By Traci Pantuso, ND, MS

Adjunct Faculty, Bastyr University, Seattle; Owner Naturopathic Doctor Harbor Integrative Medicine, Bellingham, WA

Dr. Pantuso reports no financial relationships relevant to this field of study.

SYNOPSIS: In this study, the authors demonstrated significantly improved postprandial glucose levels in patients that consumed high glycemic index meals with extra-virgin olive oil compared to meals with butter or low fat meals.

SOURCE: Bozzetto L, Alderisio A, Giorgini M, et al. Extra-virgin olive oil reduces glycemic response to a high glycemic index meal in patients with type 1 diabetes: A randomized controlled trial. Diabetes Care 2016;39:518-524.

Summary Points

  • The type of fat contained in meals may be more important than the quantity with respect to the effect on the postprandial glycemic response.
  • Consuming 2.7 tablespoons of extra virgin olive oil combined in a high glycemic meal reduces the postprandial blood glucose levels compared to butter or low fat during the 0-3 hours after the meal.

Bozzetto et al previously found that considering both the quality (fiber content) and quantity of carbohydrates, compared to only carbohydrate quantity, when calculating pre-meal insulin improves the daily blood glucose profile in type 1 diabetes (DM1) patients.1 The use of continuous glucose monitoring in patients with DM1 has demonstrated that not only carbohydrate quantity but also protein and fat content affect postprandial glucose (PPG) levels.2 There have been conflicting studies on how fat content in meals affects the PPG levels in patients with DM1. The aim of this study was to determine if there is an effect of different dietary fats on the PPG response to either a high glycemic index (HGI) or low glycemic index (LGI) meal.

Thirteen patients with DM1 (eight women and five men) were recruited at the University of Naples Federico II teaching hospital diabetes care unit in Italy. To be included in the study, the participants had to be undergoing treatment with continuous subcutaneous insulin infusion and fast-acting insulin analogs for the previous six months. Participants were excluded if they were pregnant, had a hemoglobin A1c (HbA1c) higher than 8.0%, suffered from serious micro/macrovascular complications, or had any other acute or chronic disease that seriously affected their health. Patients also were excluded if they had a diagnosis of celiac disease. The study design was a randomized, crossover trial with a one-week run-in period to provide baseline data. During the one-week run-in period, participants underwent continuous glucose monitoring (CGM) and completed a seven-day diet diary.

The HGI meals were composed of 60 grams of white rice, 75 grams of white bread, 90 grams of minced beef meat, and 180 grams of banana with the addition of the selected fat: butter (43 grams) or extra-virgin olive oil (EVOO) (37 grams) with 8 grams of EVOO in the low-fat meal. The LGI meals contained 50 grams of pasta, 100 grams of lentils, 30 grams of whole meal bread, 15 grams of ham, 185 grams of apple with either 45 grams of butter or 37 grams of EVOO, and 8 grams of EVOO in the low-fat meal.

Participants then were assigned by coin toss to a one-week period in which three lunchtime meals were consumed on three separate days that were either HGI or LGI. Then the participants were crossed over into the opposite glycemic index group. The participants underwent CGM throughout the study period and tested capillary postprandial blood glucose levels after the test meals at two, four, and six hours.

Within each glycemic index group, there were three meal groups with different types of fat: low in fat, high in saturated fat (butter), or high in monounsaturated fat (EVOO).

Participants also underwent continuous glucose monitoring during the study period. A dietician supervised the preparation of the test meals, which were then frozen and given to the patients, who were instructed on how to defrost and heat them. The test meals were consumed at lunchtime between the second and seventh day of the CGM sensor life to prevent technical difficulties with the sensor. The prandial insulin doses were individualized per patient and were different between the HGI and LGI groups but were the same between the butter, EVOO, and low-fat meals.

ANOVA testing was used to evaluate the primary outcome of PPG incremental area under the curve (iAUC) between the different groups. The secondary outcomes evaluated were the blood glucose peak level and the time to the blood glucose peak.

The participants had an average HbA1c of 7.5 ± 1.0%; other characteristics are shown in Table 1. Two participants had background retinopathy and peripheral neuropathy, while one participant only had background retinopathy. One patient’s PPG data were not included for the HGI group because of a technical issue with one of the CGM instruments.

Table 1: Participant Characteristics


Average ± SD


31 ± 11 years

Body mass index

24.8 ± 2.9 kg/m2

Diabetes duration

25 ± 3 years


7.5 ± 1.0%

Total daily insulin dose

41.1 ± 10.7 IU

Not unexpectedly, the PPG was significantly lower in the combined LGI meal group compared to the combined HGI meal group during the three hours after meal consumption (112 ± 62 vs. 337 ± 76 mmol/L x 180 min; P = 0.006). No significant difference was found in the three- to six-hour time frame between the combined HGI or LGI groups. There were no significant differences in blood glucose peak levels or time to blood glucose peak between the type and amount of fat in the LGI group.

