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Vitamin B12, also known as cobalamin, is a water-soluble vitamin that’s involved in various essential bodily processes.

For instance, your body uses it to make DNA and create new red blood cells, as well as produce energy. It may even affect your mood and memory and is instrumental in helping your brain and nervous system function properly (1, 2, 3).

Therefore, a deficiency in this nutrient can have debilitating health effects.

What’s more, some people have recently suggested that unwanted weight gain be added to the list of possible side effects.

This article reviews the latest scientific evidence to determine whether a vitamin B12 deficiency may result in weight gain.

To absorb vitamin B12 effectively, your body requires an intact stomach and gut, a well-functioning pancreas, and sufficiently high levels of intrinsic factor, a protein that binds to vitamin B12 in the stomach (1).

Adult women require 2.4 mcg of vitamin B12 per day. This requirement increases to 2.8 mcg per day during pregnancy and breastfeeding. Adult men can meet their needs by consuming 2.6 mcg of vitamin B12 per day (1).

Extra vitamin B12 can be stored in the liver, and only small amounts are lost through your urine, sweat, or stools each day. Because of this and the small daily requirements, it can take a year or longer of insufficient vitamin B12 intake to develop an overt deficiency (1).

However, once present, it can have debilitating consequences.

Symptoms of vitamin B12 deficiency include (1):

  • chronic fatigue
  • shortness of breath
  • heart palpitations
  • tingling or numbness in the extremities
  • poor balance
  • loss of concentration
  • poor memory
  • disorientation
  • mood changes
  • incontinence
  • insomnia

People most at risk of developing a deficiency include older adults, as well as those who smoke, abuse alcohol, or follow a vegetarian or vegan diet.

Gut surgery, pancreatic insufficiency, small intestinal bacterial overgrowth (SIBO), parasite infections, and certain autoimmune disorders are considered additional risk factors.

Moreover, certain medications, including metformin, proton pump inhibitors, and antacids may also reduce your body’s ability to absorb vitamin B12 from your diet (1, 4).

summary

Vitamin B12 deficiency can manifest itself through a variety of symptoms. People most at risk of deficiency include older adults, vegans, and those with specific medical conditions or taking certain medications.

Despite the numerous processes in which vitamin B12 is involved, there’s little evidence to suggest that it has any influence on weight gain or loss.

Most of the evidence giving rise to this claim comes from a few observational studies.

For instance, one study notes that people with excess weight or obesity appear to have lower vitamin B12 levels than people who have a body mass index (BMI) in the “normal” range (5).

Additional research has observed that people who voluntarily took vitamin B12 supplements gained between 2.5–17 fewer pounds (1.2–7.7 kg) over 10 years than those who did not supplement with this vitamin (6).

Still, such observational studies cannot confirm whether low vitamin B12 levels are what caused the weight gain, nor whether they’re what protected against low levels.

On the other hand, a vitamin B12 deficiency appears to cause a loss of appetite in some people, leading to weight loss rather than weight gain (7, 8).

That said, the current evidence is too weak to suggest that a vitamin B12 deficiency has any strong or specific impact on weight — be it weight gain or loss.

summary

There’s little evidence to support the notion that vitamin B12 deficiency causes weight gain. More studies are needed before such strong statements can be made.

Vitamin B12 is found exclusively in animal foods or foods fortified with this vitamin, such as:

  • Meat and chicken: especially organ meat and red meat like beef
  • Fish and seafood: especially clams, sardines, tuna, trout, and salmon
  • Dairy: including milk, cheese, and yogurt
  • Eggs: especially egg yolk
  • Fortified foods: breakfast cereal, nutritional yeast, as well as some mock meats or plant milks

Supplements, which are widely available in stores and online, are another way to meet your daily requirements.

They’re especially handy for people with low levels of intrinsic factor, a protein that helps your body absorb vitamin B12 more easily (9).

Additionally, they can help those who have difficulty consuming sufficient amounts of the foods above meet their daily vitamin B12 needs. This may include vegetarians or vegans who don’t plan their diet carefully (10, 11).

summary

Vitamin B12 can be found in animal products, as well as foods fortified with it. Supplements are a practical way for some people to ensure they meet their daily vitamin B12 requirements.

Vitamin B12 is a water-soluble vitamin that your body uses for a variety of processes, including producing energy and maintaining a healthy brain and nervous system.

Sources of vitamin B12 include animal foods, vitamin-B12-fortified foods, and supplements.

People with vitamin B12 deficiency are likely to experience a wide range of symptoms, though weight gain is unlikely one of them.

If you’re experiencing unexplained weight gain, consider discussing it with your healthcare provider or a registered dietitian to determine the root cause.