Marleen Lentjes, nutrition and dietetics expert from the University of Cambridge, looks at what reasons there might be for using supplements despite studies observing no benefit or only in sub-optimally or mal-nourished populations. Marleen is involved in the event Cod liver oil: old habits die hard.
CSF: Every day, we are bombarded with claims by companies that sell supplements that if we take a particular supplement it will help alleviate whichever ailment we may be suffering from. Is half of what we're told true, or at least based on some scientific fact?
ML: Vitamins and minerals have very specific functions in our body and the majority of the vitamins cannot be made by humans and so we depend on our food (and sun, in the case of vitamin D) as the source of these essential nutrients to prevent diseases related to deficiencies (shortage). The best known example of this is probably scurvy, a disease affecting bone, connecting tissues and skin, which was very common among sailors centuries ago. On long journeys with no supply of fresh food, vitamin C storage in the body quickly diminished. Supplementing the diet with lime juice prevented scurvy, and so the lime juice was said to have ‘a-scurvy’ effect, hence the scientific name of vitamin C: ascorbic acid.
A well-balanced diet is important to prevent nutrient deficiencies, but our diet nowadays is sufficient for the majority of the UK population (with the possible exception of vitamin D). As our diets have developed over the past centuries, so also have the diseases we get. Nutrient deficiency diseases are less common and the most common illnesses nowadays are cardiovascular diseases, cancer and diabetes. All these diseases have associations with diet, but are unlikely to be due to nutrient shortages; if anything, a major source of these illnesses is the oversupply of food, leading to obesity. So our diet as a whole is important to prevent diseases, not just sufficiency of vitamins/minerals.
CSF: In the case of cod liver oil, many people have grown up being told that taking a spoonful of cod liver oil daily will help ward off rickets or arthritis. Nowadays, it's being heralded as a wonder supplement for everything from kidney disease in people with diabetes to reducing high blood pressure. And it's supposed to be good for hair, nails and skin too. Is any of this true?
ML: Cod liver oil contains a variety of nutrients: vitamin A, vitamin D and the long chain omega-3 fatty acids (LC n 3): eicosapentaenoic acid and docosahexaenoic acid. Vitamin D has traditionally been associated with bone health, it has been used for centuries to treat arthritis and in the early 20th century it was a treatment for rickets. However, nowadays, many possible other functions of vitamin D have been identified and the nutrient has been associated with cardiovascular disease, cancer and auto-immune diseases.
Another name for vitamin A is retinol, which -like ascorbic acid- gives an indication of its main purpose, namely related to eyesight (retina). When we restrict the functionality of these vitamins to prevention of deficiency diseases, these functions have been proven. In light of prevention of diseases related to the heart, inflammatory diseases or cancer, the functions of these vitamins are less clear. Analyses which have pooled results from several trials on the use of multivitamins/multiminerals as well as the use of antioxidants have not shown benefits on all-cause mortality, cardiovascular mortality or cancer in industrialised countries.
The traditional diet of the Inuit consists of a high consumption of seal and fish (and so is rich in LC n 3); these dietary habits have been associated with less heart disease. The diet in the UK is very low in fish consumption. In the 2000/01 National Diet and Nutrition Survey (NDNS), less than 50% of the adult population consumed oily fish in a single week of recording; in the latest NDNS (2008/’12), this was 40% (however, this was over a recording period of 4 days). In 2004, the Scientific Advisory Committee on Nutrition (SACN) advised fish consumption to be increased to one portion of oily fish per week and one portion of white fish. Judging by the results from the NDNS, such recommendations are not to be met in the UK.
Observational studies across the world have studied fish consumption and observed less death due to heart disease, particularly immediate death due to heart disease; however, types of fish consumed vary widely from country to country, as does frequency and portion size of fish consumption, food preparation and contamination of fishing waters. Also, treatment/medication of cardiovascular diseases has changed over time as well as fish consumption. So the association is not as clear as one might expect and this might be why there are conflicting messages coming out of research.
CSF: How much cod liver oil should people take for it to have any benefit?
ML: Current dietary guidelines do not recommend cod liver oil supplements, but encourage the consumption of fish: one portion of white fish and one portion of oily fish per week. These will provide approximately 0.45 g of LC n 3 per week; a quantity which is safe to consume (in light of contaminants) and could potentially prevent heart disease. Whether supplements should be part of this recommendation is still to be debated.
CSF: If you were going to advise relatively healthy people in the West to take a supplement/s, which would you recommend?
ML: Diet is more than the sum of its nutrients. Only focussing on a diet sufficient in vitamins and minerals is not enough to stay healthy; the source of the nutrients is important. Obtaining your nutrients from supplements does not give you ‘carte blanche’ to eat what you want; the challenge is to obtain the nutrients we need from foods that provide a balanced source of energy, fat, carbohydrates and protein. These nutrients are often ‘translated’ into foods, which is why the consumption of fruit, vegetables and fish form an important message with regard to public health advice.
In the past, premature advice has been given to use of dietary supplements, based on trials showing promising results (e.g. vitamin E and heart). However, negative effects of high doses of nutrients are equally observed and these cannot be overlooked. Common beliefs such as ‘If one tablet works, two will work twice as well’ or ‘more is better’ are definitely not applicable! There is risk for health when a nutrient is taken in too low quantities (deficiencies), but equally when the quantity consumed is high for a long period of time (overconsumption-toxicity).
Obviously, there are risk groups in our society for micronutrient specific deficiencies. If in doubt whether this applies to you, it is best to consult your nurse practitioner or general practitioner.
CSF: What about the newest push on vitamin D?
ML: The SACN brought out a report on vitamin D and for the first time has set a recommendation for this vitamin for the general population (to be confirmed, this report is still to be finalised). The recommended nutrient intake might be set at 10 mcg/d and is likely to have to be obtained from fortified foods.
CSF: Do you take any supplements?
ML: No, my diet is rich in fruit, vegetables and the occasional fish. In this context, I am in favour of Hippocrates’ expression: “Let food be thy medicine and medicine be thy food”.