Content index
What if a simple drop of blood could tell us whether we are eating well? Although many people claim to follow a healthy diet, traditional methods used to measure what we eat—such as surveys or food diaries—are not always reliable. After all, it is easy to forget what we have eaten or even to misjudge the quantities we have consumed.
Fortunately, new alternatives are emerging. In a study we conducted at the technology centre AZTI, we analysed whether a small blood sample collected from the finger (known as a dried blood spot or DBS) can accurately reflect what we eat, particularly when it comes to the consumption of oily fish, which is rich in healthy omega-3 fats.
The study involved 18 participants—men and women aged between 18 and 65, all in good health. Over five weeks, they took part in a nutritional intervention. First, they followed their usual diet; then they avoided fish; and finally they consumed canned mackerel several times a week. At each stage, we collected blood samples and analysed the levels of different fatty acids.
The results were clear. Before the intervention began, the fatty acid profiles already revealed different dietary patterns among the participants.
On the one hand, those who regularly consumed sweetened dairy products, carbonated drinks and sugar showed higher levels of omega-6 fats and lower levels of omega-3 fats. This is important because when omega-6 levels become disproportionately high, the risk of death increases. By contrast, participants who consumed oily fish, fermented alcoholic beverages, sweeteners and nuts showed higher levels of monounsaturated fats and omega-3.
A third group—characterised by higher consumption of vegetables, whole grains, cheese and butter—showed intermediate levels of omega-3 and omega-6, and higher levels of saturated fats.
But how quickly does our fatty acid profile change after eating certain foods? Less time than we might imagine. In the experiments, we observed that when participants consumed tins of mackerel, the levels of certain omega-3 fatty acids in the blood, such as EPA, increased within a single day. Others, such as DHA, took longer to appear. This shows that the technique can detect dietary changes over both the short and long term.
Moreover, these fat levels changed differently depending on each participant’s usual dietary pattern. Among participants who normally did not eat oily fish, EPA levels increased more sharply the day after they ate fish. However, although DHA levels also increased in these participants during the two-week intervention, they did not reach the same levels as those observed in participants who regularly consumed fish. This suggests that, to achieve omega-3 levels consistent with adequate oily fish consumption, they would need to continue eating it for more than two weeks.
Above all, the new method is fast, convenient and minimally invasive. It does not require medical professionals and can easily be applied to large population samples—or even used at home. It also allows researchers to determine whether a diet is appropriate while avoiding the common errors associated with dietary questionnaires, which often fail to reflect what people actually eat. In addition, because different fats change over both short and long timeframes, the method can also help verify whether people are following the guidelines of a nutritional plan.
This research brings us one step closer to providing truly personalised nutritional recommendations. Because in the end, what matters is not what we say we eat, but what we actually eat.
This work was carried out in collaboration with the CoDiet and Optiprot projects.
