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.
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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.
Another type of ecological study is the migration study, which looks at what happens to the health of people when they relocate and acquire the food and lifestyle habits of their adopted homeland. These kinds of studies can help show whether risk for certain diseases is related more to genetics or lifestyle.
Ecological studies are riddled with problems because there are many factors that affect health outcomes and these can’t be completely controlled for in the analysis of the data. Additionally, individual food intakes can only be roughly estimated.
Better Evidence: Epidemiologic Research
Epidemiologic studies can establish that two factors occur together but not that one causes the other. They are prone to
confounding variables
, which means that there might be unidentified issues that cause two factors to be associated. For example, if researchers find out that people with low fruit intakes are more likely to get cancer, it seems logical that fruit is protective against this disease. But what if those who don’t eat
fruit also don’t exercise? It’s difficult to establish whether it is lack of fruit or lack of exercise or a combination of both that raises the risk.
Ecological studies, discussed above, are the weakest type of epidemiological studies. The following three types of epidemiology provide stronger evidence.
•
Retrospective studies
compare past eating habits between people with and without a particular disease. For example, if people with heart disease are more likely to have eaten a diet high in saturated fat, we might conclude that saturated fat has something to do with heart disease. The main drawback of these studies for nutrition research is that people’s memories of their previous diet can be faulty, especially if it has changed over the years.
•
Cross-sectional
studies compare eating habits and disease rates in groups of people at one moment in time. One problem is that people who have recently become ill may have recently changed their diet.
•
Prospective
(also called cohort) studies follow large numbers of people who are (usually) healthy when the study begins. As the population is followed, eating patterns of those who eventually get a disease are compared to those who do not. These studies require a lot of subjects—numbering in the tens of thousands—and take place over a long period of time, but they carry the most weight among epidemiologists
The
randomized controlled trial
(RCT) is the gold standard in nutrition research. It’s the most credible type of study because it randomly assigns people to different groups and then controls what they eat. Ideally, the study is double-blinded; that is, the subjects don’t know whether they are in the test group or the control group. And when the researchers collect the data, they don’t know which group it came from until all the data has been collected and analyzed. The effects of different supplements
or foods on disease markers, like cholesterol or bone density, can be studied this way. These studies can be very powerful, and ideally, everything we want to know about nutrition would be tested through RCTs. Unfortunately, they are expensive and complex, which is why they are often smaller in size and shorter in duration than is ideal.
OTHER CONSIDERATIONS
A Word about Statistics
Statistical analyses are always performed to eliminate the probability that different outcomes occurred by random chance. Generally, a finding is
statistically significant
if there is less than a 5 percent chance that it occurred by chance. When studies are small in size, it becomes difficult to show statistical significance. Even if there appears to be an effect of a treatment or differences between groups, if the differences and treatment are not statistically significant, scientists conclude that there was no effect.
One way to make good use of the data from smaller studies is to do a
meta-analysis.
This is a statistical analysis of a large number of studies for the purpose of integrating the findings. It is often done to compensate for the small size of individual studies.
Peer Review
Scientific journals make sure that studies are credible and worth publishing by having them reviewed by other researchers qualified to do so. The reviewers can recommend that the study be published or not, based on what they think of the study design and other factors.
Who Paid for the Study?
Most nutrition research is funded by the government, but some is paid for by industry. It probably doesn’t matter as much as you might think.
The results are what they are and the sponsoring party can’t change them. The only real advantage is that they get a peek at the data before it is published; that way their PR department can be ready to issue a press release.
In conclusion, we can make educated statements about vegan nutrition only by looking at what most of the studies say (rather than drawing conclusions from individual studies) and by focusing on the studies that are likely to yield the most reliable information. It’s also important that studies are published in peer-reviewed journals.
CHAPTER 2
PROTEIN FROM PLANTS
N
utrition researchers declared more than thirty years ago that plant foods can provide adequate protein.
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But “where do you get your protein?” is a question that most vegans have heard more times than they can count. Many of the questions about protein in plant-based diets stem from confusion over what it means for proteins to be “complete.”
COMPLETE AND INCOMPLETE PROTEINS
Proteins are made of chains of twenty different amino acids. Some amino acids can be made by the body (generally from other amino acids) and therefore we don’t need a dietary source of them. Others—the
essential amino acids
(EAAs)—must be supplied by the diet.
Proteins in the human body tend to have a consistent ratio of EAAs. Because the percentage of EAAs in animal products and soybeans are a close match to those in the human body, proteins from these foods are considered “complete.” Plant foods like grains, beans and nuts have a lower percentage of at least one essential amino acid, making them “incomplete.” For example, beans (other than soybeans) are low in the EAA methionine, and grains are low in lysine. But when grains and beans are consumed together, their amino acid profiles complement each other and produce a mix that is “complete” and therefore a good match to the body’s needs.
