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CME / ABIM MOC / CE
Does Vegetarian Diet Improve HbA1c; Cut CV Risk for Diabetes?
Authors:
News Author: Marcia FrellickCME Author: Laurie Barclay, MD
Faculty and Disclosures
CME / ABIM MOC / CE Released: 8/10/2018
Valid for credit through: 8/10/2019
https://www.medscape.org/viewarticle/899972
Clinical Context
Diet and lifestyle are the mainstay of diabetes management, and vegetarian dietary patterns have been shown in several prospective cohort studies to be associated with lower risk for type 2 diabetes (T2D), coronary heart disease (CHD), obesity, hypertension, cardiovascular (CV) mortality, and all-cause mortality. Similarly, several systematic reviews and meta-analyses of controlled trials show benefits of vegetarian dietary patterns for glycemic control, blood lipid profile, body weight, and blood pressure (BP).
The American Diabetes Association (ADA) and Diabetes Canada guidelines include recommendations for vegetarian dietary patterns for diabetes management but suggest the need for further research, and the European Association for the Study of Diabetes (EASD) guidelines have not specifically recommended any vegetarian dietary patterns. The goal of this systematic review and meta-analysis by Viguiliouk and colleagues of RCTs was to summarize the evidence for the effect of vegetarian dietary patterns on glycemic control and other established cardiometabolic risk factors in persons with diabetes, as an evidence base to update the EASD clinical practice guidelines for nutrition therapy.
Study Synopsis and Perspective
Vegan and vegetarian diets help lower glycated hemoglobin (HbA1c) and cholesterol levels and improve other cardiometabolic risk factors in middle-aged, overweight people controlling their type 2 diabetes with medications, said authors of a literature review published online in Clinical Nutrition.[1]
Effie Viguiliouk, MSc, with the Clinical Nutrition and Risk Factor Modification Center at St Michael’s Hospital in Toronto, Ontario, and colleagues, analyzed findings from 9 RCTs involving 664 participants who were taking oral glucose-lowering drugs, insulin, lipid-lowering agents, and/or anti-hypertensive agents.
They found that vegetarian diets compared with nonvegetarian diets improved the primary outcome of HbA1c by 0.29%.
Although the HbA1c reduction may seem moderate, Cara Schrager, MPH, RD, CDE, of the Joslin Diabetes Center in Boston, Massachusetts, pointed out that the improvement is the same as the therapeutic threshold the US Food and Drug Administration uses (FDA) when considering new medications for diabetes.
Ms Schrager told Medscape Medical News that this level of reduction suggests that patients could consider moving toward a plant-based diet with primarily vegetables, fruits, whole grains, and legumes, perhaps even before they move to diabetes medications.
Vegetarian Diet Also Helped Reduce BMI
Other results included reductions in fasting glucose of 0.56 mmol/L; lower-density lipoprotein cholesterol (LDL-C) (0.12 mmol/L); non–high-density lipoprotein cholesterol (HDL-C) (0.13 mmol/L); body weight (2.15 kg or 4 lb, 12 oz); body mass index (BMI) (0.74 kg/m2); and waist circumference (2.86 cm) with the vegetarian compared with nonvegetarian diets.
No significant differences were seen in BP, fasting insulin, HDL-C, or triglycerides (TGs).
Only diets that excluded meat and fish (from vegan [no animal products] to vegetarian diets that included eggs and dairy products) were considered vegetarian in the meta-analysis.
The authors stressed that the link between diabetes and heart disease is strong and well-established.
“Sixty to seventy percent of people who have type 2 diabetes die of heart disease,” study coauthor Hana Kahleova, MD, PhD, director of clinical research at the Physicians Committee for Responsible Medicine, said in a press release.[2]
“This study shows that the same simple prescription – eating a plant-based diet – can reduce our risk for heart problems and improve type 2 diabetes at the same time.”
The researchers acknowledged, however, that the findings had a low-to-moderate confidence level, and they encouraged further study.
Findings Build on Body of Evidence: No Downside to a Plant-Based Diet
Katherine Zeratsky, a registered dietitian nutritionist (RDN) at the Mayo Clinic in Rochester, Minnesota, told Medscape Medical News that this new review adds to a body of evidence that indicates eating less red meat and more vegetables can benefit people with T2D.
