In a recent 16-week randomised trial1 , researchers from the PCRM (Physicians Committee for Responsible Medicine) demonstrated that a plant-based diet improves factors that can lead to the development of type 2 diabetes in overweight adults.
The specific markers looked at were beta-cell function and fasting insulin resistance.
Impairment of pancreatic beta-cell function is a key factor in type 2 diabetes2 .It is typically preceded by insulin resistance in both muscle cells and liver cells
Beta-cell failure reflects:
- A relative loss of beta-cell mass due to apoptosis3
- A selective loss of sensitivity to glucose
- The loss of first-phase insulin secretion4
Beta-cell function and diet
Diabetes in vegetarians and non-vegetarians
Glycaemic control in vegans and low-calories/low-carb diets
Non-restriction of calories or macronutrient types
This study wanted to test whether a plant-based (vegan) diet intervention alone (without restricting calories or specific macronutrient intake) could improve beta-cell function and insulin sensitivity. That is, the subjects in both the intervention group and the control group were not expected to reduce the calories they consumed each day. And while the subjects were overweight, they were not yet classified as being diabetic. (Previous studies12 5 have shown improvements in beta-cell function in individuals with type 2 diabetes with an energy-restricted diet.)
The two diets – control group and intervention group
- The control group were asked not to make any dietary changes
- The intervention group was asked to follow a low-fat vegan diet (~75% of energy from carbohydrates, 15% protein, and 10% fat) consisting of vegetables, grains, legumes, and fruits
- No meals were provided
- Alcoholic beverages were limited to one per day for women and two per day for men in both groups
BMI (body mass index) and body composition
- Significant BMI decrease in intervention (plant-based) group
- Lean mass reduced in both groups
- Fat mass, and especially visceral fat volume, were reduced only in the intervention group
- Significant reductions in total, LDL, and HDL-cholesterol in intervention group
Fasting plasma glucose
- Decreased in intervention group
- Decreased in intervention group
- Decreased in both intervention and control groups
*C-peptide is a short chain amino acid released into the blood as a by-product of the formation of insulin by the pancreas. Testing its level in the blood can help evaluate endogenous production (made by the body itself) and help to differentiate it from exogenous insulin (from outside the body).
- Decrease in basal insulin secretion in the intervention group
- Increase in beta-cell glucose sensitivity
Total insulin secretion
- Increase in control group
- Marked increase in insulin secretion as a function of plasma glucose concentrations in intervention group compared with control group
- HOMA-IR index** fell significantly in intervention group but did not change significantly in control group
** HOMA (homeostatic model assessment) is a method used to quantify insulin resistance and beta-cell function.
Oral glucose insulin sensitivity
- No significant changes in either group
Discussion of results
Beta-cell sensitivity and Insulin resistance
The dietary intervention produced significant increases in meal-stimulated insulin secretion and beta-cell glucose sensitivity, along with decreased fasting insulin resistance and decreased fasting and postprandial plasma glucose concentrations, in individuals with no history of diabetes.
In the control group, beta-cell glucose sensitivity did not increase, while insulin resistance did. This combination are two main of the mechanisms involved in the development of beta-cell dysfunction, and eventually diabetes2 .
The intervention group, on the other hand, had improved metabolic markers:
- Glucose sensitivity improved
- Fasting insulin sensitivity improved
- Fasting plasma glucose levels decreased
- Mean glucose levels during meal test decreased
- Fasting insulin resistance decreased (that is, improved)
Insulin sensitivity and type 2 diabetes
The researchers consider that it is plausible that the low-fat vegan diet used in their study decreased hepatic insulin resistance and led to a subsequent improvement in beta-cell function. This is supported by other studies13 14 showing that reduced risk of developing type 2 diabetes is associated with improvements in insulin sensitivity which, in turn, has beneficial effects on the preservation of beta-cell function.
Weight loss and vegetarian/vegan diets
Interventions with plant-based diets have been shown by other studies15 16 17 to be very effective in achieving healthy weight loss and, particularly, for reduction in visceral fat and subfascial fat (fat in muscle tissue) 16 17 18 19 .
The improvements in plasma lipids (decreased total cholesterol, LDL, and HDL), along with no significant changes in triglyceride levels, are consistent with other studies20 . Vegetarian and vegan diets have been shown by additional studies21 to bring about these improvements in blood lipids – improvements considered as important cardio-metabolic benefits, also being associated with reduced all-cause mortality.
Another mechanism that may share responsibility for the improved beta-cell function in the intervention group may concern the incretin hormones. These are a group of metabolic hormones released from the GI tract. They are released from the GI tract after eating and augment the secretion of the insulin that’s being released from the pancreatic beta cells. They basically enhance meal-dependent insulin secretion from the pancreas22 .
Impaired incretin production has been shown23 to play a part in the development of beta-cell dysfunction and, hence, of type 2 diabetes. A previous study24 showed that plant-based diets improve incretin secretion, so this might be one of the reasons why the intervention diet improved beta-cell function.
