The newest revisionist trend claims that Vikings were either vegan or vegetarian, or had an oat, rather than meat, fish and dairy, based diet. The only back up for such assertion is a past unearthing of an individual, in southern Scandinavia, who had allegedly consumed oats. A finding that doesn't establish the actual diet of Vikings, and particularly, when taken out of cultural, historical, and biological context.
Even assuming the finding of some oats in an individual's stomach was a clear indication of dietatery patterns, and it is not, the typical traditional diet of a Dane in Copenhagen would not be the same as the diet of an Italian in Rome. Similarly, the diet of the same Dane in Copenhagen cannot reasonably be expected to be identical to that of a northern Norwegian in Lofoten. It isn't today in an age of globalization, a united Europe, free-trade agreements, and advanced transportation infrastructure, and it certainly wasn't over 1,000 years ago.
Even nowadays, the diet of Europeans is one of proximity, defined by terrain and geographical limitations. For example, diets in southern France are based on olive oil, while they are based on butter on northern France, only a few hundred miles away. The reality of the matter is that oats, or grains for that matter, simply didn't grow in northern Norway, due to terrain and climate, and consequently, oats and grains could not have been a staple of a Viking diet.
The most important consideration, however, is biological, and specifically, as it relates to vitamin D.
The north, and Scandinavia, not only have limited sun exposure for most of the year, but also limited daylight. This poses a challenge in providing inhabitants with Vitamin D, which is primarily acquired through skin exposure to the sun. In this case, the whiter the skin, the more efficient Vitamin D absorption will be, making white skin a major evolutionary advantage at northern latitudes.
Even with white skin, however, the northern climate and latitude do not provide enough Vitamin D. Intake must therefore be supplemented through diet. What are the best sources of Vitamin D right after sun exposure? Oily fishes (sardines, salmon, mackerel, tuna, cod liver oil, and fish eggs), dairies (milk and cheese), and eggs. Foods readily available to Norsemen during the Viking age, through proximity to the ocean, as well as livestock.
As a matter of fact, to this day, Norway remains the largest fishing nation in Europe, the world's second exporter of fish products, worth nearly US$9 billion a year and making up the second largest exported commodity right after oil. Better yet, the average Norwegian consumes nearly 120 lbs of fish a year (or about a pound every 3 days.)
Vitamin D deficiency is associated with a wide range of ailments. This includes weakening of the bones as Vitamin D is necessary for the body to use calcium (which would itself be deficient with a vegan diet or a lactose intolerance) in the building and maintenance of bones. Conditions also include rickets, a disease in which the bone tissue doesn't properly mineralize, leading to soft bones and skeletal deformities. Other health conditions associated with a lack of vitamin D comprise a weakening of the immune system, an increased risk of death from cardiovascular diseases, cancer, severe asthma in children, and cognitive impairment in older adults. During pregnancy, vitamin D deficiency dramatically increases the risk of complications such as gestational diabetes, preeclampsia, preterm birth, and low birth weight, and of course rickets in newborns.
The health conditions associated with vitamin D deficiency remain an issue today despite marked advancements in modern medicine and available supplements (see Risk of nutritional rickets among vegetarian children.) 1,000 years ago, such health ailments would have simply interfered with survival.
Considering that a vegan, vegetarian, or lactose-intolerant diet would have never been available to Vikings in the first place, and considering the absolute need for animal protein containing vitamin D for survival, the notion that Vikings could have been vegan, simply is ludicrous and without factual basis. Furthermore, a tolerance to lactose was actually essential in the settlement of various lands, including Iceland, as dairy products from livestock constituted an essential staple of the Viking diet.
So, no, Vikings never were vegan.