on 07/10/2024
Our beliefs about food and nutrition are shaped by our culture, tradition, and personal experiences. And coupled with so much noise from diet trends, social media influencers, marketers, and even well-meaning friends and family, it is sometimes easy to forget that nutrition as a field of study is based on robust scientific evidence.
Nutrition (science) teachers are in a unique position to help students recognise credible nutrition information from the overwhelming noise of pseudo-nutritional science. But to do so effectively, we must ground our teaching in the process of scientific inquiry, highlighting how different types of research contribute to our understanding of human nutrition, food and health.
Let’s reflect quickly on what the method of scientific enquiry is. It is a systematic approach to understanding the natural world through observation, experimentation, and analysis. It involves several key steps:
Observation: The observation of a phenomenon or identifying a problem, which leads to questions about why or how something occurs.
Research: Before forming a hypothesis, researchers gather existing information and data about the topic, this helps in understanding the context and avoids duplicating previous work.
Hypothesis: Based on observations and research, a testable hypothesis is formulated – which is a proposed explanation or prediction that can be tested through experiments.
Experimentation: Experiments are designed to test the hypothesis under controlled conditions. Variables are manipulated to observe their effects, and careful measurements are taken to determine whether the hypothesis is supported or refuted.
Data Analysis: The data collected from the experiments are analysed using statistical methods to see if they support the hypothesis. Researchers look for patterns, relationships, and inconsistencies in the data.
Conclusion: Based on the analysis, a conclusion is drawn. If the data support the hypothesis, it may be considered valid. If not, the hypothesis may be revised or rejected.
Replication: For a finding to be accepted as scientific knowledge, other scientists should be able to replicate the results by following the same methods.
Peer Review: Before new findings are widely accepted, they undergo peer review, where other experts in the field evaluate the research for accuracy, validity, and significance.
Publication: Once validated through peer review, the research is published in scientific journals, contributing to the broader body of scientific knowledge.
Ongoing Inquiry: Science is iterative. New questions often arise from findings, leading to further experiments and investigations. The scientific process is ongoing, with each discovery building on previous knowledge.
The Problem with Simplistic Nutrition Messages
Students today are constantly exposed to simplified and often misleading messages about food and nutrition. "Carbs are bad," “low fat is best”, "superfoods will make you live forever," or "this supplement will boost your metabolism." These messages are appealing because they’re easy to grasp and promise quick fixes. However, they are rarely supported by scientific evidence. Instead, these claims often ‘cherry-pick’ information and data from single studies or worse, rely on anecdotal evidence.
Science educators must instil a more rigorous understanding of how nutrition and food science works, emphasizing that credible nutrition advice comes from a process of evidence gathering, critical evaluation, and ongoing review and revision based on the evidence. And in here lies the difficulty, as there is a hierarchy or reliability of the different types of evidence available.
So, what is the Hierarchy of Evidence?
Figure 1: The diagram exemplifies which research designs present the strongest evidence at the top, while working down to the weaker research designs in terms of evidence presented. Credit – The Logic of Science
The Hierarchy of Evidence ranks the evidence derived from research based on how reliable and trustworthy the research is. The higher levels at the top provide more trustworthy results.
At the top of the pyramid are systematic reviews and meta-analyses, which combine data from many studies to give a clearer picture. Next come randomized controlled trials (RCTs), where participants are randomly assigned to different groups to test specific interventions. Below that are cohort and case-control studies, which observe people over time. At the bottom are case reports, expert opinions, and lab studies, which offer insights but aren’t as strong for drawing conclusions. Higher-quality evidence means more confidence in the findings. This helps guide decision making about by prioritizing the most reliable research and ensuring that health recommendations are based on the best available evidence. As an aside, you will note that those aforementioned social media influencers, marketers, and well-meaning friends and family do not make the cut.
Let’s start at the bottom!
Case reports, opinion papers, and letters are considered lower levels of evidence because they often focus on individual experiences or expert viewpoints rather than broad data. While they can provide valuable insights or highlight new issues, they lack the rigorous methods of higher-level research. These types of publications are useful for generating hypotheses but need to be interpreted with caution when making broad conclusions.
Animal trials and in vitro studies play an important role in nutrition research by helping us understand mechanisms and biological processes in very controlled environments. These studies are valuable in they help to identify how certain nutrients or compounds behave at a cellular or animal level, which is useful for early-stage discoveries. However, because humans are more complex, the results from these studies may not always apply to people. Therefore, we cannot say that an animal study which showed that Vitamin C supplements reduced the size of metastatic tumours will also occur in humans.
