One of these things is not like the other whatsoever
Sweet potatoes are a surprisingly versatile breakfast ingredient. They can be cooked into a hash, turned into toast, even stuffed into dumplings. But what do you do if you're making a recipe that calls for sweet potatoes but only have yams in your pantry. What, exactly, is the difference between yams and sweet potatoes, anyway? And can you use yams as a sweet potato substitute in your recipes? The short answer is no, you can't always substitute yams for sweet potatoes. That's because yams and sweet potatoes are different vegetables with totally different origins, even though they often get lumped together in the same category in the United States.
Sweet potatoes generally have an orange flesh and a maroon or copper skin. There's also a version of sweet potato that has a thin yellow skin and yellow flesh, though those are less common. Yams, however, are a totally different vegetable that either type of sweet potato, with a white inside and a dark, nearly black skin. Yams are native to Africa and Asia, according to the Library of Congress, and these days, they're most often imported to the United States from the Caribbean. Sweet potatoes, on the other hand, are grown in the United States.
The main reason you can't substitute sweet potatoes for yams, or vice versa, is because the two root vegetables taste entirely differently and have very different nutritional compositions. A yam is starchy and dry, but a sweet potato is, as the name suggests, sweet and moister. For some context, according to the US Department of Agriculture, a sweet potato has 5.56 grams of sugar per 100 grams, and a yam has only 0.50 grams of sugar for the same amount. Sweet potatoes are also packed with vitamin A, according to experts at the University of California, Davis, while yams have very little.
So if yams and sweet potatoes are so different, in taste and texture and even nutritional value, then why do we confuse them? Well, that's mainly because sweet potatoes are regularly mislabeled as yams in American grocery stores. That can of yams you buy for Thanksgiving? They're actually sweet potatoes that are just labeled as yams. The USDA actually requires that softer sweet potatoes are labeled with both the phrases "yam" and "sweet potato," not just yam, in order to clear up some of the confusion.
The mislabeling dates back to the 1920s, if not even earlier than that. As Joss Fong explains in a video for Vox about the difference between a yam and a sweet potato, the word "yam" is likely derived from west African languages, brought over the the United States as a result of the slave trade. "When west African people were forcibly moved to the new world," she posits, "they probably used the word 'yam' to refer to the root vegetable they found there: the sweet potato."
So, you can stop freaking out about accidentally using sweet potatoes instead of yams, because if you live in the United States, chances are good that those yams in your pantry are actually sweet potatoes after all.