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Onions

Also known as: Allium cepa, Onion, Bulb onion

Overview

Onions (Allium cepa) are a widely consumed vegetable recognized for their rich composition of bioactive compounds, including flavonoids (notably quercetin), sulfur-containing compounds, and antioxidants. They are utilized both as a culinary ingredient and as a dietary supplement to potentially promote various health benefits. Onion supplementation is primarily explored for its effects on metabolic health parameters, such as improving lipid profiles, glycemic control, and cardiovascular risk factors. Research in this area is relatively mature, featuring numerous randomized controlled trials (RCTs) and systematic reviews/meta-analyses. The available evidence suggests that onion supplementation, particularly with onion peel extract, may contribute to modest but clinically relevant improvements in body weight, lipid profiles, and blood pressure. These effects are attributed to the antioxidant, anti-inflammatory, and lipid-modulating properties of onion bioactives.

Benefits

Onion supplementation offers several evidence-based benefits, particularly for metabolic health. Studies indicate a significant reduction in body weight, body fat percentage, BMI, and waist circumference in overweight/obese individuals, especially with onion peel extract supplementation. Improvements in lipid profiles include reductions in LDL-c and total cholesterol, along with an increase in HDL-c. Longer supplementation periods (>12 weeks) and higher doses (>300 mg/day) have been associated with decreases in systolic and diastolic blood pressure. Furthermore, onion supplementation may improve liver enzyme levels and adiponectin levels, suggesting potential benefits for hepatic health and metabolic regulation. These effects are more pronounced in overweight/obese adults and those with dyslipidemia or metabolic syndrome features. While the magnitude of these changes is modest, they are clinically relevant as an adjunctive therapy.

How it works

Onion's beneficial effects are attributed to its bioactive compounds, primarily quercetin and sulfur compounds. These compounds exert antioxidant, anti-inflammatory, and lipid-modulating effects through various biological pathways. They influence adipocyte metabolism, promoting a reduction in fat accumulation and improving insulin sensitivity. Quercetin acts on AMP-activated protein kinase (AMPK) pathways, adiponectin signaling, and inflammatory cytokines. Sulfur compounds may affect lipid peroxidation and nitric oxide pathways. The absorption and bioavailability of quercetin glycosides from onion peel are moderate, with processing methods influencing the bioactive content. Onion peel extracts tend to have higher concentrations of these beneficial compounds compared to raw onion bulbs.

Side effects

Onion supplementation is generally considered safe and well-tolerated at the studied doses (up to approximately 468 mg/day of extract). The most commonly reported side effect is mild gastrointestinal discomfort, experienced by a small percentage of individuals (less than 5%). Allergic reactions are rare but possible in sensitive individuals. Due to the antiplatelet effects of onion compounds, there is a potential mild interaction with anticoagulants, warranting caution. Onion supplementation is contraindicated in individuals with a known allergy to Allium species. Limited data are available regarding the safety of onion supplementation in pregnant or lactating women, and caution is advised in these populations. Overall, the safety profile of onion supplementation is favorable, but individuals should be aware of potential gastrointestinal effects and the possibility of allergic reactions.

Dosage

The minimum effective dose of onion extract to achieve metabolic benefits is around 300 mg/day. An optimal dosage range is between 300 and 468 mg/day of onion peel extract or standardized quercetin-rich preparations. No established upper limit exists, but clinical trials have safely used up to approximately 468 mg/day for 12 weeks. Daily supplementation for at least 12 weeks is recommended to observe measurable effects. Onion peel extract capsules or powders standardized for quercetin content show superior efficacy compared to raw onion. Bioavailability may be enhanced by the food matrix and possibly by co-administration with fats. No specific cofactors are required, but general nutritional support is recommended.

FAQs

Can onion supplements cause weight loss?

Yes, modest reductions in body weight and fat have been demonstrated in overweight individuals with consistent onion supplementation, particularly with onion peel extract.

Is onion supplementation safe long-term?

Short to medium-term use (up to 12 weeks) is generally safe. Long-term safety data are limited, but no major concerns have been reported in the available research.

When should I take onion supplements?

Daily intake with meals may improve absorption and reduce gastrointestinal discomfort. Consistency is key for observing potential benefits.

Are raw onions as effective as supplements?

Supplements, especially onion peel extracts standardized for bioactives, provide more consistent and potent effects compared to consuming raw onions.

Does onion affect blood pressure?

Yes, onion supplementation can modestly reduce systolic and diastolic blood pressure, particularly with longer use and higher doses.

Research Sources

https://pubmed.ncbi.nlm.nih.gov/38056991/ – A systematic review and meta-analysis including 14 RCTs found that onion supplementation significantly improved body fat percentage, LDL-c, total cholesterol, HDL-c, blood pressure, and liver enzymes. The effects were more pronounced with supplementation lasting longer than 12 weeks and at doses exceeding 300 mg/day, suggesting a dose-response relationship.
https://pmc.ncbi.nlm.nih.gov/articles/PMC10420769/ – This systematic review and meta-analysis of five RCTs in overweight/obese adults demonstrated that onion supplementation led to significant reductions in body weight (approximately 0.74 kg), BMI, body fat, and waist circumference, particularly with onion peel extract. The consistent findings across studies with low risk of bias support the potential of onion peel extract as an adjunct for weight management.
https://examine.com/research-feed/study/94xa49/ – This source summarizes research indicating that onion supplementation can improve HDL (+2.3 mg/dL), LDL (-6.6 mg/dL), and total cholesterol (-5.4 mg/dL). The meta-analysis included ten RCTs with 446 participants, suggesting a beneficial effect on lipid profiles, although there was no significant impact on triglycerides.
https://onlinelibrary.wiley.com/doi/abs/10.1002/fsn3.3426 – This meta-analysis focuses on the antiobesity effects of onion consumption. It highlights the potential of onion, particularly onion peel extract, to reduce body weight, BMI, and waist circumference in overweight and obese individuals, reinforcing its role as a functional food with metabolic benefits.
https://onlinelibrary.wiley.com/doi/10.1002/fsn3.2309 – This meta-analysis investigates the impact of onion consumption on lipid profiles. The findings suggest that onion supplementation can lead to improvements in HDL, LDL, and total cholesterol levels, indicating a positive effect on cardiovascular health by modulating lipid metabolism.