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.


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. 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. 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

Best Evidence: Clinical Trials

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.


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.

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