Skip to content
Closer - The home of longitudinal research

Diet: what is it and how do we measure it?

Blog | | Jane Maddock

To coincide with the launch of CLOSER’s new online cross-study data guide for dietary data, Dr Jane Maddock takes a look at diet – what it is, how and why it’s hard to measure and analyse, and why bother studying it at all.

What is diet?

Vitamin C, Jammy Dodger biscuits and Paleo all comprise a small part of the taxonomy of diet. Diet refers to nutrients, specific foods or drinks, or the combination of food and drinks that people consume. You will have come across the idea that diet is important for health, whether you are a reader of books, academic journals, newspapers or… the internet. However, exactly how diet influences health and what components of diet are important often get lost in a myriad of conflicting studies, blogs and diet books.

Why is studying diet so complicated?

1. It’s hard to measure

The goal of dietary assessment is to collect an accurate record of habitual food and nutrient intake.

Think about what you ate yesterday. How much cereal did you eat? What type of bread did you use for your sandwich? Did you use oil to cook your vegetables? The point is that there are a lot of details to record. It’s hard to know or remember how much salt you put on your chips. Maybe you forgot (or didn’t want to report) that biscuit you ate while making your sandwich.

What if yesterday was unusual for you? Diets can vary depending on what day it is, what age a person is, what their working pattern is like, and what season it is. Added to this complexity is the variation in the energy, nutrients, vitamins and minerals in the foods that people eat. Nutrient composition can vary depending on things such as soil composition, fortification practices and changes in the manufacturing process.

These intricacies have been considered over the years and an array of dietary assessment tools, which are suitable for a variety of study types, have been developed. However, because of practical and financial limitations, no current dietary assessment method measures long-term diet perfectly. Two resources, Nutritools and Dapa Measurement Tookit provide a good overview of different dietary assessment methods, and their pros and cons.

2. It’s hard to analyse

Nutritional science as we know it is a relatively young discipline. A full history of nutritional science can be found in previous publications (see further reading below). In the early years, the focus was on the identification of specific nutrients and their roles in deficiency diseases (see Vitamin C for scurvy and Vitamin D for rickets). In later years, this “reductionist” approach was extended to identify single nutrients that played a role in more complex diseases such as cardiovascular disease, beginning the fat-versus-carb debates and resulting in nutrient-based dietary guidelines of the 1980s. However, people don’t eat only fat, and they (hopefully) don’t eat only butter; they eat a combination of foods containing complex combinations of nutrients that have interactive or synergistic effects. This makes it difficult to tease apart the role of a specific nutrient on a disease.

For every study, article or blog I read about diet and health, the mantra “correlation does not equal causation” plays in my head. There have been instances in nutritional epidemiology where associations observed in cohort studies have not been seen using the traditional gold-standard approach of a randomised controlled trial (see the case of antioxidants here). Although these randomised trials are suitable for figuring out that Vitamin C prevents scurvy (credit to James Lind’s 1747 scurvy trial), they are not always appropriate. It may be unethical to assign a group of people to a dangerously low intake of a particular nutrient and, randomly assigning a group of people to consume a specific dietary pattern for a very long time would be extremely difficult.

Why bother to study diet at all?

When someone asks me what my research interests are, I cautiously whisper, “I sometimes deal with dietary data”, stand back and wait for an inevitable look of horror. Yes it’s messy, yes it can be imprecise and yes it can be difficult to determine its causal role, but that doesn’t make it irrelevant.

Diet is a health behaviour that is important for health. We also know that an individual’s diet is the result of complex biological, social, economic and cultural circumstances and often those in disadvantaged positions have poorer dietary intake.

Overlooking diet leaves out a big aspect of understanding the effect that inequalities in health behaviours have on people’s lives. Having a good knowledge of how diet is measured, applying a careful approach to the analysis of observational studies and triangulating results across different study designs can support the effective use of dietary data.

What’s the purpose of the CLOSER project?

Thanks to the support of CLOSER, we have developed a comprehensive guide describing dietary data in the eight original CLOSER studies. These studies capture more than 70 years of social history and the impact of changing public health policy in the UK. They provide a valuable insight into the role and changing nature of diet over time, and when combined with the rich demographic, social and health data of study participants, can support the investigation of longitudinal and cross-study diet-related research questions.

In this guide to the dietary data, we provide a thorough description of how dietary data was collected and processed in each of the studies and how it has been used to date by researchers. This detail is enhanced with additional contextual information and more general guidance on dietary assessment methods.

We found that there is heterogeneity in dietary assessment methods across the eight original CLOSER studies, making the creation of an overall harmonised dietary variable problematic. Harmonisation depends on both the research question and the available data.

In this guide we give general suggestions on how to harmonise dietary data. We hope that these suggestions combined with the detailed documentation can act as a springboard to support and encourage researchers to answer diet-related questions in these longitudinal studies.

Explore the online dietary data guide

Further information

  • 40th Anniversary Briefing Paper: Food availability and our changing diet
  • Mozaffarian, D., I. Rosenberg, and R.J.b. Uauy, History of modern nutrition science—Implications for current research, dietary guidelines, and food policy. 361: p. k2392.
  • Carpenter, K.J.J.T.J.o.n., A short history of nutritional science: part 3 (1912–1944). 133(10): p. 3023-3032.
  • Carpenter, K.J.J.T.J.o.n., A short history of nutritional science: part 4 (1945–1985). 133(11): p. 3331-3342.

Dr Jane Maddock is a Senior Researcher at the MRC Unit for Lifelong Health and Ageing at University College London. Follow Jane on Twitter: @_JaneMaddock

Suggested citation:

Maddock, J. (2020). ‘Diet: what is it and how do we measure it?’. CLOSER. 12 November 2020. Available at: