High circulating levels of insulin-like growth factor-I (IGF-I) are associated with
an increased risk of developing prostate (Chan et al, 1998; Harman et al, 2000; Stattin
et al, 2000; Chokkalingam et al, 2001) and other cancers (Holly et al, 1999). In serum
and body fluids, IGF-I's activity is regulated by a complex system of six binding
proteins and an acid-labile subunit. Most (90%) circulating IGF-I is bound to IGF
binding protein-3 (IGFBP-3) and associations of IGFs with prostate cancer are generally
strongest with the molar ratio (IGF-I/IGFBP-3) or in statistical models controlling
for IGFBP-3 (Chan et al, 1998; Harman et al, 2000; Stattin et al, 2000; Chokkalingam
et al, 2001). Raised levels of bioavailable IGF-I may, therefore, increase cancer
risk, and raised IGFBP-3, by reducing IGF-I's bioavailability, may reduce risk.
IGF-I plays a role in energy and protein metabolism as well as modulating cell turnover
and apoptosis (Thissen et al, 1994; Holly et al, 1999). Energy restriction leads to
reduced production of IGF-I (Thissen et al, 1994), and animal experiments suggest
that this pathway mediates the cancer-protective role of diet restriction (Dunn et
al, 1997). Other dietary influences on IGFs may underlie some of the diet–prostate
cancer associations observed. While no specific food or nutrient is an established
risk factor for prostate cancer, dietary aspects most consistently related to its
risk are red meat, animal fat, calcium and dairy product consumption and lower consumption
of vegetables (Kolonel, 1996; World Cancer Research Fund, 1997; Department of Health,
1998). Diets rich in tomatoes, a major source of the carotenoid lycopene, are associated
with reduced risk (Giovannucci, 1999).
Several, generally small, cross-sectional studies have examined the association of
diet with the IGF axis (Darling-Raedeke et al, 1998; Kaklamani et al, 1999; Allen
et al, 2000; Signorello et al, 2000; Mucci et al, 2001; Holmes et al, 2002; Giovannucci
et al, 2003). The largest investigation (n=1037) (Holmes et al, 2002) reported that
higher levels of energy, protein and milk intake were associated with raised IGF-I
and high fat intake with low IGFBP-3, broadly consistent with previous, smaller, studies
(Kaklamani et al, 1999; Ma et al, 2001). Other studies have reported reduced levels
of IGF-I with tomato consumption (Mucci et al, 2001) and vegan diets (Allen et al,
2000). There has been only one (Giovannucci et al, 2003) large-scale investigation
of the association of diet with IGF in community-based men.
MATERIALS AND METHODS
Within a case–control study nested in a population-based investigation into the early
detection and management of prostate cancer (ProtecT; Donovan et al, 2002), stored
blood samples from 368 disease-free men (controls) were assayed for IGF-I and IGFBP-3.
Controls were matched to cases on age, general practice and date of recruitment. Included
in this analysis are the 344 (95%) of these disease-free men who completed a 114-item
validated food-frequency questionnaire (FFQ) (Bingham et al, 1997). Over two-thirds
(n=242) of the men also provided information on occupation, smoking and physical activity
and had height and weight measured. Ethical approval was obtained from the relevant
multicentre and local research ethics committees.
Based on FFQ responses, and using standard food tables (The Royal Society of Chemistry
and MAFF, 1991) and portion size data for men of this age (Ministry of Agriculture
and Food, 1993), we estimated weekly consumption of: energy, carbohydrate, protein,
total fat, saturated and polyunsaturated fat, calcium, red meat, dairy products, vegetables,
milk, tomatoes and foods containing tomatoes (baked beans, tomato ketchup and tomato
juice). These were selected on the basis of research findings and reviews examining
associations of diet with prostate cancer (World Cancer Research Fund, 1997; Giovannucci,
1999; Chan and Giovannucci, 2001) and the IGF-axis (Ma et al, 2001; Holmes et al,
2002). Given difficulties in measuring lycopene from FFQs (Kristal and Cohen, 2000),
we used the frequency of reported consumption of tomatoes and products with high tomato
content.
Laboratory methods
Non-fasted blood specimens, taken using standard techniques, were spun and frozen
to −80°C within 18 h. For the IGF-I assays, an ELISA kit was used (Diagnostic Systems
Laboratories, TX, USA). Assays for serum IGFBP-3 used a previously validated ‘in-house’
double antibody radioimmunoassay (Cheetham et al, 1998). The average coefficients
of variation for intra-assay variability for IGF-I and IGFBP-3 were 3 and 3.6%, and
for inter-assay variation were 15 and 14%. To measure (crudely) bioavailable IGF-I,
we multiplied the molar ratio of IGF-I/IGFBP-3 by 5.33 (molecular weights 40 000 and
7500 Da, respectively).
Statistical analysis
Using Stata (Stata Corporation, 2001) we calculated age-, centre- and energy-adjusted
levels of IGF-I, IGFBP-3 and the molar ratio in quartiles of the distribution of each
dietary factor. Intakes of individual food groups were considered in three a priori
categories. Adjustment for overall energy intake means that dietary measures relate
to dietary composition rather than absolute intake (Willett, 1998).