Cross-sectional, case-cohort and cohort studies are all classified as observational studies. While at the foundational level, observational studies do play a significant role in nutrition science. These studies look at relationships between diet and health outcomes without actively intervening in what people eat. For example, large cohort studies like the Nurses' Health Study have followed thousands of individuals for decades, tracking their eating habits and linking them to long-term health outcomes like heart disease, cancer, and diabetes. As such they are valuable for generating hypotheses. They allow scientists to identify patterns and associations quickly from a large amount of data, that might warrant further investigation. For example, observational studies have shown that populations that eat more fruits and vegetables tend to have lower rates of certain chronic diseases. However, these studies can't establish causation—only correlation. In other words, just because two things are related in an observational study (e.g., people who eat more vegetables also live longer) doesn't mean one causes the other, or it is not ‘causal evidence’. Therefore, we can’t say from these observational studies that eating vegetables will make you live longer, but we can say that people that eat more vegetables are more likely to live longer. There is also the problem of confounding factors that may also impact on the outcomes that we are interested in, for example, exercise, weight, educational status and the consumption of other foods.
Randomised Controlled Trials in humans are experimental studies. In these studies, researchers actively intervene by assigning participants to different dietary interventions, while keeping everything else the same between the groups. They then look to see how specific changes affect health outcomes. For instance, one group might be asked to follow a Mediterranean diet, while another group maintains a typical Western diet, and then both groups are monitored for changes in blood pressure, cholesterol levels, or other health markers.
RCTs are the gold standard for determining causality because they control for confounding factors by randomly assigning participants to different groups. They allow researchers to isolate the effects of a particular dietary intervention, which is something observational studies cannot do.
However, even RCTs have limitations. They are often conducted over short periods of time due to practical and financial constraints, meaning they might not capture the long-term effects of a dietary intervention. Also, people in real life don't always follow strict diets in the way participants in clinical trials are asked to, which limits the generalizability of the results.
Nutritional science teachers can use RCTs to teach students about experimental design, the importance of control groups, and how to assess whether an intervention truly makes a difference. This will help them understand that not all studies are created equal—some RCTs are better than others, and those provide stronger evidence.
Systematic Reviews and Meta-Analyses are the highest level of evidence. These studies synthesize the results of multiple studies on a particular topic. A systematic review involves a comprehensive search for all relevant studies on a particular topic, followed by a critical appraisal of the quality of the study (i.e. how well it was done). A meta-analysis takes this a step further by statistically combining the results of these studies to get a more precise estimate of the effect of a dietary intervention.
Systematic reviews and meta-analyses are powerful because they bring together data from a large body of research, reducing the influence of studies with unusual or unexpected results or studies with small sample sizes. They provide a clearer picture of what the overall body of evidence says about a particular question. For example, a single RCT might show that a high-fibre diet reduces the risk of colorectal cancer, but another RCT might show no effect. A meta-analysis that pools the data from several RCTs can help clarify whether, all together, high-fibre diets are beneficial for preventing cancer (which of course they are).
In the classroom, systematic reviews and meta-analyses can be used to teach students about the importance of looking at the "big picture" in science. They demonstrate that individual studies, while important, should not be viewed in isolation (this would be cherry-picking). Instead, we must consider the collective evidence from multiple studies before drawing conclusions about nutrition and health. Importantly this type of research provides high-level evidence to inform health-care advice and practice.
Teaching Nutrition as a Process of Ongoing Inquiry
One of the most important lessons we can impart to students is that nutrition science, like all sciences, is a process of ongoing inquiry. As new studies are conducted and more data is collected, our understanding of nutrition evolves. For example, for many years, dietary fat was vilified as a cause of heart disease, but more recent research has shown that not all fats are harmful—unsaturated fats, like those found in olive oil and avocados, and dairy fats can be beneficial for heart health. This shifting landscape of nutrition can be frustrating for those who want clear, definitive answers. But as science educators, we can use this as a teaching opportunity. We can emphasize that the changing nature of nutrition guidelines is not a sign of weakness in the science, but rather proof to the rigor of the scientific process. It shows that scientists are constantly testing, revising, and improving their understanding based on the best available evidence.
The goal of science education is to teach students how to think, not what to think. By grounding nutritional science education in the principles of scientific inquiry, we foster students who have the tools to interpret and apply the evidence so they can navigate the complex world of diet and health with critical thinking.
So, let us remember the quote from John Cleese “Science—a method of investigation and not a belief system”. Cleese J (@JohnCleese). Twitter. (https://twitter.com/JohnCleese, accessed 4 January 2016)
Dr Evangeline Mantzioris
Program Director: Bachelor of Nutrition & Food Sciences
UniSA: Clinical and Health Sciences
Alliance for Research in Exercise, Nutrition and Activity (ARENA)
University of South Australia
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