We used the DPS data to assess the association between dietary composition and change in weight and diabetes risk. The analyses revealed that a decrease in dietary fat and energy density and an increase in fibre density were associated with sustained weight reduction in a dose-dependent manner. Furthermore, low-fat and high-fibre intakes predicted decreased diabetes risk independently of body weight change and physical activity.
A debate about optimal diet for weight reduction prevails. Studies with strictly controlled energy intake have shown that weight reduction can be achieved with any dietary regimen as long as energy intake is lower than energy expenditure [21–24]. Permanently reduced energy intake, which is essential after weight reduction unless energy expenditure by physical activity is clearly increased, is more difficult to achieve . Our results offer support for the validity of the current recommendations to reduce fat and increase fibre-rich carbohydrate intake, to achieve sustained long-term weight reduction.
Reduction of fat intake has for several decades been the conventional approach for weight control. According to a review by Astrup et al. , a 4–5 kg weight loss can be achieved with 10 E% reduction in dietary fat in obese individuals, at least in short-term (<1 year) studies. Fat contains more energy per gram than other energy-yielding nutrients and also makes the diet palatable, but simultaneously less satiating, and therefore a high-fat diet is believed to promote weight gain [5, 6]; however the issue is controversial . In recent studies [27–29] a low-carbohydrate diet resulted in more pronounced short-term weight reduction compared with a conventional low-energy, low-fat dietary regimen, probably because the achieved energy deficit was larger. However, the statistically significant difference in early weight loss between the groups disappeared at 12 months [27, 30, 31], indicating that individuals are not able to adhere to such a restricted diet. Moreover, the long-term safety of low-carbohydrate diets in weight-stable individuals is not known .
Still another popular, and evidently efficient [22, 31] weight-loss approach is to increase protein intake up to 30% of total energy, with a decrease in either fat or carbohydrate. An increase in protein intake may increase patient satisfaction during a low-fat, energy-restricted diet  and has been shown to accelerate weight loss  and prevent weight regain . In our study we emphasised adequate protein intake (0.8 g/1 kg body weight) but did not advise to increase it more than that. It is impossible to say whether advocating an increase in protein would have led to larger weight reduction than was achieved in our study. We did, however, also analyse the association of the protein intake (E%) quintile with weight reduction and diabetes incidence, but did not see any significant effects (data not shown).
The intake of dietary fibre has been shown to be inversely correlated with body weight and weight reduction [8, 35, 36]. Fibre may increase satiation directly by increasing gastric distension and thereby giving a feeling of fullness, or indirectly through secretion of gut hormones (incretins), and satiety through delaying gastric emptying and nutrient absorption. Furthermore, fibre may decrease dietary energy intake by reducing absorption of fat and protein . In the present study an increase in dietary fibre intake was also associated with a reduced waist circumference, independently of weight change. A similar finding was previously seen in an observational study on American male health professionals . The mechanism explaining the inverse association between waist circumference and fibre intake is not clear. The effect of dietary fibre may be mediated by changes in insulin resistance. High-fibre intake has been shown to be associated with enhanced insulin sensitivity [38–40]. On the other hand, insulin resistance may, as discussed by Kahn and Flier  contribute to the development of obesity. Smoking is known to increase central obesity . In our study, smoking was remarkably uncommon, and thus did not confound the association between fibre intake and waist circumference. The low rate of smoking may be explained by the fact that the study participants were volunteers and probably more health-conscious than the general population.
A recently recognised factor potentially influencing nutrient intake is the energy density of the diet . People have been shown to be only partly able to compensate for changes in dietary energy density by increasing or decreasing the total amount of food, at least in short-term clinical studies [44, 45]. In our analysis, dietary energy density was highly significantly associated with sustained weight reduction; on the other hand, energy density is a composite factor of dietary fat, fibre and water. Weight reduction among the low-fat/high-fibre consumers was threefold compared with the high-fat/low-fibre group, indicating that to achieve the best results one should make extensive changes in dietary pattern, rather than focusing on any single nutrient.
Obesity is a well-established risk factor for type 2 diabetes, and we have previously shown that in the DPS population diabetes risk was reduced dose-dependently with weight reduction . Obesity increases insulin resistance, and in a subgroup analysis of the DPS study population a strong inverse correlation was found between the 4-year weight change and insulin sensitivity . In addition, according to the present analyses the composition of the diet affected diabetes risk independently of weight change. Several epidemiological studies have shown that low intake of total fat [9, 10], saturated fat [10, 11] and high intake of cereal, cereal fibre and total fibre [12–15], are associated with decreased diabetes risk during the follow-up period. In most of these studies, dietary changes during the follow-up period were typically not taken into account. In our study, the participants were advised to make changes in their diet, and dietary intake was monitored continually. Furthermore, the association between nutrient intakes during the follow-up and diabetes risk was adjusted for baseline nutrient intakes.
Type of fat (namely, high saturated and trans-fatty acid and low unsaturated fatty acid intake), rather than total fat intake, has in several studies been associated with increased diabetes risk . In our study the total and saturated fat intakes were highly correlated (r=0.81) and the intake of mono-unsaturated fat remained relatively stable throughout the study. Furthermore, the intake of trans-fatty acids was low, on average 0.7 E% (data not shown). This might explain why total fat intake was the fat-related variable most consistently associated with diabetes risk in our analyses.
Our study, like dietary intake trials in general, has several limitations. The dietary intervention was planned to encourage an increase in dietary fibre and a decrease in fat intake, and it is possible that individuals who succeeded in weight reduction were more likely to report consuming ‘the recommended diet’. In addition to dietary changes, the participants were advised to increase physical activity. Therefore, all the models to investigate the effect of dietary composition were adjusted for physical activity at baseline and during the intervention, but some residual confounding might remain. Furthermore, the energy intakes calculated from the food records revealed that under-reporting had taken place. However, this may not be too problematic, because we calculated energy proportions of nutrients and not absolute amounts. Overweight and obese people are known to be even more prone to dietary under-reporting than normal-weight individuals. Such an under-reporting has been shown to be a stable characteristic of an individual , and as the results were adjusted for the baseline, it may not cause a bias in our study. Changes in specific dietary intakes were correlated: those who decreased fat-containing foods increased carbohydrate- and fibre-containing foods, and simultaneously the energy density of the diet decreased. This real-life phenomenon is problematic in statistical analyses. When the predictors entered into a model simultaneously have multicollinearity, they tend to attenuate each other, as also shown in our data. Therefore, instead of calculating one model including all dietary variables, we used separate models, and selected the two most significant predictors for the combined analysis. The highest diabetes risk was seen among the high-fat/high-fibre and not, as would have been expected, among the high-fat/low-fibre consumers. The HRs, however, did not differ statistically significantly and thus the effect may be coincidental, or due to confounding by some unknown factors.
We have shown that a long-term weight reduction during the Finnish DPS was best achieved by reducing energy density and dietary fat and increasing fibre intake. As for the long-term health benefits, our results strongly support the validity of the current dietary recommendations to the general population: reducing the intake of saturated fat and increasing fibre-rich whole-grain cereals and fruit and vegetables, is important, not only in terms of overall health but also for sustained weight reduction and the prevention of type 2 diabetes in overweight individuals.