Data Source: The ToothGrowth dataset included in R (ToothGrowth.rds), originally sourced from “The Growth of the Odontoblasts of the Incisor Tooth as a Criterion of the Vitamin C Intake of the Guinea Pig: Five Figures” (Crampton, 1947).
Background: This dataset summarizes the results of a 1947 experiment which measured the effects of vitamin C intake on tooth cell growth in guinea pigs. 60 guinea pigs were included in this experiment. The subjects were randomized into equally-sized treatment arms based on two exposure variables: dose level (0.5, 1.0, or 2.0 mg/day) and dosing method (ascorbic acid “VC” or orange juice “OJ”). While this dataset is called ToothGrowth, the researchers were actually interested in the length of a particular dental cell type, not the teeth themselves. Odontoblasts are long cells found in the dental pulp of vertebrates; because of their length and formation in the teeth, their size could be measured microscopically using methods available at the time of the study. It is important to note that this study was undertaken in order to create and validate a bioassay that could evaluate vitamin C levels in a diet based on odontoblast size, not to determine if different doses of vitamin C have differentially beneficial effects on dental health. Accurate and reliable assays for vitamin C and other nutrients are important tools for assessing the bioavailability of these vitamins/nutrients in different food items and diets, which in turn allows doctors and nutritionists to provide accurate information and guidance on health-focused eating.
Results: In both dosing method groups, higher daily vitamin C intake was found to be associated with increased incisor odontoblast length, with successively higher cell length in each dose category (see “Odontoblast Length by Vitamin C Dose”). When comparing dosing methods, orange juice (OJ) supplementation shows a higher median cell length and smaller range than ascorbic acid (VC) supplementation (see “Odontoblast Length by Dosing Method.”) These relationships are maintained when analyzing the outcome by both exposure variables simultaneously, which may be explored further in more detail in the “Combined Plot Explorer” tab of this dashboard. Based on these analyses, we may conclude that absolute vitamin C dosage does directly affect the length of odontoblasts in a measureable way, making guinea pig odontoblast measurement a valid assay for vitamin C levels in a diet. However, it also appears that the characteristics of the vitamin C contained within a diet (explored here through the dosing method variable) can differentially affect the cell length, meaning that this assay is limited in situations where the chemical source of vitamin C (e.g. L-ascorbic acid vs sodium ascorbate) is not known.