Study Designs in the Health Sciences

What is a longitudinal study?

"In a longitudinal study subjects are followed over time with continuous or repeated monitoring of risk factors or health outcomes, or both...Most longitudinal studies examine associations between exposure to known or suspected causes of disease and subsequent morbidity or mortality. In the simplest design a sample or cohort of subjects exposed to a risk factor is identified along with a sample of unexposed controls. The two groups are then followed up prospectively, and the incidence of disease in each is measured. By comparing the incidence rates, attributable and relative risks can be estimated."[1]

Why use this type of study?

• evaluating the relationship between risk factors and the development of disease [2]
• evaluating the outcomes of treatments over different lengths of time [2]
• statistical testing may be employed to analyze change over time for both a group as a whole or for  individuals [2]

References

1. The British Medical Journal. Chapter 7. Longitudinal studies. 2020. https://www.bmj.com/about-bmj/resources-readers/publications/epidemiology-uninitiated/7-longitudinal-studies. Accessed November 6,2024.
2. Caruana EJ, Roman M, Hernández-Sánchez J, Solli P. Longitudinal studies. J Thorac Dis. 2015 Nov;7(11):E537-40. doi: 10.3978/j.issn.2072-1439.2015.10.63.

Dietary patterns and cardiorespiratory fitness in midlife and subsequent all-cause dementia: findings from the cooper center longitudinal study. 

Gerber Y, Tesler R, Byker Shanks C, Pettee Gabriel K, Pavlovic A, DeFina LF, Shuval K.

The international journal of behavioral nutrition and physical activity 2024 Sep 27;21(1):109. doi: 10.1186/s12966-024-01663-x.

Background: Identifying lifestyle factors that independently or jointly lower dementia risk is a public health priority given the limited treatment options available to patients. In this cohort study, we examined the associations between Mediterranean or Dietary Approaches to Stop Hypertension (DASH) diet adherence and cardiorespiratory fitness (CRF) with later-life dementia, and assessed whether the associations between dietary pattern and dementia are modified by CRF.

Methods: Data are from 9,095 adults seeking preventive care at the Cooper Clinic (1987-1999) who completed a 3-day dietary record and a maximal exercise test. Alzheimer's disease and related disorders or senile dementia (i.e., all-cause dementia) was identified from Medicare administrative claims (1999-2019). Illness-death models were used to estimate hazard ratios (HRs) and 95% confidence intervals (CIs) for the associations between Mediterranean or DASH diet adherence (primary exposure), CRF (secondary exposure), and all-cause dementia, adjusted for demographic and clinical factors. An interaction term was included between diet score and CRF to assess effect modification by CRF.

Results: The mean age at baseline was 50.6 (standard deviation [SD]: 8.4) years, and a majority of the study sample were men (77.5%) and White (96.4%). 1449 cases of all-cause dementia were identified over a mean follow-up of 9.2 (SD: 5.8) years. Neither Mediterranean nor DASH diet adherence was associated with dementia risk in fully adjusted models (HR per SD of Mediterranean diet score: 1.00, 95% CI: 0.94, 1.05; HR per SD of DASH diet score: 1.02, 95% CI: 0.96, 1.08). However, participants with higher CRF had a decreased hazard of dementia (HR, per metabolic equivalent of task [MET] increase, Mediterranean model: 0.95, 95% CI: 0.92, 0.98; HR, per MET increase, DASH model: 0.96, 95% CI: 0.92, 0.97). No effect modification by CRF was observed in the association between diet and dementia.

Conclusions: In this sample of apparently healthy middle-aged adults seeking preventive care, higher CRF at midlife was associated with a lower risk of all-cause dementia, though adherence to a Mediterranean or DASH diet was not, and CRF did not modify the diet-dementia association. CRF should be emphasized in multimodal interventions for dementia prevention and investigated among diverse samples.