Toxicity, oxidative stress and inflammation
It’s also possible that improvements in beta-cell function were due to a reduction in lipotoxicity, glucotoxicity, oxidative stress, and inflammation. Each of these has been shown18 25 to be influenced by diet.
Since a high-carbohydrate diet has been shown26 to improve insulin sensitivity and meal-stimulated insulin secretion (in subjects who have impaired fasting glucose), it is reasonable to assume that the substantial increase in carbohydrate intake (normal in plant-based diets) with the intervention group may relate to the demonstrated improvement in beta-cell function.
Finally, a possible contributory factor in the improvement of beta-cell function in the intervention group could be related to the fact that they cut out dietary cholesterol (i.e. animal foods). It has been shown that dietary cholesterol can cause problems in this respect, including:
- Inducing cellular and mitochondrial oxidative stress
- Causing and lipid peroxidation, which leads to
- Beta-cell dysfunction
On the other hand, plant foods have been shown27 28 to be beneficial for beta-cell function in a variety of ways, including in the provision of high levels of quercetin29 and polyphenols which have anti-apoptotic (preventing cell death), antioxidant and anti-inflammatory properties. These are considered to improve ATP-linked mitochondrial energy metabolism, and thus assist in improving meal-stimulated insulin secretion and beta-cell function.
The researchers conclude that: “…we have demonstrated that beta-cell function and fasting insulin sensitivity can be modified by a 16-week dietary intervention. Our study suggests the potential of a low-fat plant-based diet in diabetes prevention, addressing both core pathophysiologic [disordered physiological processes associated with disease] mechanisms – insulin resistance and diminished beta-cell function – at the same time.”
- A Plant-Based Dietary Intervention Improves Beta-Cell Function and Insulin Resistance in Overweight Adults: A 16-Week Randomized Clinical Trial. Hana Kahleova, Andrea Tura, Martin Hill, Richard Holubkov, Neal D. Barnard. Nutrients. 2018 Feb; 10(2): 189. Published online 2018 Feb 9. doi: 10.3390/nu10020189. PMCID: PMC5852765.
- Ferrannini E., Gastaldelli A., Miyazaki Y., Matsuda M., Pettiti M., Natali A., Mari A., DeFronzo R.A. Predominant role of reduced beta-cell sensitivity to glucose over insulin resistance in impaired glucose tolerance. Diabetologia. 2003;46:1211–1219. doi: 10.1007/s00125-003-1169-6.
- Butler A.E., Janson J., Bonner-Weir S., Ritzel R., Rizza R.A., Butler P.C. Beta-cell deficit and increased beta-cell apoptosis in humans with type 2 diabetes. Diabetes. 2003;52:102–110. doi: 10.2337/diabetes.52.1.102.
- Gastaldelli A., Ferrannini E., Miyazaki Y., Matsuda M., DeFronzo R.A. Beta-cell dysfunction and glucose intolerance: Results from the San Antonio metabolism (SAM) study. Diabetologia. 2004;47:31–39. doi: 10.1007/s00125-003-1263-9.
- Solomon T.P.J., Haus J.M., Kelly K.R., Rocco M., Kashyap S.R., Kirwan J.P. Improved Pancreatic Beta-Cell Function In Type 2 Diabetics Following Lifestyle-Induced Weight Loss Is Related To Glucose-Dependent Insulinotropic Polypeptide. Diabetes Care. 2010 doi: 10.2337/dc09-2021.
- Tonstad S., Butler T., Yan R., Fraser G.E. Type of Vegetarian Diet, Body Weight, and Prevalence of Type 2 Diabetes. Diabetes Care. 2009;32:791–796. doi: 10.2337/dc08-1886.
- Snowdon D.A., Phillips R.L. Does a vegetarian diet reduce the occurrence of diabetes? Am. J. Public Health. 1985;75:507–512. doi: 10.2105/AJPH.75.5.507.
- Cersosimo E., Solis-Herrera C., Trautmann M.E., Malloy J., Triplitt C.L. Assessment of pancreatic β-cell function: Review of methods and clinical applications. Curr. Diabetes Rev. 2014;10:2–42. doi: 10.2174/1573399810666140214093600.
- Vang A., Singh P.N., Lee J.W., Haddad E.H., Brinegar C.H. Meats, processed meats, obesity, weight gain and occurrence of diabetes among adults: Findings from Adventist Health Studies. Ann. Nutr. Metab. 2008;52:96–104. doi: 10.1159/000121365.
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- Barnard N.D., Gloede L., Cohen J., Jenkins D.J.A., Turner-McGrievy G., Green A.A., Ferdowsian H. A low-fat vegan diet elicits greater macronutrient changes, but is comparable in adherence and acceptability, compared with a more conventional diabetes diet among individuals with type 2 diabetes. J. Am. Diet. Assoc. 2009;109:263–272. doi: 10.1016/j.jada.2008.10.049.
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- Quercetin Food Chart.