Fasting has become an increasingly popular way to lose weight, improve health, and simplify life. Proponents of intermittent fasting, alternate day fasting, and extended fasts claim benefits like better blood sugar control, reduced inflammation, and increased longevity. However, critics argue that fasting may lead to nutrient deficiencies over time. One nutrient of particular concern is vitamin B12.
Vitamin B12 plays vital roles in red blood cell formation, cell metabolism, nerve function, and DNA synthesis. A deficiency can cause fatigue, weakness, constipation, loss of appetite, weight loss, neurological problems, and megaloblastic anemia. B12 deficiency is estimated to affect up to 15% of the general population.
This article will explore whether fasting can cause or contribute to B12 deficiency. We’ll look at:
- How B12 is obtained and utilized in the body
- Groups at risk for deficiency
- Potential impact of fasting on B12 status
- Preventing and treating deficiency on a fasting regimen
Gaining this understanding will allow you to fast safely and maximize the health benefits.
How Is B12 Obtained and Used in the Body?
Vitamin B12 is unique among vitamins in that it is synthesized exclusively by certain bacteria and archaea. Most animals obtain B12 by eating animal products like meat, eggs and dairy. Some plant foods may be contaminated with B12 due to bacterial interaction in the soil. However, plant products are not considered a reliable source.
B12 absorption is a complex multistep process that begins in the stomach. When food is ingested, B12 is freed from proteins by stomach acid and digestive enzymes. It then combines with a protein called intrinsic factor and travels to the end of the small intestine where it enters circulation.
Once in the bloodstream, B12 is transported throughout the body and taken up by cells. It serves two primary functions:
- A coenzyme called methylcobalamin helps convert the amino acid homocysteine to methionine. This reaction is essential for proper cell division and DNA synthesis.
- Adenosylcobalamin facilitates rearrangement of methylmalonyl CoA to succinyl CoA. This provides substrate for the Krebs cycle which generates cellular energy.
Without adequate B12, both cellular replication and energy metabolism are impaired.
Who Is at Risk for B12 Deficiency?
While B12 deficiency can theoretically occur in anyone with low intake, certain groups are especially vulnerable:
- The elderly – decreased stomach acid impairs absorption from food
- Individuals with pernicious anemia – an autoimmune condition where antibodies target intrinsic factor
- Those with gastrointestinal conditions – diseases like Crohn’s and celiac damage the intestine and prevent absorption
- People taking medications – drugs like metformin and proton pump inhibitors interfere with absorption
- Strict vegetarians and vegans – no intake of meat, eggs or dairy
Genetic polymorphisms affecting B12 metabolism may also predispose certain ethnic groups like Mexicans and Indians to deficiency.
Pregnant women need extra B12 to support fetal development, and deficiency can cause neurological problems in the baby. Exclusively breastfed infants of vegan/vegetarian mothers are at high risk since milk contains very little B12.
With inadequate intake, liver stores of B12 can deplete in as little as 6 months. Fat malabsorption syndromes like celiac disease can accelerate depletion. Those already deficient are extremely sensitive to any further reductions in body stores.
Can Fasting Regimens Deplete B12?
To understand if fasting may cause B12 deficiency, we must consider its impact on absorption and utilization of the vitamin.
Absorption
Remember that intrinsic factor and stomach acid are essential for B12 absorption. Fasting impacts these factors in a few key ways:
- Reduced food intake means less trigger for gastric acid secretion. Over time, prolonged fasting may inhibit acid production.
- Less digestion of protein can also lower intrinsic factor levels since its release is stimulated by amino acids.
- Although absorption occurs in the small intestine, overall gut function often declines with fasting. This can impair transport of B12 into circulation.
For these reasons, the major fasting regimens like intermittent, alternate day and extended fasting could theoretically reduce B12 absorption, especially if prolonged. However, research on whether fasting truly lowers B12 levels has been mixed.
A 2016 study compared B12 status between intermittent fasting and daily calorie restriction groups. After 8 weeks, only the calorie restriction group saw a significant drop in B12. This suggests short-term intermittent fasting may spare B12 levels better than daily restriction.
Longer fasts may have a different impact. Case studies of people undergoing complete water fasts of 15 to 40 days documented significantly decreased B12 levels by the end. However, concentrations returned to normal after resuming eating.
