Protein intake is associated with plasma insulin-like growth factor (IGF)1 in postmenopausal women but not in premenopausal women

It has been recently reported that older adults between the ages of 50 and 65 who consumed over 20% of their calories as protein had a 75% increase in overall mortality compared with those consuming low to moderate amounts of protein. This was a cross-sectional study aimed at: 1) determining the association between protein intake and insulin-like growth factor (IGF)1 and 2) determining the association between body mass index (BMI) status and age on plasma concentrations of IGF1 in women participating in three independent breast cancer prevention clinical trials (WISER, WISER sister and Minnesota Green Tea Trial (MGTT)). In all three trials, dietary data were collected using the NIH/NCI diet history questionnaire. IGF1 was measured using commercial sandwich enzyme immunoassay kits. Data from the three trials were analyzed separately. Linear regression models were used to examine the association between plasma concentrations of IGF1 and protein intake, BMI status, and age. Protein intake was divided into three categories: low (less than 12% total calories), medium (12–19% of total calories) and high (20% or more total calories). Age was divided into two categories as follows: WISER trial: 18–25 and 26–30 years, WISER sister trial: 18–30 and 31–50 years, MGTT: 50–65 and 65 and older. Data were available from 303, 113, and 935 women in the WISER, WISER sister, and MGTT, respectively. While no associations between protein intake and plasma IGF1 were found in the WISER and WISER sister trials, women in the highest category of protein intake in the green tea trial had significantly higher concentrations of plasma IGF1 (mean ± SE, 97.9 ± 3.1 ng/mL) compared to those in the medium (86.7 ± 1.2 ng/mL, p=.0004) and low (78 ± 3.3 ng/mL, P<0.0001) categories of protein intake, after adjusting for energy intake, animal protein intake, BMI and age. In the WISER trial, the mean plasma IGF1 in women 25–30 years was 358.8 ± 7.7 ng/mL compared with 411.8 ± 9.4 ng/mL in women younger than 25 years (P<0.0001). Similarly, in WISER sister, the mean plasma IGF1 in women 31–50 years was 96.5 ± 3.2 ng/mL compared with 123.5 ± 6 ng/mL in women 18–30 years (P<0.0001). In the MGTT, the mean plasma IGF1 in women older than 65 years was 83.4 ± 1.9 ng/mL compared with 90.5 ± 1.1 ng/mL in women 50–65 years (P=0.0007). We also found that the mean plasma IGF1 of obese women was lower than that of normal weight women in the WISER sister trial (118.9 ± 3.9 ng/mL in normal weight women compared to 94.2 ± 6.2 ng/mL in obese women, P=0.0006) and in the green tea trial (90.7 ± 1.3 ng/mL in normal weight women compared to 81.4 ± 2.4 ng/mL in obese women, P=0.0004). No significant differences in plasma IGF1 concentrations were found between obese and normal weight women in the WISER trial. Our findings indicate that while dietary intake of protein is not associated with plasma IGF1 levels in women younger than 50 years, a significant association exists in women older than 50 years. Similar to previous reports, plasma IGF1 concentrations were lower in older women and in those with higher BMI. Additional research to assess the relationship between amount and type of dietary protein and IGF1 concentrations is needed to better understand how protein intake may affect mortality.