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When it comes to cod liver oil, taking a lot may be too much of a good thing.

A group of prominent vitamin D researchers has issued a commentary that cautions against consuming large amounts of the supplement because it may lead to excessive exposure to vitamin A.

Taking cod liver oil, a ritual in many households during the 1950s and 1960s, isn't as common as it once was.

But with renewed interest in the health benefits of omega-3 fatty acids, found in fish oils, the practice may be becoming more extensive.

The authors of the commentary are recommending consumers refine their strategy to derive the greatest benefits from supplements.

"Basically, if you're taking your teaspoon full of cod liver oil, it's fine ... but higher levels are not associated with health," says Reinhold Vieth, a professor in the department of nutritional sciences at the University of Toronto and one of the authors.

The recommendation appears in the current edition of the Annals of Otology, Rhinology and Laryngology, and is based on concerns that the high amount of vitamin A in cod liver oil undermines the effectiveness of vitamin D, which is also found in the supplement.

The researchers believe many people who are taking both multivitamins and cod liver oil may reach such high levels of vitamin A that they could experience bone toxicity, leading to hip fractures and other problems.

Prof. Vieth says consuming fish oils specifically marketed for heart-healthy omega-3 fatty acids is an "excellent" idea because they generally don't contain any vitamin D or vitamin A.

The commentary said that most people should be able to maintain healthy vitamin A levels without supplements, because a diet rich in carrots, sweet potatoes, cantaloupe and other colourful fruits and vegetables will supply more than enough of the nutrient.

But the picture for vitamin D is different, and insufficiency is widespread, according to the commentary.

Part of the problem stems from the common practice of sun avoidance. Vitamin D is made naturally in skin exposed to sunlight. Many people shun the summer sun because of the risk of skin cancer. And most people's diets aren't rich in the vitamin either. Small amounts are present in oily cold-water fish and some types of mushrooms, but not enough of these foods are consumed by most people to raise their levels of the sunshine vitamin sufficiently.

"We wish that our diets were as rich in vitamin D as they are in vitamin A. With the exception of infants on formula or toddlers drinking large amounts of milk or vitamin-D-fortified juice, adequate amounts of vitamin D are virtually impossible to obtain from diet," the authors said.

Among those who signed the commentary were prominent nutrition experts, such as Walter Willett from Harvard University's school of public health and Joan Lappe, a U.S. researcher who wrote an influential paper last year indicating that high doses of vitamin D appear to help prevent cancer.

According to the commentary, cod liver oil sold decades ago tended to have higher amounts of vitamin D than products available today. It speculated that modern deodorization removes the nutrient, which is then added back in by processors at lower doses.

It cited one brand that the manufacturer said had only 3 to 60 international units of vitamin D for each tablespoon and up to 6,000 IU of vitamin A.

The commentary called for higher rates of vitamin D supplementation, particularly for children. It said children who don't receive significant sun exposure probably need about 1,000 IU of vitamin D a day for every 11 kilograms of body weight.

The Canadian Cancer Society recommends that adults take a supplement of 1,000 IU of vitamin D each day during winter months, and year-round for those with dark skin or people aged 50 or older.