In the HGI group, the PPG iAUC from 0-3 hours was significantly decreased in the EVOO meal compared to both the meal containing butter or the low-fat meal (P < 0.05). (See Table 2.) No significant differences were observed in PPG iAUC during the three- to six-hour time frame. The time to blood glucose peak for the EVOO group was statistically significantly delayed compared to the butter or lowfat meal groups (P = 0.035). The type and amount of fat did affect the iAUC in the HGI significantly. The high-fat EVOO meal decreased the PPG level significantly compared to the low-fat EVOO and butter meals (P < 0.0001). During the late postprandial phase, the low-fat meal group returned to fasting blood glucose levels, while the meal with butter demonstrated elevated blood glucose levels compared to fasting.

Table 2: Postprandial Blood Glucose (0-3 hours) Levels in Participants Consuming HGI Meals with EVOO, Butter and Low Fat

HGI Meal type of Fat

Blood glucose
iAUC 0-3 hours


198 ± 274


398 ± 355

Low fat

416 ± 329


This study demonstrated that the type and amount of fat in HGI meals affects postprandial glucose during the 0- to 3-hour time but not in the late postprandial phase (3-6 hour). This is significant because DM1 patients maintain blood glucose levels through the use of insulin and calculating prandial insulin requirements, which is mostly based on calculating the amount of carbohydrates. The American Diabetes Association recommends that further education on the glycemic impact of fat and protein be provided after patients have mastered carbohydrate counting.3 In a recent systematic review, the authors concluded that higher-fat meals require more insulin coverage than lower-fat meals with the same carbohydrate content.2 This research study demonstrated that the effect of fat on PPG depends on the type of fat consumed and that monounsaturated fats, such as EVOO, may improve PPG levels compared to saturated fats such as butter, which affects overall insulin requirements. There was no significant effect of fat on PPG found in the LGI group.

The strengths of this study are the randomized, controlled, crossover trial design. Further, the authors also performed a power calculation that predicted 13 participants were needed to detect an 80% power and a 5% significance level for this study. The limitations of this study were the short duration and the lack of an EVOO biochemical profile. The study was performed in Naples, Italy, where access to high-quality EVOO may be greater than in the United States. However, the European Union top 10 foods at risk for food fraud listed olive oil as the highest risk.4 In the United States between 1980-2012, oils have been rated the top category of fraudulent foods.4 The main source of fraud with olive oil is the substitution of a lower cost alternative of either olive oil or, in certain cases, another nut or seed oil than what the label designates.4 The International Olive Council provides guidelines for standards, both sensory and chemical, to assess olive oil quality. EVOO is considered the top grade of olive oil followed by virgin olive oil, refined olive oil, and olive oil, which is a blend of refined and virgin olive oil.

The United States is not a member of the International Olive Council. To further understand the quality of olive oil in the United States, the Olive Center at the University California, Davis published a report in 2011 investigating the quality of olive oils sold in California.5 They concluded that 73% of the oils from the imported EVOO top five U.S. brands failed the International Olive Council’s sensory panel test and 70% failed the chemistry test investigating the content of the 1,2 diacylglycerol and pyropheophytins.5 A higher 1,2 diacylglycerol content in olive oil is an indicator of its better quality and freshness, while a higher pyropheophytins content indicates increased oxidization or adulteration with refined oil.5 When recommending olive oil, it is important to counsel patients on the importance of using a good quality EVOO instead of buying the most cost-efficient type. This can be challenging, but there are a number of resources available to assist with the purchasing of quality EVOO.6

An estimated 1 million Americans have been diagnosed with DM1. Commonly diagnosed in childhood, adolescence, or early adulthood, DM1 is a chronic condition that is associated with micro/macrovascular and neuropathic complications. Maintenance of normal blood glucose levels with insulin greatly reduce the development and progression of DM1 complications.

This research study shows that LGI meals have better PPG levels than HGI meals and that the types and quantity of fats adds little to no extra benefit in LGI meals. The amount of carbohydrates is still the most important variable for PPG levels. However, it is possible to mitigate the adverse effects on PPG levels of HGI meals with EVOO, and even low-fat is better than high saturated fat. The results from this study illustrate some advice for patients with DM1, including an emphasis on LGI meals, the addition of high quality EVOO to HGI meals, and the reduction of dietary intake of saturated fats.


  1. Bozzetto L, Giorgini M, Alderisio A, et al. Glycaemic load versus carbohydrate counting for insulin bolus calculation in patients with type 1 diabetes on insulin pump. Acta Diabetol 2015;52:865-871.
  2. Bell KJ, Smart CE, Steil GM, et al. Impact of fat, protein and glycemic index on postprandial glucose control in type 1 diabetes: Implications for intensive diabetes management in the continuous glucose monitoring era. Diabetes Care 2015;38:1008-1015.
  3. American Diabetes Association: Approaches to glycemic treatment. Sec.7 In Standards of Medical Care in Diabetes-2015. Diabetes Care 2015;38(Suppl 1):S41-S48.
  4. Johnson R. Food Fraud and “Economically Motivated Adulteration” of Food and Food Ingredients. Congressional Research Service; 2014. Available at: Accessed Dec. 1, 2016.
  5. Frankel EN, Mailer RJ, Wang SC, et al. Evaluation of extra-virgin olive oil sold in California. UC Davis Olive Center at the Robert Mondavi Institute; 2011. Available at: Accessed Dec. 1, 2016.
  6. Truth in Olive Oil. Available at: Accessed Dec. 1, 2016.