Pregnant woman smiling with hand on bump

Is this amino acid stopping you getting pregnant?

31 Mar 2023


One in seven couples have trouble conceiving, but new research shows an amino acid could have a role to play.

If you’re trying for a baby, it can be frustrating trying to figure out why you’re not getting pregnant. In 25% of cases, no underlying cause can be identified,1 but experts now believe, homocysteine, an amino acid may be important in some cases.

What is homocysteine?

The body produces homocysteine when it breaks down proteins in the food we eat. From this, the body makes cysteine using vitamin B6, which is then converted into the useful antioxidant glutathione.2 If we are deficient in B6, B9 (folate) or B12 vitamins, the body lacks the required nutrients to convert homocysteine into cysteine, leading to high levels of homocysteine in the blood. This can encourage the production of free radicals – harmful molecules thought to contribute to ageing, heart disease, dementia and damage to our DNA.3

The fertility link

After reviewing available research, scientists from Yale University School of Medicine concluded that high homocysteine levels may trigger the release of free radicals that can damage developing embryos4, while Italian researchers found women with fertility problems also had high homocysteine levels. In 2007, researchers from the University of Naples and Fatebenefratelli Hospital of Naples measured blood homocysteine levels in 20 healthy women, 20 women who had experienced repeated miscarriages and 20 women with unexplained infertility. Homocysteine levels were more than twice as high in the groups with recurrent pregnancy loss and infertility.5 Another study of 50 women undergoing IVF published in the Journal of Assisted Reproduction and Genetics in 2012 revealed that those who went on to become pregnant had lower levels of homocysteine.6

How might B12 help with conception?

Vitamin B12 helps reduce the amount of harmful homocysteine circulating in the bloodstream. It does this by ‘recycling’ homocysteine into methionine – an essential amino acid which helps the liver, bones and blood vessels to function.7 Raised levels of this amino acid may be due to a low intake of vitamin B12. A 2001 study by Ha’Emek Medical Center in Israel analysed the medical histories of 14 patients with low B12 levels, and found a link with infertility in four cases and recurrent miscarriage in 11 cases. The study concluded that B12 deficiency might affect ovulation or egg development, or trigger ‘changes leading to defective implantation’.8

What about male infertility?

There is evidence to show that vitamin B12 also plays a role in male fertility. A review of 23 studies by Jordan University of Science and Technology in 2017 concluded that B12 can increase sperm count, boost sperm motility and reduce sperm DNA damage, and that these effects may be partly due to decreased homocysteine toxicity.9
Advice is for information only and should not replace medical care. Please consult a doctor or healthcare professional before trying any remedies.
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1 NHS Choices. Infertility causes. Available from:
2 University of Maryland Medical Center. Cysteine. Available from:
3 Jack Norris. Mild B12 deficiency. Available from:
Harvard TH Chan. The nutrition source: three of the B vitamins: folate, vitamin B6 and vitamin B12. Available from:
Emedicinehealth. Homocysteine blood test. Available from:
WebMD. Homocysteine. Available from:
4 Riley JC, Behrman HR. Oxygen radicals and reactive oxygen species in reproduction. Available from:
5 D’Uva M, et al. Hyperhomocysteinemia in women with unexplained sterility or recurrent early pregnancy loss from Southern Italy: a preliminary report. Available from :
6 Ocal P, et al. The association between homocysteine in the follicular fluid with embryo quality and pregnancy rate in assisted reproductive techniques. Available from:
7 Jack Norris. Mild B12 deficiency. Available from:
Harvard TH Chan. The nutrition source: three of the B vitamins: folate, vitamin B6 and vitamin B12. Available from:
Dr Axe. Food is medicine. What is L methionine? L methionine benefits & top foods sources. Available from: ]
8 Bennet M. Vitamin B12 deficiency, infertility and recurrent fetal loss. Available from:
9 Banihani, SA. Vitamin B12 and Semen Quality. Available from:

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