Least-squares linear regression models investigated change in growth factor levels
for a one standard deviation increase in each dietary factor. Log or square-root transformations
were used for the latter due to positive skewness, and sampling weights adjusted for
the dependence on the age distribution of cases. Tests for trend were based on the
continuous variable (for nutrients and food groups) or three-level category (for tomato
products and milk).
We assessed possible confounding by exercise, smoking, body mass index (BMI) and socioeconomic
position in the 242 men with complete data.
RESULTS
Mean age was 62.2 years (range 50–70) and most men (90%) were nonsmokers and came
from nonmanual social classes (64%). Mean (s.d.) blood levels of IGF-I, IGFBP-3 and
the molar ratio (IGF-I/IGFBP-3) were 126.6 ng ml−1 (36.9), 3393.6 ng ml−1 (1049.8)
and 0.21 (0.08), respectively. Median daily intakes were as follows: energy: 10.3 MJ;
carbohydrate: 314.8 g; protein: 89.5 g; fat: 77.6 g; red meat: 46.9 g; dairy products
344.9 g; calcium 1126.3 g; vegetables 271.0 g.
Raised IGF-I levels were seen in men consuming higher levels of polyunsaturated fat
(P
trend=0.017) and calcium (P
trend=0.035) (Table 1
Table 1
Age-, centre- and energy-adjusted levels of IGF-I, IGFBP-3 and IGF-I/IGFBP-3 molar
ratio in relation to quartiles of increasing intake of dietary variables (n=344)a
Quartile
1 (low intake)
2
3
4 (high intake)
Change in growth factor (95% CI) per s.d. increase in each dietary variable
P-value (linear trend)b
IGF-I (ng ml−1)
Energy intake (MJ)
119.9
133.8
129.3
125.8
1.2 (−3.6 to 5.9)
0.63
Carbohydrates (g)
109.2
128.6
129.6
141.5
10.0 (−1.2 to 21.2)
0.08
Protein (g)
115.7
137.4
131.5
124.9
2.6 (−6.5 to 11.8)
0.57
Fat (g)
121.1
125.4
125.3
137.2
5.0 (−4.6 to 14.6)
0.30
Polyunsaturated fat (g)
114.4
122.6
136.2
136.1
9.1 (1.6 to 16.5)
0.017
Saturated fat (g)
125.3
121.4
128.5
133.9
1.8 (−6.5 to 10.2)
0.67
Red meat (g)
125.2
134.2
135.3
117.8
−2.7 (−8.3 to 2.9)
0.35
Dairy products (g)
117.8
130.7
125.6
134.1
4.4 (−0.8 to 9.7)
0.09
Calcium (mg)
120.5
119.1
137.2
131.8
6.5 (0.5 to 12.5)
0.035
Vegetables (g)
134.1
125.9
122.8
126.5
−2.1 (−8.3 to 4.1)
0.50
IGFBP-3 (ng ml−1)
Energy intake (MJ)
3281.6
3479.5
3494.2
3328.1
15.4 (−101.6 to 132.4)
0.80
Carbohydrates (g)
3017.2
3423.7
3541.0
3598.2
184.5 (−131.6 to 500.7)
0.25
Protein (g)
3120.7
3408.3
3651.0
3395.1
150.0 (−107.2 to 407.2)
0.25
Fat (g)
3564.5
3387.7
3299.6
3335.9
−85.8 (−329.8 to 158.3)
0.49
Polyunsaturated fat (g)
3272.0
3288.7
3357.0
3667.2
228.1 (−1.2 to 457.4)
0.05
Saturated fat (g)
3465.4
3514.0
3444.0
3162.4
−170.4 (−374.7 to 33.9)
0.10
Red meat (g)
3469.4
3389.2
3521.4
3196.7
−78.0 (−196.5 to 40.4)
0.20
Dairy products (g)
3348.8
3450.2
3385.1
3396.2
3.9 (−139.4 to 147.3)
0.96
Calcium (mg)
3345.5
3326.7
3540.2
3368.1
81.2 (−95.6 to 258.1)
0.37
Vegetables (g)
3164.8
3637.4
3401.5
3374.8
93.8 (−55.7 to 243.4)
0.22
IGF-I : IGFBP-3 molar ratio
Energy intake (MJ)
0.206
0.221
0.210
0.213
−0.001 (−0.012 to 0.010)
0.85
Carbohydrates (g)
0.202
0.214
0.209
0.226
−0.009 (−0.034 to 0.016)
0.48
Protein (g)
0.204
0.231
0.204
0.213
−0.002 (−0.021 to 0.018)
0.88
Fat (g)
0.193
0.208
0.214
0.235
0.011 (−0.007 to 0.029)
0.23
Polyunsaturated fat (g)
0.200
0.212
0.231
0.208
−0.002 (−0.017 to 0.013)
0.84
Saturated fat (g)
0.208
0.196
0.210
0.238
0.012 (−0.006 to 0.030)
0.19
Red meat (g)
0.204
0.224
0.221
0.209
0.001 (−0.008 to 0.011)
0.82
Dairy products (g)
0.205
0.209
0.212
0.225
0.007 (−0.003 to 0.018)
0.18
Calcium (mg)
0.207
0.204
0.218
0.222
0.006 (−0.008 to 0.019)
0.39
Vegetables (g)
0.236
0.206
0.201
0.209
−0.011 (−0.022 to −0.000)
0.045
a
All values are controlled for age, study centre and energy intake, except that for
energy intake, which is controlled for age and study centre only. All models are weighted
by the inverse of the sampling probability in relation to age.