Overall, periodic fasting and shorter fasts (Cellular Utilization
Another consideration is whether fasting alters cellular utilization of B12.
During fasting, cells shift from using glucose to fatty acids and ketones for energy. This “metabolic switching” spares blood glucose for the brain but can affect methylmalonyl CoA mutase, one of the B12-dependent enzymes.
Some research shows that ketones may inhibit activity of methylmalonyl CoA mutase. The implications of this are uncertain. It’s possible that functional B12 deficiency could develop in some tissues during extended fasting or nutritional ketosis even if body stores are adequate.
The risk depends on the length of the fast and degree of ketosis. Intermittent fasting generally doesn’t produce high, sustained ketones so is unlikely to affect B12 utilization. But longer fasts may potentially impair cellular uptake and metabolism of the vitamin.
Preventing B12 Deficiency When Fasting
The bottom line is that periodic, time-restricted fasting presents little risk of B12 deficiency for most people. However, those following long-term fasting protocols or repetitive extended fasts may need to take precautions to avoid depletion. Strategies include:
Consume B12 During Eating Windows
Make sure to get adequate B12 from food or supplements during feeding periods. Good sources include:
- Meats like beef, chicken and fish
- Eggs and dairy products like milk and cheese
- Nutritional yeast
- Fortified foods like cereal, plant milks and breakfast bars
- B12 supplements
The RDA for B12 is 2.4 mcg daily for adults, but many experts recommend at least 10 mcg daily, especially for those over 50 years old. Taking doses of 500-1000 mcg 2-3 times per week will help replenish liver stores.
Optimize Absorption
To aid absorption during eating windows:
- Take B12 supplements on an empty stomach
- Use sublingual forms that dissolve under the tongue
- Increase intake of protein foods to stimulate intrinsic factor
- Avoid excessive alcohol, caffeine and antacids which hinder absorption
- Consider supplementing with betaine HCL if you have low stomach acid
Monitor Status
Have your B12 level tested regularly to catch a potential deficiency early. Optimal levels are around 600-900 pg/mL. If your level starts to drop, increase your intake and consider sublingual or injectable supplementation which bypass GI absorption.
Supplement During Extended Fasts
When undertaking longer fasts of 3 days or more, supplement with a daily B12 pill or sublingual dose of at least 500 mcg. This will help counteract reduced absorption and utilization during the fasted state. Cease supplementation when you resume normal eating.
Treating B12 Deficiency
If a B12 deficiency does develop, swift treatment is important to reverse symptoms and avoid permanent neurological damage. Treatment involves:
- Oral or sublingual B12 of 1000-5000 mcg per day
- Intramuscular injections of 1000 mcg weekly for 1 month, then monthly
- Improving diet and absorption factors after stopping fasting
- Addressing any underlying gastrointestinal conditions
With prudent repletion, most symptoms will begin to improve within weeks. However, complete neurological healing may take up to a year.
Takeaways
Here are the key points to remember about fasting and B12 status:
- Intermittent and short-term fasting are unlikely to impact B12 levels in most people
- Prolonged fasting and chronic fasting may increase risk for deficiency over months to years
- Certain populations like vegetarians/vegans are at higher baseline risk
- Getting adequate B12 during feeding periods is crucial
- Monitor levels periodically and supplement if needed during extended fasts
- Rapidly treat any deficiency that develops with aggressive oral or injectable supplementation
With some simple precautions, periodic fasting can be maintained safely without compromising vitamin B12 status. Being aware of the risks allows you to structure your fasting practice in a smart, evidence-based way.
The Bottom Line
B12 is an essential vitamin that plays critical roles in red blood cell formation, cellular energy metabolism, neurological function and DNA synthesis. While periodic intermittent fasting is unlikely to affect levels, chronic fasting and extended fasting may increase risk for deficiency over months to years.
Those already at risk due to low intake or absorption issues should take particular care to maintain adequate B12 status when fasting. This includes vegetarians/vegans, the elderly, and those with gastrointestinal disorders.
Getting sufficient B12 from fortified and animal source foods during feeding windows is important. Supplementing with sublingual or injectable B12 periodically or during prolonged fasts can also prevent depletion.
With some basic monitoring and supplementation practices, fasting can be undertaken safely without compromising vitamin B12 status for most people.