This prospective clinical study investigated the contribution of diet in developing GDM in a comprehensive way. Our study revealed that a higher adherence to the heathier dietary pattern was associated with a lower risk of GDM, and a high inflammatory potential of the diet (measured by E-DII) was associated with an increased risk of GDM. In the evaluation of the intakes of nutrients, higher intakes of total fat, SFAs, and trans fatty acids were significant predictors for an elevated GDM risk. In contrast to expectations, we found no association between eating frequency or overnight fasting time and the onset of GDM.
Our study, together with previous reports, indicates that a healthy diet reduces the risk of GDM. We found that a higher adherence to the healthier dietary pattern characterized, for example, by rye bread, vegetables, fruits, and berries was associated with a decreased risk of developing GDM. In a previous study  in 168 normal weight, overweight, and obese women, a prudent dietary pattern, including vegetables, seafood, and pasta, was associated with a decreased risk of GDM. Additionally, the relationship was confirmed when only women with overweight or obesity were included in the analysis. Further support is evident from a prospective cohort study , in which four different dietary patterns during pregnancy (traditional, sweet food, fried food-beans, and whole grain-seafood) were identified from data collected from 1014 women. Of these, a traditional pattern, resembling the healthier pattern of our study, characterized by vegetables, fruits, and rice was associated with a decreased risk of GDM. Additionally, in another large (n = 3853) study , four pre-pregnancy dietary patterns (Meats, snacks and sweets, Mediterranean-style, Cooked vegetables, Fruit, and low-fat dairy) were identified. They found that a Mediterranean pattern, including increased consumption of vegetables, nuts, and rye bread, was associated with a decreased risk of GDM. These findings altogether support the idea that an overall healthy diet is of importance in decreasing the risk of developing GDM, although, compared to our study (36 E%), fat intake was higher (40 E%), e.g., in the Mediterranean diet pattern . Interestingly, in our study, it was found that higher adherence (4th quintile), but not the highest adherence (5th quintile) to the healthier diet, was associated with a lower GDM risk. The reason for the lack of association for the highest quintile remains unknown, calling for further investigations. Also, we did not detect any difference in an overall dietary quality, as measured by IDQ, between the women developing GDM and those not developing GDM. Although IDQ is a validated tool, it measures an overall diet quality by a short stand-alone questionnaire in reference to diet recommendations, whilst dietary patterns were determined based on the intakes of food groups calculated from food diaries, thus likely providing more detailed description of the dietary intake.
Considering the mechanisms behind the pathophysiology of GDM, it seems that low-grade inflammation as a mediator for insulin resistance  could contribute to the onset of GDM, particularly in overweight and obese women, as obesity promotes inflammation in the body and is a known risk factor for GDM . Diet may act as a major contributor, because various foods and nutrients are associated with elevated levels of inflammatory markers . The findings from our study support this concept as a higher E-DII score, i.e., a more inflammatory diet, was found to be associated with an elevated GDM risk. Our finding is in accordance with two prior studies, indicating that a higher DII score is associated with an increased risk of GDM  particularly in women with overweight or obesity . The lower inflammatory potential of foods could partially explain the observed relation between the healthier dietary pattern and a reduced risk of GDM in our study . Food groups, such as rye bread, vegetables, fruits and berries, fish, margarine high in unsaturated fatty acids, and vegetable oils, had high loadings in the healthier pattern. These food groups are rich in vitamins, minerals, fiber, and/or unsaturated fatty acids, and thus have a low inflammatory potential. Indeed, intakes of vitamin C, folic acid, and magnesium, nutrients with lower inflammatory potential [44,45,46], were higher in women not developing GDM compared to women developing GDM, although they were not associated with a risk of GDM in the logistic regression models. Interestingly, E-DII and DII correlated negatively with IDQ, again indicating that an overall healthier diet also is less inflammatory. Another mechanism how nutrients could affect the development of GDM is their effects on glucose and insulin metabolism as these are important factors in the pathophysiology of GDM . For example, fiber intake has been shown to have beneficial effects on these, as well as body weight and satiety . Although, in our study, only a small difference was seen in the intake of fiber between the women developing and not developing GDM, the previous studies have indicated that higher fiber intake is associated with a lower risk of GDM [12, 13]. Furthermore, the intake of salt  may unfavorably affect glucose and insulin metabolism, although no difference in salt intake was detected in our study.
In a more detailed examination of diet, we demonstrated that a higher intake of dietary fats is associated with an elevated GDM risk. More closely, the intakes of SFAs, and trans fatty acids were significantly higher in women developing GDM compared to women not developing GDM. Dietary fats could be one explanatory factor behind the relationship between inflammation and the onset of GDM. Total fat, SFAs, and trans fatty acids have a high inflammatory potential ; thus, they are able to increase the presence of chronic, systemic inflammation . Our findings are in line with previous evidence, indicating that the quality of fat is an important factor in the development of GDM [9, 10]. Previous studies have linked a high intake of SFAs in early pregnancy with an elevated risk of developing GDM [9, 10]. Additionally, one study  found that women diagnosed with GDM had higher intakes of SFAs, and trans fatty acids as a percent of energy than women without GDM, and that total fat intake as a percent of energy was associated with an elevated GDM risk. More detailed breakdown of the consumed foods, e.g., based on source of protein, i.e., animal or vegetable protein could yield further insight into the onset of GDM, as indicated by a study in which higher intake of animal protein was associated with an increased GDM risk, while intake from vegetable source was associated with a decreased risk . In another study, intake of total protein and animal protein increased the risk of GDM but no relation was found with intake of vegetable protein . Also, evidence from a meta-analysis indicates that higher levels of circulating branched-chain amino acids may be associated with an increased risk of GDM .
