Vitamin B12 is an essential nutrient that plays many critical roles in the body. It helps make DNA, nerve cells, and red blood cells. It also supports energy metabolism and brain function. Low levels of B12 can cause fatigue, memory problems, and neurological issues. Many people are deficient in this important vitamin. So how long does it take to restore B12 levels after they have dropped too low?
What is Vitamin B12?
Vitamin B12, also known as cobalamin, is a water-soluble essential vitamin. This means that the body requires it for normal functioning but cannot produce it on its own. B12 must be obtained from food or supplements. The recommended daily intake is 2.4 mcg for adults, although requirements increase slightly during pregnancy and breastfeeding.
B12 is unique among vitamins in that it contains a metal ion, cobalt. It’s the only vitamin that contains a metallic element. It has a complex chemical structure that allows it to participate in a variety of enzymatic reactions.
There are a few different forms of B12 used in supplements:
- Cyanocobalamin: This synthetic form is used in the majority of B12 supplements.
- Methylcobalamin: A naturally occurring and active form of B12.
- Adenosylcobalamin: Another naturally occurring form that is readily usable by the body.
- Hydroxocobalamin: The form of B12 administered via injection.
In the body, B12 is converted between different forms as needed. Vitamin B12 is found almost exclusively in animal products, so vegans and vegetarians are at a higher risk of deficiency. It can also be an issue for the elderly due to reduced absorption.
B12 Function and Deficiency
Vitamin B12 plays essential roles in many bodily processes:
- DNA and red blood cell production
- Energy metabolism
- Neurological function
- Homocysteine metabolism
- Cardiovascular health
When B12 levels drop too low, it can lead to neurological disorders and symptoms like:
- Fatigue
- Weakness
- Constipation
- Loss of appetite
- Weight loss
- Depression
- Poor memory
- Confusion
- Neuropathy
- Dementia
Deficiency occurs for a number of reasons:
- Lack of dietary B12 from plant-based diets or malnutrition
- Medications – proton pump inhibitors, metformin
- Malabsorption from gut disorders like IBS, Crohn’s, celiac disease
- Pernicious anemia – inability to absorb B12
- Other genetic or immune conditions affecting absorption
- Alcohol abuse
When deficiency is suspected, doctors can test B12 blood levels to confirm. Levels below 300 pg/mL indicate a vitamin B12 deficiency.
How is B12 Absorbed and Transported?
In order to understand how long it takes B12 levels to increase, it helps to know how the vitamin is absorbed and used in the body.
Intake and Absorption
Dietary B12 is bound to proteins in food. When it reaches the stomach, hydrochloric acid separates vitamin B12 from proteins. Once dissociated from food, B12 binds with a protein called haptocorrin. This protects the B12 from being broken down by stomach acid.
As B12 enters the upper small intestine, pancreatic enzymes break down the haptocorrin. The freed B12 can then bind with intrinsic factor, another protein made by specialized cells in the stomach lining. The B12-intrinsic factor complex makes its way to the end of the small intestine, where receptors enable its absorption into the bloodstream.
Transport and Storage
Once in the blood, B12 detaches from intrinsic factor and binds to another protein called transcobalamin II. This protects B12 and transports it to the tissues of the body. The liver stores approximately 50% of vitamin B12 reserves in the body. The rest is stored in muscle tissue.
B12 remains safely tucked into these storage sites until it is needed and released by the liver into the bloodstream. On average, 2–5 mg of B12 is stored in the human body, mostly in the liver. This can provide a buffer when intake is low. But stores can become depleted over time without adequate intake from diet or supplements.
Cellular Uptake and Metabolic Roles
For B12 to be used at the cellular level, it must detach from transcobalamin II and enter cells. Inside cells, it is converted into the active forms methylcobalamin and adenosylcobalamin. These active forms carry out important metabolic roles:
- Methylcobalamin helps convert homocysteine to methionine, needed for red blood cell production and neurological function.
- Adenosylcobalamin facilitates conversion of methylmalonyl-CoA to succinyl-CoA, needed for DNA synthesis.
How Long Do B12 Deficiency Symptoms Last?
The onset of B12 deficiency symptoms is gradual. Symptoms may not appear until B12 levels have been low for an extended period. In fact, deficiency can take years to develop because the body recycles and reuses this vitamin once absorbed.
When deficiency does occur, symptoms may include:
- Chronic fatigue and weakness
- Tingling in hands and feet
- Difficulty walking or moving
- Vision loss or light sensitivity
- Cognitive troubles like memory loss and confusion
- Mood changes like depression or apathy
- Neuropsychiatric issues like paranoia or hallucinations
These symptoms of B12 deficiency can persist and worsen over years if levels are not corrected. Prompt treatment is important to relieve symptoms and avoid permanent nerve damage.
How Soon Does B12 Replenish?
