Chapter 5. DISCUSSION
5.6 Integrated effects of multiple dietary components on overall metabolic health 90
Although vegetarianism is defined by avoidance of meat, fish, and possibly other animal products, such as eggs and dairy (for vegans), the beneficial effect of a vegetarian dietary pattern on diabetes appear to go beyond just the avoidance of animal products. In our study, the effect of food groups on diabetes risk appears to be small and mostly insignificant, while the effect of vegetarian pattern is large and robust. It is most likely the combination of low harmful components from animal products and the healthful plant components that act additively to improve metabolic health.
Figure 5 – 4 proposes how a healthful vegetarian diet may act through various metabolic
pathways to influence the twin cycles of diabetes. Vegetarian diet may decrease liver fat via lower body weights due to lower metabolizable energy in some plant foods(154,155,156), and potential change in microbiome(70,72,209). The lower iron store(158,159) and higher magnesium and soy intake may all contribute to lower insulin resistance(83,84,87,159). Due to higher carbohydrate intake, TG may not necessarily be reduced. However, vegetarians may minimize β-cell
dysfunction by lowering consumption of saturated fat(18) and environmental contaminants(200). In addition, the lower iron store(158,159) will likely reduce oxidative stress to β-cell(19). Finally, plant polyphenol and microbial fermentation of fiber to short chain fatty acid may stimulate GLP-1 secretion, improve glucose control, and enhance β-cell function(70,93).
Figure 5 – 4. Potential mechanisms on how a vegetarian diet affects metabolic health in the context of the twin cycle for diabetes. TG = triglyceride, GLU = glucose, SCFA = short chain fatty acids, GLP-1 = glucagon-like- peptide-1, ER = endoplasmic reticulum, Mg = magnesium. Modified from Taylor’s twin cycle model(66).
Vegetarian diet appear to benefit metabolic health via different pathways. However, in our study, vegetarians tend to consume a high carbohydrate, low protein, low fat diet. This may offset the decreased TG that is expected with lower body weight. Vegetarians may benefit from replacing some carbohydrates (particularly refined carbohydrates and simple sugar) with plant protein, as this may improve TG and HDL-C profile, leading to further protection for diabetes.
5.7 Study strength and limitations
The large sample size and detailed health examination enable us to study the effect of vegetarian diet on diabetes risk in the context of cardiometabolic risk factors, including metabolic syndrome and fatty liver. The homogenous population of smokers and non-alcohol drinkers from the same religious community may reduce unmeasured confounding and strengthen internal validity, although the generalizability to other population will require further confirmation from other studies. To date, there are only a handful of cohorts with sufficient number of vegetarians to prospectively investigate the impact of vegetarian diets on health, and most of these studies are from Western countries(210), and based only on questionnaire without health examination data.
The prospective design with high follow-up rate (93%) of our study reduces recall and selection biases. The majority (75%) of participants have their diabetes status confirmed by HbA1C or two fasting blood glucose, or use of diabetes medication (through medical records).
This practice has reduced misclassification of disease outcome.
Baseline diet was assessed by a validated FFQ, and interviewed by trained research assistants. The FFQ had been shown to have good relative validity in ranking nutrient intakes, but is not accurate for exact nutrient assessment, and our estimation of food and nutrient intake may be subjected to systematic error. Future calibration study is needed to better estimate nutrient and food intake. The FFQ was interviewed instead of self-administered, and this
prevents missing data on dietary intakes. Unfortunately, follow-up dietary assessment was made through a simple questionnaire. The lack of detail diet prevented us from analyzing detail dietary changes, except that meat and fish intake changed from small to zero for the converted.
Nevertheless, we captured dietary changes pertinent to our study aim (vegetarian vs
nonvegetarian dietary patterns), providing more insights than most cohorts that rely only on one baseline dietary assessment.
The use of ultrasound could determine presence of fatty liver but could not distinguish severity of fatty liver. However, a meta-analysis concluded that ultrasonography has good reliability and accuracy for detecting moderate to severe fatty liver, compared against
biopsy(211), which is invasive and impractical in epidemiological settings. We attempted to assess fatty liver severity by calculating the NAFLD Fibrosis Score. Although this is not a direct
assessment, it has good validity for determining liver fibrosis (126) and predicts mortality(162).
CHPATER 6. CONCLUSION
Vegetarian diet is associated with better metabolic profile, lower prevalence of fatty liver, and reduced risk of diabetes among Taiwanese. The inverse association between vegetarian diet and diabetes is independent of BMI, while the association with fatty liver is BMI-dependent.
Although it is difficult to separate the effect of animal components from plant components when examining a vegetarian dietary pattern as a whole, it is likely that the lack of harmful animal components and healthful plant components together drive the protective effect of a vegetarian diet.
There is, however, room for improvement in the current vegetarian dietary practice in Taiwan. About 70% of vegetarians did not meet the recommendation for vitamin B12. In addition, intakes of protein, calcium, magnesium, and zinc may be suboptimal among some vegetarians. Dietary planning should aim to increase more plant protein, whole grains, nuts and seeds, as well as vitamin B12 supplements or fortified foods, to improve the nutritional status of Taiwanese vegetarians.
The negative association between vegetarian diet and nonalcoholic fatty is mainly related to BMI. Besides limiting caloric intake, substituting meat or fish with soy, or substituting refined sugar with whole grains may help prevent fatty liver.
Plant-based diets with minimal animal products serve as a frame for diabetes prevention, but more researches on how plant functional components target the diabetes pathophysiology
(such as impaired insulin secretion and function) will be needed to disclose etiology for diabetes prevention.
Our consistent finding with Western populations has a far-reaching public health and environmental implication. The large and consistent protective effect of plant based diets and the over-consumption of meat with inadequate consumption of fruits, vegetables and whole grains by the majority today suggest enormous population-attributable protection potential of
vegetarian diets. At the same time, shifting toward plant based diets is estimated to reduce food-related greenhouse gas emissions by 29 – 70%(212). Vegetarian diet may be a stunning dietary solution to the diet-environment-health trilemma that our globe urgently need to tackle, for the welfare, if not the survival, of many who are deeply threatened by climate change and
noncommunicable chronic diseases such as diabetes.
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