Studies have investigated the effect pistachio nuts specifically have during pregnancy on gestational diabetes mellitus (GDM). Assaf-Balut et al. conducted a prospective, randomized, controlled trial (San Carlos Gestational Diabetes Mellitus Prevention Study) to evaluate the incidence of GDM with either a control diet (standard diet with limited fat intake) or a Mediterranean diet supplemented with extra virgin olive oil and pistachios. One thousand normoglycemic (<92 mg/dL) pregnant women at 8–12 gestational weeks (GW) were recruited for the study to assess the effect of the intervention on GDM incidence at 24–28 GW. A total of 874 women completed the study.

The supplemented Mediterranean diet reduced the incidence of GDM rate from 23.4 percent to 17.1 percent and improved maternal and neonatal outcomes, significantly lowering rates of insulin-treated GDM, prematurity, emergency Cesarean sections, perineal trauma, and small and large for gestational age newborns.¹ Health benefits to the infant persisted during the first two years of life.  The offspring of mothers receiving the supplemented Mediterranean diet during pregnancy had fewer hospital admissions requiring antibiotic and corticosteroid treatment, and admissions related to asthma/bronchiolitis, especially in women who had pre-gestational BMI <25 kg/m.² A sub-analysis of the data collected in the St. Carlos Gestational Diabetes Mellitus Prevention Study showed that mothers receiving the enriched diet had over 50 percent lower risk of composite adverse maternal fetal outcomes.³

Pistachios consumed alone have a minimal effect on post-prandial glycemia and when taken with a carbohydrate meal they attenuate the glycemic response of the test meal.⁴ In individuals with metabolic syndrome, pistachios added to the meal attenuate postprandial glycemia, increase glucagon-like-peptide levels and may have insulin-sparing properties.⁵ 

Feng et al evaluated the acute effects of two isocaloric test meals, 42 g pistachios and 100 g whole-wheat bread (WWB) on postprandial glucose, insulin, and gut derived incretin levels in Chinese women with gestational impaired glucose tolerance (GIGT) or GDM.⁶ In this randomized, controlled, crossover study, 73 women 23–39 years of age, with singleton pregnancies, and having a gestational age of 24–30 weeks at the time of screening were recruited from the prenatal care clinic in the Shanghai Jiaotong University Affiliated Sixth People's Hospital (Shanghai, China). All women underwent screening for GDM using a standard 50 g glucose tolerance test. Women with a positive screening were given a 2 hr, 75 g oral glucose tolerance test (OGTT) after an 8–12 hr overnight fast. Women who fulfilled at least one of the following criteria were diagnosed GDM when fasting glucose ≥5.1 mmol/L, 1 hr glucose ≥10 mmol/L or 2 hr glucose ≥8.5 mmol/L during the 2 hr 75 g OGTT. The diagnosis of GIGT was made when subjects tested positive with 50-g glucose tolerance test but did not meet the criteria of GDM. Thirty women with GIGT and 29 with GDM completed the study.

On the study day, fasting blood samples were collected to determine baseline levels of glucose, insulin, and incretins. Women were then given either WWB or pistachios and consumed it within 15 min. Blood samples were collected at 30, 60, 90, and 120 min. after ingestion. Each intervention was followed by 1-week washout and women were crossed over to the other intervention.

Pistachio consumption did not increase the blood glucose and insulin levels compared to baseline in both GIGT and GDM women. Conversely, WWB consumption significantly increased blood glucose and insulin levels. Compared to pistachio consumption, blood glucose and insulin levels at 30, 60, 90, and 120 min postprandial as well as AUCglucose0−120min and AUCinsulin0−120min were significantly higher after WWB consumption in both GIGT and GDM participants. The low carbohydrate, and high fat content of the pistachios most likely contributed to the observed differences of postprandial glucose and insulin responses between pistachio and WWB intake.

In both GIGT and GDM women, significant higher GLP-1 levels were observed at 90 and 120 min after pistachio compared to WWB intake. Significant lower gastric inhibitory polypeptide (GIP) levels were observed at 30 and 60 min in GDM patients or 120 min in GIGT patients after pistachio compared to WWB intake. The gut incretin hormones GLP-1 and GIP are released to lower blood glucose levels after a meal. GLP-1 and GIP possess strong glucose-dependent insulin regulatory properties and augment glucose-dependent insulin secretion after a meal. This potentiation of insulin secretion by gut hormones such as GLP-1 after an oral glucose load is crucial for controlling postprandial glucose excursions. Impaired postprandial GLP-1 response was previously reported in pregnant women with GDM.

Pistachios are nutrient dense foods high in polyunsaturated fatty acids (PUFAs) and monounsaturated fatty acids (MUFAs)], high in protein, fiber (both soluble and insoluble), potassium, phytosterols, γ-tocopherol, vitamin K, and xanthophyll carotenoids.  These studies provide evidence that pistachio nuts, with their healthy nutrient profile, can be an ideal choice for healthy snacks during pregnancy.