How long it takes to restore B12 status depends on the cause of deficiency, its severity, and repletion method.
Oral Vitamin B12
For mild deficiencies, oral B12 supplements can be used to correct levels. Vitamin B12 is absorbed under the tongue and in the stomach via passive diffusion. Only about 1.2 mcg of a 500 mcg oral supplement gets absorbed via this method. Absorption is similarly low for B12 obtained from foods.
With oral supplementation, B12 blood levels begin to increase within 1-2 days of taking a supplement. But it takes longer for tissue storage sites and metabolic levels to reach optimal levels.
For deficient individuals using oral B12 tablets of 1000-2000 mcg daily, it can take 1-2 months for symptoms to improve and B12 levels to return to normal. Some studies show that only around 19-29% of people taking oral B12 restore their levels within 3 months.
High-Dose Oral B12
Higher oral doses are more effective for replenishing B12. One study gave 1000 mcg oral B12 daily to deficient individuals and compared results to those given intramuscular injections.
Within 4 days, average plasma B12 levels increased:
- Oral group: From 149 to 349 pg/mL
- Injected group: From 141 to 355 pg/mL
While injections increased levels slightly faster initially, by day 10 the oral group’s levels surpassed the injections. After 3 months, 94% of the oral group and 100% of the injected group were B12 replete.
This demonstrates that an aggressive oral B12 dosing strategy of 1000 mcg per day can correct deficiency similar to injections.
Intramuscular Injections
For severe deficiency, B12 is administered via intramuscular injection, usually of 1000 mcg. This bypasses absorption issues and provides immediate increases in blood levels as the B12 is absorbed directly into circulation.
With injections, B12 blood levels begin to increase within 1-2 days, peak around day 5, and remain elevated for about a month. It generally only requires one injection to restore B12 levels to normal range.
Clinical improvement of neurological symptoms may begin within a few days of the first injection if deficiency is significant. It typically takes 1 month for most neurological symptoms to substantially improve or resolve with B12 injections.
Sublingual and Sprays
Sublingual sprays and tablets are applied under the tongue for direct absorption into the bloodstream. They provide better absorption than standard oral supplements but are less effective than injections.
One study found that 500 mcg of sublingual B12 taken daily increased blood levels after 1 month from 288 to 445 pg/mL. Results improved further after 3 months of continued dosing. So sublingual B12 may require 1-3 months to normalize levels.
Who Should Increase Their B12 Intake?
The below populations are more prone to B12 deficiency and may require supplementation or injections to prevent or treat low levels:
- Vegans and vegetarians
- Breastfed infants of vegan/vegetarian mothers
- People over 50 years old
- Those with gastrointestinal disorders like Crohn’s or celiac
- People who have had gastrointestinal surgery such as weight loss procedures or bowel resection
- Those taking heartburn medications long-term
- People with autoimmune disorders like diabetes, lupus, or thyroid disease
- Those with a family history of B12 deficiency
- Heavy drinkers
Pregnant and breastfeeding women also have increased B12 needs, as it’s critical for infant development.
How to Tell if Your B12 Levels Are Rising
Blood tests are needed to definitively assess B12 levels. But there are also signs and symptoms you can watch for to evaluate if your levels are increasing in response to supplementation or injections:
Signs of Improving Levels
- Energy levels increase
- Mental fog begins to lift
- Tingling and numbness in hands/feet improves
- Balance and coordination improve
- Vision gets sharper
- Memory improves
- Mood lifts
Symptoms may start gradually improving within 1-2 weeks if deficiency was severe. In milder cases, it can take 1-3 months for symptoms to resolve.
Signs Levels Are Still Low
- Fatigue, weakness, and headaches persist
- Tingling or numbness in extremities
- Walking and balance issues
- Forgetfulness and brain fog
- Neuropsychiatric issues like depression or paranoia
- Sore mouth or tongue
- Vision problems like blurry vision or light sensitivity
- Poor appetite and weight loss
If deficiency symptoms continue after 3 months of oral B12 or 1 month of injections, it’s a sign your B12 levels remain insufficient. Follow up with your doctor to recheck levels. You may require a higher dose or more frequent B12 administration.
Conclusion
Vitamin B12 plays so many vital roles in the body. When levels fall too low, it can lead to fatigue, neurological issues, psychiatric problems, and symptoms like tingling or numbness in the extremities.
Luckily, correcting B12 deficiency is possible with proper supplementation and treatment. With oral tablets or sublingual B12, blood levels start increasing within 1-2 days but can take 1-3 months to fully restore cellular B12 status if deficiency is mild.
For severe deficiency, intramuscular B12 injections offer the fastest way to normalize levels. Symptoms often improve within 1-2 weeks. However, if deficiency is not treated promptly and for long enough, some effects like nerve damage may become permanent. That’s why it’s critical to catch and reverse B12 deficiency as soon as possible.