Smoothies are innocent? Is it better to eat the whole fruits or not?
Introduction
For quite some time, smoothies have been praised as a healthy choice, often touted for their convenience and nutritional value. However, a recent study has cast doubt on their innocence. In this article, we’ll take a closer look at why the perception of smoothies is changing and explore the potential benefits of opting for whole fruits instead. It’s time to reconsider whether our go-to health drink is truly as wholesome as we’ve been led to believe.
This exploration is grounded in a recent scientific paper, shedding light on findings that challenge the prevailing notions about the health benefits of smoothies.
Insulin resistance, a key factor in metabolic syndrome affecting a third of US adults, is influenced by hyperinsulinemia. Understanding how food choices and processing impact blood sugar and insulin levels is crucial for preventing metabolic disorders. Dietary fiber, linked to a lower risk of metabolic syndrome, reduces post-meal glucose and insulin responses. Soluble fiber, by increasing food viscosity, slows gastric emptying and glucose absorption. While blending is a popular method for consuming nutrients, its impact on fiber’s protective benefits is unclear.
So let’s dive into the study. Here are highlights to see the key points and understand what, how and why:
Study criteria:
- Participants: 20 young, healthy college students (12 female, 8 male)
- Comparison: Whole fruit (apple and blackberries) vs. blended fruit (apple and blackberry smoothie)
- Sex Consideration: Recognized sex-specific differences in postprandial glycemic response, prompting inclusion of both males and females for analysis.
- Real-life Relevance: Postprandial glycemic response measured approximately 3.0–3.5 hours after lunch to reflect practical, non-fasting conditions.
Study Participants and Design:
- Participant Selection: Twenty healthy college students (n = 12 female, n = 8 male) aged 19 to 21, mean age 20.4 ± 1 year, with non-obese BMI under 30 kg/m2.
- BMI Breakdown: Male participants had BMI ranging from 17.3 to 25.8 kg/m2, mean 22.3 ± 2.6 kg/m2. Females’ BMI ranged from 19.6 to 28.1 kg/m2, mean 23.0 ± 3.1 kg/m2.
- Health Screening: Participants prescreened for absence of health conditions related to digestion, nutrient absorption, or metabolism.
- Study Design: Repeated measures design with each participant undergoing both whole fruit and blended fruit treatments. Trials conducted from 4:00 PM to 5:10 PM, on different days (no more than one week apart). Whole fruit trial preceded blended fruit trial for efficient food preparation.
- Timing Relevance: Postprandial glycemic response measured after lunch, considering participants’ shared lunchtime between 12:00 PM and 1:00 PM.
Fruit Treatments:
- Ingredients: One serving size of gala apple (190 g) and blackberries (148 g) used for both whole fruit and blended fruit treatments. Central core and seeds removed, skin included.
- Blended Fruit Preparation: Blended fruit treatment made with 100 mL water, 100 g ice, totaling 248 mL. To maintain consistency, 200 mL water consumed with the whole fruit treatment.
- Fruit Selection Rationale: Gala apples and blackberries chosen for common use in fruit smoothies, and both are rich in dietary fiber, particularly blackberries.
- Fiber Content: A single serving of apple contains 3.23 g insoluble fiber and 0.38 g soluble fiber; blackberries have 6.66 g insoluble and 0.76 g soluble fiber.
- Carbohydrate Composition: Gala apples: 57.2% fructose, 26.8% sucrose, 16% glucose. Blackberries: ~50% fructose, 48% glucose, <1% each of sucrose and maltose.
- Enzyme Treatment: Blended apple and blackberry samples incubated with invertase to cleave sucrose into glucose and fructose.
- Glucose Measurement: Glucose oxidase/peroxidase assay performed on blended apples and blackberries using spectrophotometer readings. Gala apples contained 6.27 g glucose, blackberries had 3.76 g glucose, totaling 10.03 g glucose per fruit treatment.
- Quality Assurance: Enzyme assays confirmed glucose percentages align with reported values for apples and blackberries.
Experimental Trial Procedure:
- Pre-Trial Instructions: Participants instructed to have lunch between 12:00 PM and 1:00 PM, three to four hours prior to the experimental trial, with only water allowed thereafter.
- Baseline Measurements: Baseline glycemic measurements taken before the trial initiation.
