Vegan for Life - Jack Norris [8]
Adequate Intake (AI): When there isn’t enough data to establish an RDA, the Institute of Medicine sets an AI, which is based on both studies and observations of what healthy populations consume. The recommendation for calcium, for example, is an AI because the data regarding calcium needs is conflicting.
Tolerable Upper Intake Level (UL): This is the maximum daily intake of a nutrient that is likely to be safe. Some nutrients can be extremely toxic at higher than normal levels, although excessive intakes are almost always associated with supplements.
Daily Values (DV): These are values used strictly for food labeling purposes and they are based on much older RDAs. The amounts of vitamins and minerals in a food are listed as a percent of the DV. For example, the DV for calcium is 1000 milligrams, so if a food contains 10 percent of the DV for calcium per serving, it provides 100 milligrams of calcium. The problem is that without knowing what the DV is for a specific nutrient—and the food label doesn’t tell you—it’s hard to know exactly what these numbers mean. As we discuss the nutrients that are especially relevant to vegans in the next several chapters, we’ll give you the information you need to decipher food labels.
UNDERSTANDING NUTRITION RESEARCH
The amount of nutrition information in the media and on the Internet is staggering. Much of it is conflicting and often studies looking at the same question come up with completely different answers.
In fact, for essentially all heavily researched areas, you can build a case for just about anything by picking and choosing the studies that support your point. Some advocates do this to make vegan diets look more beneficial. And some vegan detractors pick a completely different set of studies to make vegan diets look bad.
The key to understanding nutrition research is to look at the entire body of evidence and see what most studies say. Rarely can a single study provide a definitive answer to a question. There are always inconsistencies and there are always study flaws. In addition, different types of studies carry different weight. So the strengths and weaknesses of certain types of studies have to be balanced against the strengths and weaknesses of others.
TYPES OF STUDIES
Weakest Evidence
These types of studies don’t provide conclusive evidence but are conducted primarily to determine if further research is warranted.
• Neither in vitro (studies conducted in test tubes or cell culture often using single cells) nor animal studies can serve as the basis for conclusions about diet and disease. Aside from any ethical considerations and despite their widespread use, findings about nutrition from animal studies often can’t predict what is going to happen in humans.
• A case study is an observation about one or perhaps several patient histories and their treatment and disease outcomes that is published in a scientific journal. Often these types of reports can be used as a basis for hypothesis generation, but they don’t provide definitive answers. In contrast, if a report isn’t published in a peer-reviewed journal, it is merely an anecdote and has little or no value in contributing to nutrition knowledge. A great deal of nutrition information on the Internet and in books—including books by doctors and other health professionals—is based on anecdotes rather than actual science.
• Ecological (also called correlational) studies compare food habits and disease rates among different groups of people. One ecological study that is familiar to many vegans looked at rates of hip fracture and protein consumption in different countries.4 The results showed that as protein intake increases, so does the rate of hip fracture. But contrary to popular opinion, that study didn’t show that high protein intake causes weak bones. (We talk more about why that is in Chapter 4.) It did set the stage for clinical studies on how protein might impact calcium metabolism.