We found no differences in eating frequency or overnight fasting time between the women developing GDM and those not developing GDM. No previous studies have investigated the association between eating frequency and the development of GDM. We hypothesized that eating frequency could contribute to the risk of GDM through its effect on glucose metabolism. Human bodies comply with a 24-h cycle, i.e., circadian clock regulated by light–dark cycle. Various physiological events such as insulin and glucose metabolism follow daily light–dark cycle. It has been shown that eating according to circadian clock, i.e., food intake during light phase and fasting during dark phase, has beneficial effects on the glucose regulation. Also, proper fasting intervals between the meals may enhance glucose homeostasis.  Hence, an adequate night-fasting period in addition to lower numbers of meals per day could decrease the risk of developing GDM through their effects on glucose metabolism. This relationship has previously been postulated by Loy and colleagues in pregnant women . They found that each additional meal was associated with an increased 2-h post-prandial glucose level, while an hourly increase in overnight fasting period was associated with lower fasting glucose in pregnant women. It is of note that population in these two studies differed, which may partly explain the differing study results. It does seem that the relationship between eating frequency and development of GDM requires further investigation.
This study has various strengths; we evaluated a relatively large number of pregnant women in a prospective clinical study setting. This also allowed a detailed collection on women’s background and lifestyle habits, which were taken into account in “Statistical analysis”. Moreover, because the dietary assessment was conducted in early pregnancy, the temporal relationship between dietary intake and the risk of developing GDM was readily observable. We also used multiple tools to assess dietary intake; total energy intake, energy-yielding nutrients, nutrients from dietary supplements, dietary patterns, index of diet quality, and the inflammatory potential of the diet. This comprehensive evaluation of the women’s diets made it possible to find putative connective factors between the early pregnancy diet and the development of GDM later in her pregnancy.
Despite its strengths, this study has some limitations. First, because all subjects were either obese or overweight, the results can be generalized only for that population. Nonetheless, we took into account pre-pregnancy BMI by adjusting in the analyses; thus, the results may be potentially generalized to an overall population of pregnant women in Finland. Nevertheless, as obese women are more prone to develop GDM compared to normal-weight women and obesity is becoming more of a global problem, it is important to increase our knowledge in this population. Second, one possible source of error can be traced from underreporting of dietary intake. It is known that especially women with overweight or obesity tend to underreport their dietary intake . Usually, underreporting focuses on high-caloric foods, such as fat and sugary foods, and thus could affect estimation of total energy and nutrient intakes . Also, evaluation of eating behavior domains  could bring insight into the diet GDM associations. Third, the excluded women had higher median pre-pregnancy BMI and more frequently GDM diagnosis in previous pregnancy compared to women included in the study. Thus, this might have influenced the results as the included women were generally healthier than the excluded women. A fourth limitation arises from the analysis for defining dietary patterns. Two components explained about 15% of the total variance in the data, which is a relatively low value. However, similar percentages are also described in other studies depending on the number of components selected. In one previous study, one component explained 8.2% of the total variance , while in another study, four components explained 45.6% of the total variance . In our study, the 4th quintile, but not the 5th quintile, was associated with a lower risk of GDM. The finding could relate to the study technical issues, e.g., study power to detect differences between the quintiles. However, our finding is supportive to the findings from other studies, which indicate that an overall healthier diet, and also lower intake of nutrients that have a capacity to promote inflammation, such as SFA, could decrease the risk of GDM. It is noteworthy that, in addition to diet, other factors such as obesity, age, and previous GDM are involved in the development of GDM. Additionally, because healthy lifestyle markers typically cumulate, it is possible that the women whose diet is healthy also exercise and sleep more, which might contribute to the association between diet and development of GDM. Finally, in addition to inspecting dietary fat and carbohydrate quality by separating intakes of SFA, MUFA, PUFA, trans fatty acids, cholesterol, and fiber, and sucrose, evaluating food sources of these nutrients could have provided deeper insight into the onset of GDM. Overall, our findings are in line with those in the literature; a healthy diet with anti-inflammatory effects decreased the risk of GDM, while a higher intake of nutrients with a pro-inflammatory potential, total fat, SFAs, and trans fatty acids increased the risk of GDM.
Taken together, the results suggest that pregnant women with overweight or obesity consuming an overall healthy diet and one with a low inflammatory potential experience a lower risk of developing GDM compared to their counterparts who consume a more pro-inflammatory, less healthy diet. It was also demonstrated that avoiding an excess intake of dietary fats, especially SFAs, and other nutrients promoting inflammation in the body, might be associated with a decreased risk of developing GDM. It is likely that considering these aspects in the dietary counselling would benefit these women.