In terms of omega-3 fatty acid supplementation, UK recommendations are to consume 2 portions of fish per week, one of which should be oily. This equate to around 450mg of EPA + DHA day. Some studies which show beneficial effects use much higher doses, such as the average of 3.5g/day in rheumatoid arthritis studies I mentioned. The main ‘side-effect’ people tend to report with taking fish oil is ‘fishy burps’ which can put people off them completely. This can be reduced somewhat by taking capsules with a meal rather than on an empty stomach.

Seaweed such as Nori can be a great source of iodine though can be high in salt (483mg sodium in 15g dried wakame seaweed). But for ALA I would suggest linseeds (also known as flaxseeds) or their oils. Seeds and nuts are a good source of ALA, particularly walnuts. Many other foods also contain ALA, such as avocadoes and edamame beans.

This is an interesting question and one which is a little controversial in the research field. It is broadly reported that adults have a limited ability to convert ALA to DHA, with some estimates of it suggesting it is below 1% in humans. However, the ability to convert ALA to EPA+DHA is known to vary with age, background diet, and hormonal status. I have written a few papers on this subject! It is my hypothesis that we are able to effectively upregulate EPA+DHA synthesis at the time/location it is required. In my own research for example, I found that the enzymes involved in the conversion were upregulated by progesterone, and so could ensure a supply of DHA to the developing infant brain during pregnancy.

A good question – and one which is hard to unpick. Eating fish is about more than just eating some EPA/DHA. Fish are also a source of protein, iodine and vitamin D for example. So algal oil or ALA in the diet cannot replace those (although of course there are other vegetarian sources of those nutrients).
Given that most people in the UK never eat oily fish, I suggest all of us, whether vegan or not, consider what non-fish sources of omega-3 we can include in our diet. For those who do eat oily fish, the health benefits from doing so may be further increased by substitution effects e.g. by choosing salmon over meals containing red and processed meats.

You can increase your omega-3 intakes by making changes to the cooking oils you use, adding seeds and nuts to your diet, and eating green vegetables. For non-fish eaters who are not vegan, other meats such as chicken or beef also contain EPA and DHA. You might be interested to look at a public engagement ‘Omega-3 Top Trumps’ game developed by some of my colleagues in the Institute of Developmental Sciences which will be released at part of the Southampton Science and Engineering Festival this year – more details at https://www.sotsef.co.uk/.

These kind of population studies, known as nutritional epidemiology, are often the starting point for further research into nutrition and immune function. For example, it was Horrobin’s hypothesis in 1987 that Eskimo populations had lower incidence of heart disease, psoriasis, asthma and rheumatoid arthritis, and that that this may relate to their consumption of large quantities of oily fish. More localised historical comparisons can also be useful, for example considering what dietary factors may underlie the differences seen in asthma prevalence between children who grew up in East or West Germany. There are some natural current population comparisons which can be studied such as of Amish vs. Hutterite farming communities in the USA. To move beyond hypotheses based on observed associations, it is important to confirm effects seen using randomised controlled trials. But this is not always possible, as for example it would not be possible (or ethical) to recruit parents to a study and randomise them to follow vegan or fish-eating diets so that we could study allergic disease rates in their children.

Vitamin D has a role in immune function, with vitamin D receptors found on immune cells, and seasonal variations in vitamin D status linked to variations in rates of winter cold and flu like symptoms. For more detail on the role of vitamin D in immunity ad COVID, I would recommend you read the recent Nutrition Bulletin from Buttriss & Lanham-New (https://onlinelibrary.wiley.com/doi/full/10.1111/nbu.12430) which provides an excellent overview of the role of vitamin D in health.

Each year since the 2016 UK government advice on vitamin D was published I have asked my undergraduate students if they know about this recommendation, and virtually none report having ever heard it before. Yet in some communities, the message was so effectively actioned by GPs that researchers planning vitamin D interventions studies struggled to identify participants who were not already taking supplements! So the effectiveness of this recommendation is currently driven at a grass roots level in my experience. It is worth noting that the current recommendations are based on levels found to be optimum for preventing rickets, osteomalacia and falls, and to benefit muscle strength and muscle function, and not on immune measures, where further research is required.