Long-term effects of a reduced fat diet intervention on cardiovascular disease risk factors in individuals with glucose intolerance
Introduction
People with impaired glucose tolerance (IGT) are known to be at high risk of developing type 2 diabetes [1], and have an independent risk of atherosclerosis nearly three times that of people with normal glucose tolerance (NGT), after 5 years of follow up [2]. The rate at which people with IGT die from cardiovascular disease is also higher than normoglycaemic controls [3], [4].
High dietary fat intake has been consistently linked to the development of IGT and diabetes [5], [6]. A high fat diet promotes passive over-consumption of total energy [7] and weight gain is a critical factor in both the development of IGT [8] and its progression to diabetes [9], increasing diabetes risk approximately, 2–12-fold [10].
There is substantial current debate about the impact of reducing total dietary fat. Proponents of ad libitum reduced fat diets maintain that they promote weight loss [11], [12], [13]. Weight loss is known to reduce blood pressure, serum triglycerides, total and LDL cholesterol, blood glucose and increase HDL cholesterol [14], all of which are important for people with IGT [15].
However, metabolic studies with isoenergetic substitution of carbohydrate for fat have produced mixed results on blood lipids. One such study in people with IGT reported adverse changes in triglyceride and HDL cholesterol concentrations [16], and another similar study in people with type 2 diabetes reported persistent elevation of triglyceride levels [17]. However, others have found no adverse effects with isoenergetic studies in obese and/or type 2 diabetes participants [18] or healthy normal weight volunteers [19].
Short-term dietary manipulations that precisely replace energy from fat with carbohydrate may not provide appropriate evidence for recommendations on reducing fat for health gains in free living individuals or populations. Reducing total fat intake whilst maintaining an otherwise ad libitum diet without attempting to replace energy removed, does reduce energy intake [20] and either decreases weight in the overweight or prevents weight gain in normal weight people [21]. A previous report of our glucose intolerant group found that the energy deficit from reduced fat eating was not replaced with carbohydrate [22].
We have previously reported on the 5 year effects of a 1 year intervention for reduced fat eating on blood glucose and glucose tolerance status [23]. A significant positive effect of diet was found on 2 h glucose over a 5 year period (P<0.0001) and on glucose tolerance status at l year (P=0.015). Also the people who were most compliant with the program showed long-term reductions in both fasting (P=0.041) and 2 h (P=0.024) blood glucose levels. We now examine the long-term impact of the reduced fat ad libitum intervention on other known cardiovascular disease risk factors.
Section snippets
Participants
Participants were selected from a previous Workforce Diabetes Survey [24] in which glucose tolerance tests were performed in 4833 workers aged 40 years and over, from 41 work sites around Auckland between 1988 and 1990. One-hundred and forty-five (3%) of those surveyed were classified as having impaired glucose tolerance by the 1985 WHO criteria (2 h blood glucose 7.8–11.0 mmol/l) [25] and a further 114 (2%) had high normal blood glucose concentrations (7.0–7.8 mmol/l). In this combined ‘glucose
Participants
The baseline characteristics of participants who completed the 1 year intervention are presented in Table 1. There were no significant differences found between the groups except that there were more smokers in the reduced fat diet group (P=0.02). Forty people did not complete the 1 year intervention (four died, one became pregnant, seven developed serious illness, four moved out of Auckland and 24 dropped out of the study). More Maori and Pacific Islands participants did not complete the
Discussion
The medium term effects (1 year) of the reduced fat education program were a reduction in fat intake with no compensatory increase in absolute intake of carbohydrate. This resulted in a reduction in total energy intake (84% from fat reduction), weight loss and no change in the P:M:S ratio. Proportionally, more energy was obtained from carbohydrates-, protein- and fibre-rich foods. These changes resulted in overall lower total cholesterol, LDL cholesterol and blood pressure during the 5 year
Acknowledgements
M. Wilson is acknowledged for his support in collection of data and carrying out the intervention. This study was funded in part, by the National Heart Foundation of New Zealand, the Auckland Medical Research Foundation, and the Lotteries Medical Board. P.A. Metcalf was supported by the Health Research Council of New Zealand.
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