- Preparation of Fruit Treatment: Participants consumed a fruit treatment over 10 minutes, consisting of 200 mL water.
- Post-Consumption Measurements: Blood glucose values measured every 10 minutes for one hour after fruit treatment ingestion.
- Measurement Method: Glucometer (Auvon Blood Glucose Monitor) used for blood glucose readings, taken in triplicate during each measurement interval.
- Accuracy Assurance: Auvon glucometers reported to function within ±10 mg/dL of lab values over 95% of the time, surpassing ISO passing standards.
- Outlier Handling: If one of the three glycemic values deviated by 10% or more from the average, a fourth measurement replaced the outlier.
- Blood Sample Collection: Blood drawn from the index, middle, or ring finger of either hand using a lancet fitted for the glucometer.
Statistical Analysis:
- Data Adjustment: Baseline glycemic values subtracted from all data, and the trapezoidal method employed to calculate incremental area under the curve (iAUC).
- Dependent Variables: Analysis focused on three variables — maximum blood glucose, 60-minute blood glucose, and iAUC.
- Software and Version: SPSS Statistics (version 28.0.1.0) utilized for statistical analysis.
- Experimental Factors: Fruit treatment (whole vs. blended) considered as the within-subjects variable, and sex as the between-subjects factor.
- Data Transformation: Maximum blood glucose values square root transformed, and iAUC data log10 transformed to achieve normality, confirmed through Shapiro–Wilkinson normality test.
- Significance Level: Statistical significance set at p ≤ 0.05.
- Analysis Methods: Repeated measures two-way ANOVA employed to assess the impact of fruit treatment and sex on the three dependent variables.
Results
The results really shocked, but here they are:
Blending apple and blackberries reduces the glycemic response compared to consuming them whole. Baseline glycemic values were within the normal range for both treatments. The maximum blood glucose was markedly lower in the blended fruit group (28.8 mg/dL) compared to the whole fruit group (42.5 mg/dL). Similarly, the incremental area under the curve (iAUC) for glucose was significantly lower in the blended fruit treatment (850 mg/dL × min) compared to the whole fruit treatment (1269 mg/dL × min). At 60 minutes, glucose levels were marginally lower in the blended fruit group (6.7 mg/dL) compared to the whole fruit group (12.1 mg/dL). These findings suggest that blending may have a favorable impact on postprandial glycemic response.
Sex as a Factor in Glycemic Response to Whole vs. Blended Fruit
The study found that sex did not have a statistically significant impact on the glycemic response to consuming whole fruit versus blended fruit. Analysis of the three measured dependent variables (glucose maximum, glucose iAUC, and 60-minute glucose) did not show significant differences between males and females (p > 0.05). While there was a noticeable difference in the glycemic response to blended fruit, with males exhibiting a faster return to baseline and lower 60-minute blood glucose compared to females, this difference did not reach statistical significance (p = 0.15). Therefore, the study suggests that, although there is an observable trend, the influence of sex on glycemic response may not be statistically significant in the context of whole versus blended fruit consumption.
Discuss the results
Blending apples and blackberries reduces postprandial glycemic response, potentially due to the release of fiber, polyphenols, fats, and proteins from blackberry seeds. Similarities with studies involving mango and apple support the idea that blending seeded fruits decreases glycemic response. The high polyphenol content in blackberry seeds may inhibit glucose transport and digestive enzymes, contributing to reduced glycemic response. Additionally, the blending process may enhance fiber solubility, influencing chyme viscosity and glucose absorption. Although the study did not observe significant sex-specific differences, the faster return to baseline in males after blended fruit consumption suggests a potential trend. Future studies should explore insulin responses, separate fruits in treatments, extend monitoring duration, and consider participant lunch variations.
The study’s findings may not directly apply to commercial smoothies with added sugars, and caution is advised in generalizing results to larger serving sizes or diverse smoothie compositions.
Conclusion
This study affirms the potential health benefits of fruit smoothies without added sugars, aligning with recommended daily fruit intake when serving sizes mirror whole fruits. To enhance nutritional content and mitigate glucose rises, incorporating protein powder, nut butter, and seeds into smoothies is recommended. This addition introduces proteins and fats, contributing to nutrient richness and potentially reducing postprandial glycemic response. Such dietary modifications could play a role in preventing insulin resistance and type 2 diabetes in healthy individuals.
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