b
Tests for trend based on continuous measure of diet.
). There were weaker positive associations with carbohydrate and dairy products. IGFBP-3
levels were weakly positively associated with polyunsaturated fats (P
trend=0.05) and inversely associated with saturated fats (P
trend=0.10). The molar ratio was inversely related to vegetable intake (P
trend=0.045).
Controlling for BMI, social class, smoking and exercise attenuated the associations
of IGF-I and IGFBP-3 with carbohydrates, polyunsaturated fats and, to a lesser extent,
vegetables (not shown). Associations of dairy products and calcium with IGF-I and
saturated fat with IGFBP-3 were not confounded.
IGF-I tended to be lower and IGFBP-3 higher in those who ate tomatoes or tomato-containing
products more frequently, although evidence for a trend was only clear for IGF-I/IGFBP-3 molar
ratio (Table 2
Table 2
Age-, centre- and energy-adjusted mean levels of IGF-I, IGFBP-3 and IGF-I/IGFBP-3
molar ratio in relation to increasing levels of intake of tomatoes, tomato-rich products
and milka
Weekly consumption
IGF-I (ng ml−1) (no. with data)
<Once per week
1–4 times per week
5+ times per week
P-trend
Tomatoes (n=342)
139.2 (n=45)
125.3 (n=232)
126.3 (n=65)
0.19
Baked beans (n=341)
129.3 (n=139)
126.8 (n=196)
119.6 (n=6)
0.55
Tomato ketchup (n=343)
130.3 (n=233)
121.6 (n=100)
120.4 (n=10)
0.10
Tomato juice (n=343)
127.9 (n=324)
122.9 (n=14)
87.0 (n=5)
0.14
Daily consumption
<½ pint
½−3
4 pint
1+ pints
Milk (n=342)
117.8 (n=95)
126.2 (n=173)
140.7 (n=74)
0.004
Weekly consumption
IGFBP-3 (ng ml−1)
<Once per week
1–4 times per week
5+ times per week
P-trend
Tomatoes (n=342)
3648.8
3322.2
3504.9
0.70
Baked beans (n=341)
3377.9
3424.4
3743.0
0.63
Tomato ketchup (n=343)
3275.7
3732.1
2968.9
0.09
Tomato juice (n=343)
3383.1
3958.7
2715.9
0.47
Daily consumption
<½ pint
½−3
4 pint
1+ pints
Milk (n=342)
3343.9
3394.6
3465.7
0.62
Weekly consumption
IGF-I/BP-3 molar ratio
<Once per week
1–4 times per week
5+ times per week
P-trend
Tomatoes (n=342)
0.221
0.214
0.204
0.28
Baked beans (n=341)
0.214
0.212
0.177
0.64
Tomato ketchup (n=343)
0.224
0.187
0.223
0.005
Tomato juice (n=343)
0.214
0.178
0.177
0.004
Daily consumption
<½ pint
½−3
4 pint
1+ pints
Milk (n=342)
0.206
0.209
0.230
0.17
a
All values are controlled for age, study centre and energy intake and are weighted
by the inverse of the sampling probability in relation to age.
). Men consuming higher levels of milk had raised levels of IGF-I (P
trend=0.004). There was no association between milk intake and IGFBP-3 and the molar
ratio was highest in those men drinking at least one pint of milk per day. These associations
were little changed after adjustment for BMI, social class, smoking and exercise (not
shown). Associations of IGF-I with milk were not confounded by calcium intake, whereas
associations with calcium intake were attenuated in models controlling for milk intake.
