Chapter 1: Apply epidemiological concepts

Nutritional epidemiology is a vital field for nutrition and dietetics professionals in Kenya, providing the tools to study the distribution and determinants of diet-related health outcomes in populations. Understanding epidemiological concepts enables practitioners to design effective interventions, assess nutritional risks, and contribute to public health policy. This chapter introduces fundamental epidemiological concepts, tracing their historical development and outlining the scope and assumptions that guide their application in nutrition-related research and practice across diverse Kenyan settings.

1.1 Key concepts in epidemiology

Epidemiology gives a nutrition professional a shared language for describing how health problems are distributed in a population and why. The terms in this topic are used in every later chapter of the unit: surveillance, screening, measures of occurrence and research all depend on them. A dietitian who reads a nutrition survey, a clinic register or a research report meets these words daily, so each must be understood exactly.

1.1.1 Definition of terms

A definition fixes what a term includes and what it leaves out, so that two workers describing the same situation reach the same conclusion. The seven terms below move from the discipline itself, to the way disease is counted, to the three words that describe how far a disease has spread, and finally to the group that a measure refers to.

1.1.1.1 Epidemiology

Epidemiology is the study of the frequency, distribution and determinants of health-related states or events in specified populations, and the application of that study to control health problems. Each part of the definition has a practical meaning.

• Frequency: how often a condition occurs, counted as new cases or as existing cases.
• Distribution: how the condition varies by person (age, sex, occupation), place (village, ward, settlement) and time (season, year).
• Determinants: the factors that raise or lower the chance of the condition, such as diet, infection, income and water supply.
• Specified population: a defined group, for example children aged 6 to 59 months registered at one health centre, rather than individual patients.
• Application to control: findings must lead to action, such as a feeding programme or a counselling campaign.

Example: a dietitian notes that cases of stunting cluster in one ward of a county. Describing the cluster is distribution, finding poor harvests and low protein intake is determinants, and planning a feeding programme and counselling for caregivers is application.

1.1.1.2 Nutrition epidemiology

Nutrition epidemiology applies epidemiological methods to diet, nutritional status and their links with health and disease in populations. It asks questions such as how much of a population has inadequate iron intake, whether a food pattern is linked with a disease, and whether a programme has improved nutritional status.

Its subject matter differs from general epidemiology in one important way: the exposure is something that everyone has, namely food, and it is difficult to measure exactly. People forget what they ate, eat differently on different days, and change their reports when they know they are being studied.

Concepts specific to nutritional epidemiology

• Dietary exposure: the foods, nutrients or eating patterns whose effect on health is being studied, such as intake of fortified maize flour.
• Confounding: a third factor that is linked with both the diet and the outcome and distorts the apparent link between them. Household income can confound the link between fruit intake and child growth, because wealthier households may buy more fruit and also have better access to health care.
• Bias: a systematic error in collecting or interpreting data. Recall bias occurs when mothers who have a sick child remember the child's foods differently from mothers of healthy children.
• Causality: a judgement that a dietary factor produces a health outcome, not merely accompanies it. It needs consistent findings, a correct time order (the diet comes before the disease) and a plausible biological explanation.

1.1.1.3 Disease occurrence

Disease occurrence is the number of cases of a disease or condition found in a population, described in terms of how many are new and how many exist. Two forms are used.

• Incidence: the number of new cases that appear in a population at risk during a stated period.
• Prevalence: the number of existing cases, old and new, in a population at a stated time or over a stated period.

Example: in a catchment area with 900 children who were not wasted at the start of the quarter, 18 become wasted during the next three months. These 18 are incident cases: 18 ÷ 900 = 0.02, which is 2 new cases per 100 children over three months. If a screening on the last day finds 40 wasted children in a different group of 1,000 children, the prevalence on that day is 40 ÷ 1,000 = 0.04, or 4 per 100. Incidence tells how fast a problem is arising, while prevalence tells how large it is at one moment, and prevalence rises when children stay sick for longer. The full set of measures is taught in Morbidity measures.

1.1.1.4 Endemic

A disease is endemic when it is constantly present at a usual, expected level in a particular area or population. The word describes steady presence, not low or high numbers: a disease can be endemic and still cause many deaths each year.

Example: goitre linked with iodine deficiency persists in communities living on iodine-poor soils and is described as endemic there, because the level changes little from year to year unless iodised salt reaches households. Endemic conditions are tracked by comparing today's figures with the usual level for that area.

1.1.1.5 Epidemic

An epidemic is the occurrence of cases of a disease clearly above the level normally expected in a community, region or season. An epidemic is therefore defined against a baseline, which is why surveillance records must be kept before it begins. It may be short, as in food poisoning after one shared meal, or last for months.

Example: a health centre usually sees about 5 children a week with watery diarrhoea. If it records 40 children in one week from a single settlement, the number is far above the expected level and the dietitian joins the response by assessing the nutritional status of the affected children. A small number of cases can also be an epidemic when the disease is new to the area.

1.1.1.6 Pandemic

A pandemic is an epidemic that spreads across several countries or continents and usually affects a large number of people. What separates it from an epidemic is its geographical reach, not the seriousness of the illness.

Nutrition is affected even when the disease itself is not nutritional. During the COVID-19 pandemic, movement restrictions and lost incomes disrupted food supply, feeding programmes and routine child growth monitoring in many countries. Dietitians therefore treat a pandemic as a threat to food security and care services as well as to health.

1.1.1.7 Population at risk

The population at risk is the group of people who are able to develop the condition being studied. It forms the denominator when incidence is calculated, so it must not include people who already have the condition or who cannot get it.

• Example 1: in measuring new cases of wasting, the population at risk is children aged 6 to 59 months who are not wasted at the start of the period.
• Example 2: in measuring new cases of rickets in infants, adults are not in the group at risk and are left out.
• Example 3: in a food poisoning outbreak after a function, the population at risk is everyone who attended and ate, not everyone in the neighbourhood.

A wrongly chosen population at risk makes a rate look smaller or larger than it is, and it hides the groups with the greatest need.

Chapter Summary

This chapter introduced the fundamental concepts of epidemiology, defining key terms such as epidemiology itself, nutrition epidemiology, disease occurrence, and classifications like endemic, epidemic, and pandemic, as well as the idea of a population at risk. It traced the history and scope of epidemiology, highlighting its evolution and the foundational assumptions that guide its practice. The chapter then outlined the general and specific goals of epidemiology, emphasizing its role in understanding and controlling health-related events in populations. Different categories of epidemiological diseases were examined, including communicable, occupational, acute, chronic, and nutritional deficiency diseases, each with distinct characteristics and implications for public health. The integration of these concepts provides a comprehensive framework for applying epidemiological principles in nutritional contexts and broader health settings.