In the early 1970s, the idea that vegetarian meals should contain these specific complementary pairings was popularized in
Diet for a Small Planet
by Frances Moore Lappé.
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Today we know that the theory about what happens when protein foods are combined in this way wasn’t wrong; it just turned out to be unnecessary. Newer research has shown that the body maintains its own storage supply of the essential amino acids.
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We need to keep replenishing that storage with all of the amino acids, and so it’s important to eat a variety of plant foods. But the old idea that certain combinations of plant foods—the complementary pairings—must be consumed together isn’t true.
While fruit is extremely low in protein, and oils don’t provide any, all other plant foods contain protein. One common misconception is that plant foods are completely without one or more amino acids. That’s not true. All plant sources of protein contain at least some of every essential amino acid. In fact, you could get enough protein and all of the essential amino acids by eating just one type of food like pinto beans. You’d need to eat a lot of them, though—about four cups per day. That’s not practical, partly because it would be boring, but also because all those beans are likely to displace foods that are needed to satisfy other nutrient requirements. So eating a variety of protein sources makes better nutritional sense.
PROTEIN RDA FOR VEGANS
Protein needs are calculated on the basis of healthy (or “ideal”) body weight—that is, what a person with a healthy amount of body fat weighs. Scientists use the metric system, so U.S. protein needs are determined using your healthy weight in kilograms.
The protein RDA for adults is 0.8 grams of protein per kilogram of ideal body weight, but along with most other vegan dietitians, we recommend a slightly higher protein intake for vegans. This is because plant proteins are not digested as well as animal proteins.
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Since both cooking and processing often improve protein digestibility, this may
be less of an issue for vegans who consume more foods like tofu or veggie meats made from processed soy protein. For those who are depending on whole foods like legumes, nuts, and grains for most of their protein, the digestibility factor comes into play.
It’s not a big difference, but vegans should strive for a protein intake of 0.9 grams per kilogram of body weight. For ease of calculation, this translates to around 0.4 grams of protein per pound of healthy body weight. So a vegan whose healthy weight is 150 pounds would need 60 grams of protein (150 x 0.4) per day.
Since protein needs vary considerably among individuals, the RDA is designed to cover the needs of 97 percent of the population and is therefore more than what many people need. Without any way of knowing where you fall on the protein-need spectrum, and because recent research suggests that protein recommendations could be too low,
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it’s a good idea to play it safe and aim for the RDA.
For children and teens, we would use an RDA aimed at the needs of different age groups and calculated specifically for vegans.
PROTEIN RECOMMENDATIONS FOR YOUNG VEGANS
Age (years)
| Females (grams/day)
| Males (grams/day)
|
|---|
1–2
| 18–19
| 18–19
|
2–3
| 18–21
| 18–21
|
4–6
| 26–28
| 26–28
|
7–10
| 31–34
| 31–34
|
11–14
| 51–55
| 50–54
|
15–18
| 50–55
| 66–73
|
While the chart on page 19 shows that many plant foods are good protein sources, legumes are especially rich in protein. Legumes include beans, peas, lentils, soyfoods (like tofu, soymilk, and veggie meats), and peanuts. (Most people think of peanuts as nuts, but they are botanically legumes and, from a nutritional standpoint, they have more in common with pinto beans and lentils than walnuts and pecans.) Our food guide specifies at least three to four servings per day of these foods. A serving is pretty modest: ½ cup of cooked beans, ½ cup of tofu or tempeh, a one-ounce veggie burger, one cup of soymilk, or two tablespoons of peanut butter. Planning menus that include these foods isn’t difficult, and we give you several tips for doing so in Chapter 7.
In addition to being protein-rich, these foods are the only good plant sources—with a few exceptions—of the essential amino acid lysine. A diet that gets most of its protein from grains, nuts, and vegetables is likely to be too low in lysine. And while some popular resources suggest that very low protein intakes—as little as 5 to 6 percent of total calories—can meet our nutritional needs, it’s actually difficult to get enough lysine (or total protein) on a diet that isn’t more protein-dense.
You can get a rough idea of how much lysine you need by multiplying your weight (in pounds) by 19. This calculation includes a small factor that makes up for the slightly lower digestibility of protein from whole plant foods. For example, a person weighing 140 pounds would need 2,660 milligrams of lysine per day. The chart on page 21 shows that the best sources of lysine are legumes, quinoa, pistachios, and cashews.
If you follow our recommendations to consume at least three to four servings of legumes per day, you’ll meet lysine needs with ease. That doesn’t mean that beans, peanuts, and soyfoods are absolutely essential in vegan diets. While it is difficult to meet protein and lysine needs without them, it’s possible, and we provide guidance on how in Chapter 7, when we look at meal planning guidelines for vegans.