Excluding men (n=95) reporting low levels of energy intake in relation to their estimated
basal metabolic rate (ratio of energy intake/basal metabolic rate <1.2 (Joint FAO/WHO/UN
Expert Consultation, 1985)) did not change the associations with milk intake, tomato-rich
products or vegetable intake.
DISCUSSION
In a group of healthy, community-sampled, middle-aged men, we found associations of
the IGF-axis with several aspects of diet linked previously to prostate cancer. Positive
relations were seen with dairy products, milk and calcium intake, all of which were
associated with raised IGF-I levels. High intakes of vegetables and tomatoes or tomato-containing
products were associated with lower levels of IGF-I or its molar ratio. In contrast
to some other studies (Kaklamani et al, 1999; Holmes et al, 2002), we found only weak
associations with saturated fat and no evidence of an association with red meat.
Associations were not confounded by socioeconomic position or lifestyle. While we
have examined associations with a range of dietary variables and three different measures
of the IGF-axis, thereby increasing the possibility of chance results, our findings
are consistent with previous studies. In a cross-sectional study, it is not possible
to determine whether dietary associations arise as the result of long-term intake
of particular foods/nutrients or reflect patterns of intake around the time of blood
sampling.
Association of IGF-I levels with dairy products, milk and calcium are consistent with
some (Heaney et al, 1999; Ma et al, 2001; Holmes et al, 2002) but not all (Mucci et
al, 2001) previous analyses. The strongest evidence of a causal association between
higher levels of milk consumption and IGF comes from a randomised trial of dietary
milk supplementation, reporting a rise in IGF-I in those supplemented but not the
controls (Heaney et al, 1999). While some research suggests that neonates absorb IGF-I
from breast milk (Diaz-Gomez et al, 1997), there is no strong evidence that bovine
IGF-I in cows milk could be similarly absorbed from the gut (Holmes et al, 2002).
Dietary intake of animal protein (essential amino acids) is known to stimulate IGF-I
production (Thissen et al, 1994), but we found no evidence of associations with animal
protein intake, nor that controlling for animal protein intake attenuated associations
with milk (not shown). This contrasts with the findings of Giovannucci et al (2003)
and Holmes et al (2002). In Giovannucci et al's analysis, associations with vegetable
protein were, however, of similar magnitude to those for animal protein.
Associations of calcium, milk and dairy products with IGF-I suggest a possible pathway
linking dietary intake of these factors with prostate cancer (Chan and Giovannucci,
2001). The relation of these dietary aspects with prostate cancer risk are, however,
in the opposite direction to their association with colorectal cancer (Ma et al, 2001;
Wu et al, 2002), another neoplasm associated with raised IGF-I levels (Ma et al, 1999).
The IGF–cancer associations seen for a range of different cancer sites (Holly et al,
1999; Yu and Rohan, 2000) may not therefore be explained in terms of common dietary
influences on the growth factor axis. Nevertheless, our finding that vegetable intake
was weakly related to lower molar ratios is consistent with the observation that vegetable-rich
diets appear to protect against colorectal, breast and prostate cancer (World Cancer
Research Fund, 1997), although associations of vegetable intake with IGF-I or IGFBP-3
have not been found in other studies (Kaklamani et al, 1999; Holmes et al, 2002).
The weak associations of IGFs with tomatoes and tomato-containing products support
those reported for 112 Greek men (Mucci et al, 2001), where a strong inverse association
was found between cooked tomato consumption and IGF-I. Likewise, in the Nurses Study
intake of lycopene was positively associated with circulating levels of IGFBP-3 (but
not IGF-I). These findings hint at the possible importance of the IGF axis in mediating
the protective effect of higher levels of tomato or lycopene intake on prostate cancer
reported in several investigations (Giovannucci, 1999). A possible biological mechanism
lies in the reported inhibitory effects of lycopene on IGF-I receptor signalling and
cell cycle progression (Karas et al, 2000), but a small trial of lycopene supplementation
found no difference in IGF-I levels in supplemented vs control subjects (Kucuk et
al, 2001).
International comparisons of cancer incidence and changes in incidence in migrants
moving between different continents, indicates large dietary influences on epithelial
cancer incidence (World Cancer Research Fund, 1997). Our study adds to evidence that
aspects of diet previously linked to prostate cancer may influence cancer risk through
the IGF-axis. Trials of dietary interventions aimed at reducing bioavailable IGF-I
are now required. Identification of relevant aspects of diet could then lead to trials
of dietary interventions against cancer incorporating measurements of IGF-I.