A two-week trial reveals that soups made from broccoli, cabbage, and kale reduce post-meal glucose spikes more effectively than starchy vegetables like potatoes and pumpkin, pointing to the unique power of cruciferous vegetables for metabolic health.
Study: Cruciferous vegetables improve glycaemic control compared to root/squash vegetables in a randomized, controlled, crossover trial: The VEgetableS for vaScular hEaLth (VESSEL) study. Image credit: Pixel-Shot/Shutterstock.com
People who eat more cruciferous vegetables have a reduced risk of type 2 diabetes (T2DM) and cardiovascular disease. A recent study in Diabetes, Obesity and Metabolism compared the effect of cruciferous vegetables on glucose regulation with that of root vegetables and squash, in adults without diabetes but with hypertension.
Introduction
About 541 million people worldwide struggle with impaired blood glucose control, putting them at risk for T2DM and cardiovascular disease. The resulting illness and healthcare costs make improving glycemic control an urgent priority.
Dietary strategies, especially the consumption of fruit and vegetables, are essential in reducing the risk of T2DM. However, not all vegetables are created alike. Cruciferous vegetables like broccoli are often considered protective against diabetes. However, there is little hard data to support this. A previous study by the authors of the present paper concluded that regular cruciferous vegetable consumption was associated with a reduction in systolic blood pressure and serum triglycerides.
The current study was a randomized controlled trial with 18 participants, mostly women, whose median age was 68. They had mild hypertension, both systolic and diastolic, but no diabetes. They were randomized to receive 300 g a day of either cruciferous vegetables or roots/squash, in the form of 600 mL of soup, each day, for two weeks each. The cruciferous vegetable soup comprised 40% broccoli, 25% cabbage, 25% cauliflower, and 10% kale. The roots/squash soup was made of 20% carrot, 40% potato, 30% pumpkin, and 10% sweet potato.
Despite attempts to match nutrient composition, the cruciferous soup had slightly fewer carbohydrates (-4 g), more protein (+3 g), more fiber (+1.5 g), and slightly lower energy (-12 kJ) per day than the control soup, which the authors noted could have contributed to the effects. They also received standardized lunch and dinner, frozen meals from the local supermarket that did not include any cruciferous vegetables.
Participants were blinded to the allocation, and continuous glucose monitors were taped to prevent them from viewing readings. The glucose levels were measured every 15 minutes throughout the intervention period. The two arms of the trial crossed over after a two-week washout period.
Study findings
The mean fasting blood sugar at baseline was 5.5 mmol/L. At this point, the habitual intake of cruciferous vegetables was about 26 g per day, while the total vegetable intake was about 327 g per day. This emphasizes the current low vegetable intake despite the guidance offered by most national health associations.
The cruciferous vegetables group showed more stable blood sugar levels than the roots/squash group, with a 2% lower mean variability in interstitial glucose levels. Notably, greater fluctuation in glucose levels is linked to more oxidative stress and poorer prognosis, including increased cardiovascular risk, irrespective of the development of diabetes.
The small size of the effect might have been because this group did not have diabetes. With patients who have diabetes, amplified impacts might be expected, with cruciferous vegetables stabilizing the blood sugar and improving the clinical outcome.
In addition, the 2-hour postprandial glucose response (PPGR-2) was lower in the cruciferous group after lunch and dinner. So was the area under the curve, the mean difference being -0.14 mmol/L and -20.1 mmol/L x minute, respectively. This difference was mostly due to the after-dinner PPGR; the lunch-time differences were not statistically significant.
These results agree with prior studies showing that stronger-tasting and more bitter cruciferous vegetables improve insulin resistance and fasting blood glucose levels, compared to other vegetables or dietary controls. In this same cohort, previous analyses showed higher plasma sulforaphane concentrations during cruciferous intake. However, oxidative stress and inflammation markers did not differ significantly, likely due to the small sample size. Such compounds produce antioxidant and anti-inflammatory effects by inhibiting the pro-inflammatory NF-ĸB pathway while activating the Nrf2 pathway.
However, the mean continuous glucose did not vary between the groups. Again, this might be because of the absence of T2DM in this group. Alternatively, the time and the meal at which crucifers are served might affect the physiological effects of these vegetables.
Crucifers also contain more vitamin K and nitrate than others. These have also been linked to reductions in the risk of T2DM and a better prognosis for patients with this condition. Both groups had comparable weight loss and reduced calorie consumption, and the amount of weight loss did not differ between arms, suggesting the glycemic benefits of crucifers were independent of weight change.
These findings underline the public health recommendations for eating more vegetables. Concerningly, vegetable consumption remains low in the population at large, and cruciferous vegetables are among the least consumed.
Conclusions
Glycemic control was a pre-specified secondary outcome of this vascular/metabolic trial. The trial was short-term, included only 18 participants (mostly women), and was conducted in non-diabetic individuals, so generalizability is limited.
“Cruciferous vegetable consumption improved postprandial glycemic control compared with root/squash vegetables.”
However, further study is required to understand how this affects the clinical prognosis in people with impaired glucose control.
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