Many of the noted reductions in this study fit with what others have shown, she said.
“We know that weight in and of itself is a strong risk factor for diabetes – so it makes sense that a plant-based diet will have a large impact on health overall,” she said, although countering that she wished the evidence “was stronger.”
Still, as with all nutrition research, she said, "It’s very difficult to say with absolute certainty that if you do this, this will happen."
Nonetheless, for most patients, she added, there isn’t a downside to trying a plant-based diet.
It Doesn’t Have to Be All or Nothing: How to Move Away From Meat
Still, fewer than 10% of people in North America and Europe have adopted vegetarian diets, according to national survey data cited by the authors.
Ms Zeratsky emphasized that moving away from meats and toward vegetables, fruits, and whole grains does not have to be an all-or-nothing effort.
A good start is cutting meat consumption at a meal from 8 oz to 6 oz and filling the plate with more fruits and vegetables to curb hunger, she said.
Ms Schrager suggested adding “meatless Mondays” or trying the diet one day a week at first.
“It is a shock to the system if you suddenly stop eating a certain group of foods,” she said.
Some foods, such as lasagna or chili, have comparable meatless versions, she noted, and beans can add bulk. Adding protein sources such as tofu and tempeh can also give dishes a meaty texture.
More Studies Will Help Answer Questions
Mr Viguiliouk and colleagues said more RCTs will help answer questions this one could not, including what kind of plant-based diets have the most beneficial effects and how a vegetarian diet might affect people with type 1 diabetes.
Ms Schrager said she also suspects, though this study did not address it, that the microbiome has an important role in the mechanism that links vegetarian diets and improved diabetes outcomes.
The mechanism may be related to more than the low-calorie and low-fat aspects and may be rooted in the better balance of gut bacteria produced by the amount of fiber in the vegetarian diet, she hypothesized, adding that she would like to see if further research sheds light on this.
The review was funded by the Diabetes and Nutrition Study Group of the EASD. A complete list of disclosures is available on the journal website.
Clinical Nutrition. Published online June 13, 2018.
Study Highlights
A search of Medline, Embase, and Cochrane databases through February 26, 2018 identified 9 RCTs (total N=664) that examine the effect of vegetarian dietary patterns for at least 3 weeks in persons with diabetes.
Vegetarian diets were defined as those excluding meat and fish, either vegan (no animal products) or those allowing eggs and dairy.
Most participants were middle-aged, overweight or obese, and had T2D controlled by medications, including insulin, oral glucose-lowering, lipid-lowering, and/or anti-hypertensive drugs.
HbA1c was the main study endpoint in these trials, and secondary outcomes were other glycemic control markers, blood lipids, body weight/adiposity, and BP.
2 independent reviewers determined risk for bias and extracted data, which were pooled using generic inverse variance method and expressed as mean differences (MD).
The Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach was used to evaluate the overall certainty of the evidence.
Vegetarian dietary patterns were associated with significantly lower HbA1c (MD, −0.29% [95% CI: −0.45, −0.12%]), fasting glucose (MD, −0.56 mmol/L (95% CI: −0.99, −0.13]), LDL-C (MD, −0.12 mmol/L [95% CI: −0.20, −0.04]), non-HDL-C (MD, −0.13 mmol/L [95% CI: −0.26, −0.01]), body weight (MD, −2.15 kg [95% CI: −2.95, −1.34]), BMI (MD, −0.74 kg/m2 [95% CI: −1.09, −0.39]) and waist circumference (−2.86 cm [95% CI: −3.76, −1.96]).
Fasting insulin, HDL-C, TGs, and BP were not significantly affected by vegetarian dietary patterns.
Although the overall certainty of evidence was moderate, it was low for fasting insulin, TGs, and waist circumference.
According to their findings, the investigators concluded that vegetarian dietary patterns improve glycemic control, LDL-C, non-HDL-C, and body weight/adiposity in persons with diabetes.
Although the findings support inclusion of vegetarian dietary patterns in diabetes management, more research is needed to improve confidence in the pooled estimates, which was only moderate to low.
