a ceiling on nutrient consumption “likely to pose no risk of adverse health effects to almost all individuals.” Presumably, intakes higher than the UL might be bad for a person.

For instance, copper is a mineral needed for the functioning of many important enzymes. Too much copper, however, can foster oxidation—a chemical process that underlies many chronic diseases. An easy-to-read table in the new book gives copper an EAR for most adults of 700 micrograms per day, an RDA of 900 µg/day, no AI—since an RDA exists for all age groups except infants—and a UL of 10,000 µg/day. Pregnant and lactating women get somewhat higher EARs and RDAs.

Although people typically think of RDAs in terms of vitamins and minerals, the institute has developed DRIs for a host of other dietary constituents—including fats, carbohydrates, protein, water, salt, fiber, cholesterol, amino acids, carotenoids, and fluoride.

For each nutrient or food-constituent category, the new book includes a chapter containing a page or two of “key points.” These highlights typically identify the primary function of the food constituent, its richest sources, problems associated with getting too much or too little, and populations most vulnerable to getting too little.

Perhaps the biggest surprise for most people will be the fact that this book has an RDA for something no one eats: exercise. Meyers explains that because the DRIs suggest limits on how many calories people should consume, it made sense for the book to also address how many calories they should burn.

The eight-page chapter on exercise distills information that was buried in an earlier volume on macronutrients. Among the new chapter’s guidelines: All healthy adults and children should get 60 minutes of moderately intense physical activity a day. To illustrate what that might mean, the new book includes a chart describing activities that provide: mild energy expenditures (such as billiards and walking the dog), moderately intense activities (gardening, golf, and raking the lawn), and vigorous activities (chopping wood, jogging, and aerobic dancing).

Work in progress

Meyers emphasizes that as a result of advances in nutrition science and other health-care disciplines, scientists have begun discussing a possible need for DRIs for additional dietary ingredients. Among those mentioned, for instance, are lycopene (see Looking for lycopene? Tomatoes are okay, but. . . ), and the antioxidants in many plants (SN: 1/8/05, p. 27). “What I’m hearing,” she says, “is that we need criteria for determining when a new substance should be considered for a DRI.”

Toward that end, the institute will be hosting a series of workshops over the next year or so to assess how well the DRI-development process has—or hasn’t—worked. Shortly after these meetings, Meyers says, “we will presumably begin updating the DRIs.”

“My personal view,” she told Science News Online, “is that vitamin D should be first” to get an update. In fact, she says, “it’s at the top of everyone’s list that I’ve talked with.” The reason: When the institute set its DRI for this vitamin, a host of studies were completed or in the works—but not yet published—indicating that the value was probably too low. Subsequent research has only reinforced that view.

However, the earliest any new DRIs will emerge is many years from now. Until then, the new book offers an authoritative and concise—presuming that that word can apply to a 543-page volume—handbook for designing a healthy diet.