Supplements are a multi-billion dollar industry. But, unlike pharmaceutical companies, manufacturers don’t have to prove that their products are effective, only that they are safe – and that’s for new supplements only.

We wanted to know which supplements are worth our attention (and money) so we asked six scientists – experts in everything from public health to exercise physiology – to name a supplement they take each day and why they take it. Here is what they said.

Vitamin D is a peculiar vitamin in that it is synthesized in our bodies with the aid of sunlight, so people who live in cold countries, or who spend a lot of time indoors, are at risk of a deficiency. People with darker skin tone are also more at risk of vitamin D deficiency as melanin slows down skin production of vitamin D. It is estimated that about a billion people are deficient in the vitamin.

Most people are aware we need enough vitamin D to maintain healthy bones, but, over the past few years, scientists have become increasingly aware of other important roles of vitamin D. We now believe vitamin D deficiencies can result in a less efficient immune system, impaired muscle function and regeneration, and even depression.

Vitamin D is one of the cheapest supplements and is a really simple deficiency to correct. I used to test myself for deficiencies, but now – because I live in the UK where sunlight is scarce between October and April, and it doesn’t contain enough UVB radiation during these cold months – I supplement with a dose of 50 micrograms, daily, throughout the winter. I also advise the elite athletes that I provide nutrition support to, to do the same.

Prebiotics are non-digestible carbohydrates that act as a “fertiliser” to increase the growth and activity of beneficial bacteria in the gut. This is turn can have positive effects on inflammation and immune function, metabolic syndrome, increase mineral absorption, reduce traveller’s diarrhoea and improve gut health.

I first came across prebiotics in my research to target the gut microbiota in athletes suffering from exercise-induced asthma. Previous research had shown asthma patients to have altered gut microbiota, and feeding prebiotics to mice had been shown to improve their allergic asthma. Taking this as our launching point, we showed that taking prebiotics for three weeks could reduce the severity of exercise-induced asthma in adults by 40 per cent. Participants in our study also noted improvements in eczema and allergic symptoms.

Omega 3 is a form of fatty acid. It comes in many forms, two of which are very important for brain development and mental health: EPA and DHA. These types are primarily found in fish. Another type of omega 3 – ALA (alpha-linolenic acid) – is found in plant-based foods, such as nuts and seeds, including walnuts and flax seeds. Due to my busy schedule as a lecturer, during term time my diet is not as varied and enriched with omega 3 fatty acids as I would like, forcing me to choose a supplement. I take one 1,200mg capsule, daily.

Studies of multivitamins show regular users are more likely to die of cancer or heart disease, for example. The only exception is supplements for preventing blindness due to macular degeneration, where randomized trials have been generally positive for a minor effect with a mixture of antioxidants.