Uncertainty resulted from serious imprecision in pooled estimates for HbA1c, fasting glucose, fasting insulin, LDL-C, non-HDL-C, TGs, and BP; indirectness for fasting insulin, body weight and adiposity outcomes; and inconsistency for HDL-C and TGs.
Future research should include more high-quality randomized trials of the effect of vegetarian dietary patterns on glycemic control and cardiometabolic outcomes, particularly in persons with type 1 diabetes, and to examine possible differences in outcomes for different forms of vegetarianism.
Large randomized trials should also address clinically significant outcomes such as cardiovascular disease (CVD), nephropathy, retinopathy, and mortality in people with diabetes.
The 0.29% improvement in HbA1c with vegetarian vs nonvegetarian diets was only moderate but similar to the therapeutic threshold used by the FDA when considering new medications for diabetes.
The findings therefore suggest that adopting a plant-based diet rich in vegetables, fruits, whole grains, and legumes could be a reasonable first step in diabetes management, even before a trial of pharmacotherapy.
Given the strong association between diabetes and heart disease, a vegetarian diet may also reduce CV risk and improve T2D.
For most patients, a vegetarian diet will have no adverse effects.
For persons unwilling to avoid animal products altogether, it could still be beneficial to reduce meat intake and proportionately increase fruits and vegetables in the diet.
Potential mechanisms underlying the association of a vegetarian diet with better outcomes in diabetes may include a better balance of gut bacteria related to increased fiber, as well as reduction of calorie and fat intake.
Study limitations include serious imprecision in pooled estimates across several outcomes, with inability to rule out clinically important harm related to TGs and BP or clinically trivial effects of lowered HbA1c, fasting glucose, LDL-C, non–HDL-C, and waist circumference.
In addition, only one trial enrolled persons with type 1 diabetes, and the small number of available trials precluded subgroup analyses or estimation of publication bias for any outcome.
In several of the included trials, patients reduced their use of oral anti-hyperglycemic agents, suggesting that vegetarian dietary patterns might lower the need for medications.
In Europe and North America, approximately less than 10% of the population follow vegetarian diets, suggesting that there could be important public health benefits if more people adopted vegetarian diets and lowered their risks for uncontrolled diabetes and CVD.
The acceptability, adherence and suitability for long-term use of vegetarian diets are similar to those of other therapeutic diets.
Vegetarian dietary patterns also are less expensive than those based on animal products and have environmental benefits, which may promote adoption and adherence.
Clinical Implications
Vegetarian dietary patterns improve glycemic control, LDL-C, non–HDL-C, and body weight/adiposity in persons with diabetes, according to a systematic review and meta-analysis of RCTs.
The findings support inclusion of vegetarian dietary patterns in diabetes management, but more research is needed to improve confidence in the pooled estimates, which was only moderate to low.
Implications for the Healthcare Team: In several of the included trials, patients reduced their use of oral anti-hyperglycemic agents, suggesting that vegetarian dietary patterns might lower the need for medications.
CME Test
To receive AMA PRA Category 1 Credit™, you must receive a minimum score of 75% on the post-test.
Question 1 of 2
You are a member of the healthcare team for a 62-year-old man newly diagnosed with T2D. According to the systematic review and meta-analysis of RCTs by Viguiliouk and colleagues, which of the following statements about the evidence for the effect of vegetarian dietary patterns on glycemic control and other established cardiometabolic risk factors in persons with diabetes is correct?
Vegetarian dietary patterns did not affect the primary endpoint of HbA1c
Vegetarian dietary patterns were associated with significantly lower fasting insulin, HDL-C, TGs, and BP
Vegetarian dietary patterns were associated with significantly lower body weight and BMI
The overall certainty of evidence was high
Question 2 of 2
According to the systematic review and meta-analysis of RCTs by Viguiliouk and colleagues, which of the following statements about clinical implications of effects of vegetarian dietary patterns on glycemic control and other established cardiometabolic risk factors in persons with diabetes is correct?
The findings do not support inclusion of vegetarian dietary patterns in diabetes management
The meta-analysis proves that person with type 1 diabetes will have lower mortality on a vegetarian diet
Long-term use of vegetarian diets is less feasible than that of other therapeutic diets
The 0.29% improvement in HbA1c with vegetarian diets was similar to the therapeutic threshold used by the FDA when considering new diabetes medications
