Bioavailability of vitamin C from kiwifruit in non-smoking males: determination of 'healthy' and 'optimal' intakes

Abstract

Vitamin C is an essential nutrient in humans and must be obtained through the diet. The aim of this study was to determine vitamin C uptake in healthy volunteers after consuming kiwifruit (Actinidia chinensis var. Hort. 16A), and to determine the amount of fruit required to raise plasma vitamin C to 'healthy' (i.e. >50 µmol/l) and 'optimal' or saturating levels (i.e. >70 µmol/l). Leucocyte and urinary vitamin C levels were also determined. A total of fifteen male university students with below average levels of plasma vitamin C were selected for the study. Weekly fasting blood samples were obtained for a 4-week lead-in period and following supplementation with, sequentially, half, one, two and three Gold kiwifruit per d for 4-6 weeks each, followed by a final 4-week washout period. The results showed that addition of as little as half a kiwifruit per d resulted in a significant increase in plasma vitamin C. However, one kiwifruit per d was required to reach what is considered healthy levels. Increasing the dose of kiwifruit to two per d resulted in further increases in plasma vitamin C levels as well as increased urinary output of the vitamin, indicating that plasma levels were saturating at this dosage. Dividing the participants into high and low vitamin C groups based on their baseline plasma and leucocyte vitamin C levels demonstrated that it is critical to obtain a study population with low initial levels of the vitamin in order to ascertain a consistent effect of supplementation.

Keywords: DTPA, diethylene triamine pentaacetic acid; Human saturation levels; Kiwifruit supplementation; Leucocytes; Plasma vitamin C; RDI, recommended dietary intake.

Fig. 1.

Study design which consisted of a lead-in phase of 4 weeks, an intervention phase of 20 weeks and a washout phase of 4 weeks. * Fasting blood samples taken. † 24 h urine collection and total leucocyte preparations carried out and when food and beverage diaries completed.

Fig. 2.

(a) Daily fruit and vegetable consumption and (b) vitamin C intake by the study participants. (■), Total fruit and vegetable intake or vitamin C intake; (), total minus kiwifruit intervention. Data are means, with standard errors represented by vertical bars. The numbers of participants are indicated in parentheses. * Mean value was significantly different from that at baseline (P < 0·05; one-way repeated-measures ANOVA with the Fisher least significant deviation pairwise multiple comparison procedure). WO, washout.

Fig. 3.

Weekly plasma vitamin C levels of the combined group (▼; n 14), low vitamin C participants (•; n 7) and high vitamin C participants (■; n 7). Data are means, with standard errors represented by vertical bars. * Mean value was significantly different from that at baseline (P < 0·05; one-way repeated-measures ANOVA with the Fisher least significant deviation pairwise multiple comparison procedure).

Fig. 4.

(a) Plasma vitamin C levels of study participants and (b) urinary excretion of vitamin C by the study participants as a function of daily kiwifruit (KF) intake. Data are means, with standard errors represented by vertical bars. The numbers of participants are indicated in parentheses. * Mean value was significantly different from that at baseline (P < 0·05; one-way repeated-measures ANOVA with the Fisher least significant deviation pairwise multiple comparison procedure). WO, washout. (c) Correlation of plasma vitamin C with urinary excretion of vitamin C. Data points (n 62) were obtained from participants at each stage of the study (baseline, 0·5 KF/d, 1 KF/d, 2 KF/d, 3 KF/d, WO).

Fig. 5.

(a) Leucocyte vitamin C levels of the low vitamin C group as a function of daily kiwifruit (KF) intake. Data are means, with standard errors represented by vertical bars. The numbers of participants are indicated in parentheses. * Mean value was significantly different from that at baseline (P < 0·05; two-tailed paired t test). WO, washout. (b) Correlation of plasma vitamin C with leucocyte vitamin C. Data points (n 58) were obtained from participants at each stage of the study (baseline, 0·5 KF/d, 1 KF/d, 2 KF/d, 3 KF/d, WO). Linear regression analysis provided an R value of 0·374 and a P value of 0·004.