We're still waiting to hear! Texts don't really tell us much, since they were written mostly by monks who were much more concerned with viking axes than with their diets. So where should we turn? Archaeology! You can tell a lot about what a person ate and how their diet changed over the course of their life based upon certain elements and the ratios of various isotopes which are preserved in the human skeleton. Tooth enamel preserves information from childhood, femurs have a slow cell turnover and preserve information from several years before a person's death, and ribs turnover much more quickly and give you a snapshot of a person's last months.

So the first step is finding vikings to test. This depends on the hazards of the archaeological record, our ability to determine whether those skeletons that do survive can be identified as viking raiders, and having the money and resources to actually sample the skeletons. Right now, this perfect set of circumstances has only happened once—for the Ridgeway burials (sometimes also referred to as the Dorset viking grave).

The skeletons at Ridgeway can be carbon dated to right around 1000 AD, which is rather late in the Viking Age. Isotope analysis shows that the individuals buried at Ridgeway grew up in lots of different places ranging from Iceland to Russia, but most spent their childhoods in Norway. They ate all sorts of local diets during their youths, as might be expected. Then, when most of them were in their mid-teens they all started eating a heavily maritime diet. The group was mostly young, with an average age probably in the low-20s but some older members among the group, when they were executed by beheading and dumped into an old gravel pit at Ridgeway. They seem to have been naked when killed and buried, so none of our favorite viking artifacts (swords, axes, spears, shields, etc.) were found on site. But it seems reasonable to assume that this was a captured viking crew receiving a judicial punishment. There are remarkable few signs of traumatic injury, so it looks like most gave up and accepted their fate without much of a fight.

But that doesn't really answer your question. The Ridgeway vikings seem to have been something like a conscript crew, which makes a lot of sense for the later Viking Age when 'vikings' were no longer simple pirates but more often standing troops serving Western-style kings. The 8th-9th centuries were completely different. Fortunately, archaeologists turned up a different grave which will help us better understand what raiding parties looked like in the early Viking Age.

Although most Viking Age history begins with raids on England, this particular grave comes from Salme in Estonia in about 700—that is, a raid in the opposite direction almost a hundred years before the sack of Lindisfarne. These guys came from central Sweden around Stockholm, but they had a rough day on arrival. Their bones seem to have been hewn in battle, and then they were stacked like cordwood in their boats. Fortunately for us, the tides soon covered their boats with sand, protecting the bodies from scavengers and the wastes of time so that we can study them today.

Analysis of these bodies is still incomplete, although the Estonian archaeologists who first worked on the site immediately recognized its significance and began publishing interim reports. (And note also that some of the Wikipedia information comes from the initial articles in 2011, and much has changed since then but the Wikipedia page hasn't been comprehensively updated.) The Swedish government recently awarded a large research grant that should help us understand how these Swedes abroad compare with contemporary cemeteries in central Sweden. But at present, there's only a few things we can say about their diet.

Isotope analysis shows that these people had similar diets in their youth, which means that unlike the diversity of the Ridgeway crew, these guys all grew up in the same place and ate the same things, which suggests they were also all of a similar social status. We're still waiting for this analysis to be published, though, so that's all the publications tell us

Osteological analysis can tell us a bit more. Among one sample of skeletons, osteologists found only one cavity, suggesting a diet low in sugar. Although this is fairly common for premodern Europeans (cavities only became a major problem with the rise of sugar plantations in the Caribbean), it's a bit surprising giving the emphasis on mead in poems like Beowulf and in later Scandinavian poetry and picture stones. There's also hints that several of the Salme skeletons suffered from scurvy—too little fruit. In the modern period, scurvy was primarily a problem for sailors spending months at sea, but that obviously wasn't a case for sailors on the early medieval Baltic. Instead, it seems to have been more a consequence of lifestyle.

That's it at present! Not as much mead as we thought, very little fruit. Hofstadir in Iceland shows that beef was an important part of some people's diets, even when local conditions discouraged it, so presumably there was an international beef culture which may have included 9th-century viking raiders; if so, then this must have changed by the 10-11th century Ridgeway burials. Hopefully we'll get more from the Salme research soon!