Circulating Insulin-Like Growth Factor I (IGF-I) has been associated with higher risk of several common types of cancer, such as prostate, breast, and colorectal cancer^{(Reference Pollak1)}, and dietary intake has been suggested to influence IGF-I concentrations^{(Reference Crowe, Key and Allen2)}. However, previous observational studies investigating the association of nutrients with circulating IGF-I have been limited by small sample sizes. We assessed the association of macronutrients and fibre intake with circulating IGF-I concentrations in an observational analysis in the UK Biobank; a large cohort of British adults.

In this analysis, participants were selected if they completed at least four (maximum of five) web-based 24-hour dietary assessments³ and had serum IGF-I measured (n = 12,000). Usual macronutrient and fibre intakes were determined by the mean of completed 24-hour dietary assessments. Multivariable linear regression was used to assess the associations of these dietary factors with circulating IGF-I. Sensitivity analyses were conducted in participants who had a second blood sample five years after recruitment, using the mean of the two IGF-I measurements (n = 2,581).

The mean circulating IGF-I concentration was 21.96 nmol/L. Consumption of 2.5% higher energy intake from total protein, dairy protein, milk protein, and yogurt protein were associated with 0.57 (95% confidence interval (CI): 0.47–0.66), 0.71 (95% CI:0.50–0.92), 1.17 (95% CI:0.87–1.48), and 1.33 (95% CI:0.80–1.85) nmol/L higher circulating concentrations of IGF-I respectively, whereas cheese protein was not associated with IGF-I concentration. A 5 gram/day higher intake of fibre was associated with 0.46 (95% CI:0.35–0.57) nmol/L higher concentration of IGF-I. When analyses were restricted to participants with two blood samples, results remained largely the same, with milk protein being most strongly associated with the average measurements of IGF-I concentration. Carbohydrates and fats were not materially associated with IGF-I concentrations.

We found that dairy protein was the macronutrient most strongly associated with circulating IGF-I concentrations. However, this association varied by dairy protein source, with the strongest association being for milk and yogurt protein. Moreover, dietary fibre intake was also positively associated with IGF-I, which warrants further investigation.