Dietary fibre, weight gain and risk factors for cardiovascular disease

Common to many cardiovascular disease risk factors (age, obesity, smoking and physical inactivity) are insulin resistance and hyperinsulinaemia, which may, in turn increase the risk for cardiovascular disease. This paper examines the possibility that diet, in particular fibre, also affects insulin secretion. It investigates whether fibre consumption is independently associated with insulin levels, weight gain and other cardiovascular disease risk factors, compared with fat and other dietary components.

Bottom line

Fibre consumption was inversely associated with insulin levels, weight gain and other cardiovascular disease risk factors (central adiposity, blood pressure, HDL and LDL cholesterol, fibrinogen and triglycerides).

Reference

DS Ludwig et al. Dietary fiber, weight gain, and cardiovascular disease risk factors in young adults. Journal of the American Medical Association 1999 282: 1539-1546.

Study

Participants were 2,909 healthy black and white adults, from four US states, aged 18 to 30 years, from the Coronary Artery Risk Development in Young Adults Study. They were followed for ten years (1986 to 1996) with follow-up examinations at years two, five, seven and ten. Excluded were those with extreme values of dietary intake; individuals with diabetes; those taking medication for blood pressure or lipid control; those who had not fasted for at least eight hours before the clinic visit; and women who were lactating or pregnant.

Before each examination participants were asked to fast for 12 hours and to avoid smoking and heavy physical activity for two hours. The following were measured: body mass index; waist-to-hip ratio; fasting insulin, two-hour insulin; systolic and diastolic blood pressure; high- and low-density lipoprotein cholesterol; fibrinogen; and triglycerides.

Diet was assessed at years 0 and 7 with an interviewer-administered food frequency questionnaire. Energy intake, alcohol intake, dietary fibre, percentage of daily energy intake from saturated fats, unsaturated fats, carbohydrates and protein were measured. Dietary intake was divided into quintiles.

Results

Results were adjusted for age, gender, education, energy intake, vitamin supplementation use, smoking, alcohol intake and physical activity. Associations between diet and insulin levels were also adjusted for body mass index. To examine whether the associations between diet and risk factors may be mediated by insulin level, these results were adjusted for fasting insulin.

All results compared the lowest with the highest quintile of dietary intake.

Body weight

In white adults: body weight was inversely associated with fibre intake (lowest quintile 78.3 kg; highest quintile 75 kg) and carbohydrate intake and positively associated with protein intake. Fat consumption was not associated with body weight.
In black adults: body weight was inversely associated with fibre (lowest quintile 83.5 kg; highest quintile 79.9 kg) and carbohydrate and positively associated with total fat, but not saturated fat.
Fibre remained independently associated with body weight after adjustment for carbohydrate and protein in white adults and fat and carbohydrate in black adults.
At all levels of fat intake, individuals eating the most fibre gained less weight than those eating the least fibre. In white adults, the largest difference in weight gain was seen comparing those with the lowest fibre and fat consumption to those with the greatest fibre and fat consumption (9.59 kg versus 5.72 kg).

Waist-to-hip ratio

Fibre intake was inversely associated with waist-to-hip ratio in both white adults (lowest quintile 0.813; highest quintile 0.801) and black adults (lowest quintile 0.809; highest quintile 0.799). Fat consumption was not associated with waist-to-hip ratio.

Insulin

In white adults: fasting insulin was inversely associated with fibre intake (lowest quintile 11.2 µU/mL; highest quintile 10.4 µU/mL) and positively associated with saturated fat intake. When included in the same analytical model, fibre remained significantly associated with fasting insulin; fat did not.
In black adults: only fibre was associated with fasting insulin (lowest quintile 13.3 µU/mL; highest quintile 11.9 µU/mL).
Fibre, but neither total nor saturated fat, was associated with two-hour insulin in both white adults (lowest quintile 37.6 µU/mL; highest quintile 33.8 µU/mL) and black adults (lowest quintile 53.3; highest quintile 37.4 uU/mL).

Other risk factors

In white (but not in black) adults, fibre was associated with the following (quoting lowest quintile; highest quintile): systolic blood pressure (109.1; 106.9 mm Hg); diastolic blood pressure (72.4; 69.7 mm Hg); triglycerides (88.5; 80.5 mg/dL); high-density lipoprotein cholesterol (46.5; 49.0 mg/dL); low-density lipoprotein cholesterol (112.8; 108.0 mg/dL); and fibrinogen (264; 248 mg/dL).
After adjusting for fasting insulin these associations were no longer apparent, with the exception of diastolic blood pressure.
Fat was not associated with any of these cardiovascular risk factors.

Comment

Fibre was consistently associated with insulin levels, weight gain and other cardiovascular disease risk factors, whereas fat was not, which raises the interesting possibility that fibre may play a greater role in determining cardiovascular disease risk than either total or saturated fat.

Furthermore, the results suggest that high-fibre diets may protect against obesity and cardiovascular disease by lowering insulin levels. Fibre reduces insulin secretion by slowing the rate of nutrient absorption after a meal. In this study, individuals eating the most fibre gained the least weight. Although fibre could influence body weight in ways unrelated to insulin, e.g. by promoting satiety, the association between fibre and insulin remained after adjusting for body mass index. This suggests that higher insulin levels in those consuming low-fibre diets did not result from excessive weight alone. Secondly, this study found that fasting insulin attenuated the associations between fibre and blood pressure, lipids and fibrinogen, providing support for an intermediary role of insulin.

There are several details needing comment.

Preliminary analysis found that associations were similar for men and women, hence their results were pooled.
Although a study such as this cannot prove causality because high-fibre and low-fat diets are typically associated with a generally healthy lifestyle, this study did adjust for several other health behaviours, e.g. vitamin supplementation use, smoking, alcohol intake and physical activity.
There is the possibility that fat could have been less accurately measured than fibre. However, this study used a standardised, validated food frequency questionnaire which included approximately 700 foods.
Results differed between white and black adults, raising the possibility that racial differences exist; although more likely is that the food frequency questionnaire is less suited to the black population (its validity was reported to be lower for black than for white adults).
This study did not examine the effects of soluble versus insoluble fibre and the extent to which the reduced risk of cardiovascular disease observed with diets high in fruit, vegetables, wholegrains, etc. is attributable to the high fibre content and/or the substances within them, such as antioxidants and phytochemicals.