From the Guidelines
Vitamin B12 deficiency is a significant concern in obese individuals, and I strongly recommend getting B12 levels tested before starting any supplementation, as evidenced by the most recent study in 2025 1.
Importance of B12 Testing
Obese individuals are at a higher risk of nutrient deficiencies, including vitamin B12, due to their diet and potential underlying health conditions. The study published in Advances in Nutrition in 2025 highlights the importance of assessing nutritional status in obese patients, as they may experience deficiencies despite not being undernourished 1.
Supplementation and Treatment
For those with confirmed deficiency, typical supplementation includes oral B12 at 1,000-2,000 mcg daily or weekly B12 injections (1,000 mcg) until levels normalize. However, it's essential to follow the guidelines for treatment, as outlined in the 2020 study published in Obesity Reviews, which recommends immediate treatment of vitamin B12 deficiency using specific protocols, including hydroxocobalamin injections 1.
Relationship Between B12 and Obesity
Obesity may affect B12 absorption and metabolism through several mechanisms, including gastric bypass surgery, metformin use, and chronic inflammation. The 2023 study published in Diabetes Care notes that long-term use of metformin may be associated with biochemical vitamin B12 deficiency, emphasizing the need for periodic measurement of vitamin B12 levels in metformin-treated individuals 1.
Comprehensive Approach
While adequate B12 levels may support weight management, taking B12 supplements without a confirmed deficiency will not cause weight loss. A comprehensive approach, including balanced nutrition, regular physical activity, and medical supervision, is crucial for obese individuals rather than relying on B12 supplementation alone.
Key Considerations
- Get B12 levels tested before starting supplementation
- Follow guidelines for treatment of vitamin B12 deficiency
- Consider the potential impact of metformin on B12 levels
- Focus on a comprehensive approach to weight management, including nutrition, physical activity, and medical supervision
From the Research
Vitamin B12 and Obesity
- Vitamin B12 deficiency is a common issue that can lead to various health problems, including megaloblastic anemia and neuropsychiatric symptoms 2
- Obesity is associated with a higher risk of vitamin B12 deficiency, as seen in studies where obese individuals had lower vitamin B12 levels compared to non-obese individuals 3, 4
- The relationship between vitamin B12 levels and body mass index (BMI) has been observed, with vitamin B12 levels negatively correlated with BMI 3
Vitamin B12 Deficiency in Obese Individuals
- A study found that 37.6% of obese women had vitamin B12 deficiency, compared to 24.7% of non-obese women 3
- Another study found that 34.1% of obese adolescents had vitamin B12 deficiency, compared to 12.5% of healthy control adolescents 4
- Vitamin B12 deficiency can have significant effects on the body, including impaired neuronal migration, metabolic reactions, and other systems 4
Diagnosis and Treatment of Vitamin B12 Deficiency
- Screening for vitamin B12 deficiency is not recommended for average-risk adults, but may be warranted in patients with risk factors such as obesity 2
- Diagnosis of vitamin B12 deficiency can be made using serum vitamin B12 levels, as well as other tests such as holotranscobalamin, serum methylmalonic acid, and plasma homocysteine measurements 5
- Treatment of vitamin B12 deficiency can be done using oral or intramuscular administration of vitamin B12, with oral administration being comparable to intramuscular administration in terms of efficacy 2, 6