For the later Middle Ages, the best evidence for sufficiency of diet comes from the period of the worst insufficiency: the Great Famine. From 1315 to 1322, northern Europe was absolutely devastated by year after year of utter, catastrophic crop failure. Numerous causes have been posited, from unchecked/unsupportable population growth to temporary climate changes to structural failures in terms of food distribution. Heavy spring downpours made planting impossible; wars diverted manpower and transportation away from importing food from the south. What's important is that famine lasted a number of years, and left a huge mark in contemporary sources and in cultural memory.

Except--we don't see problems right away. The first year, sources note the weather problems and crop failure, but not in the terms of fames and caristia that will come to be endemic later. William Chester Jordan describes 1315 as a "buffer year" before disaster and despair set in. It seems that localities had storage and other systems in place to last for one year of insufficient food, whether the cause was famine, war, or increased taxation.

Except the Great Famine lasted seven years, and people suffered drastically. We see widespread mortality to the extent that entire villages go deserted.

Fast forward twenty-odd years. In 1348-1351, the Black Death ravaged much of the Eastern Hemisphere, and particularly Western Europe. Although demographic loss is extremely difficult to calculate and controversial (we're not even sure about overall mortality; 1/3 is kind of a guesstimate), one notable pattern is discernible that stands out here.

The towns of the Low Countries like Flanders actually seem to have suffered less from pestilence than other areas. Intriguingly, these are the same areas that suffered epidemics during the Great Famine which wiped out large portions of their younger population. In other areas, the children born into or growing up during the Famine survived in larger numbers, but would have suffered what we now know are long term consequences of childhood malnutrition. They reached adulthood (30-40) by the plague years--but significantly weaker than their older and younger counterparts due to the Famine of their childhood days and the resulting effects of malnutrition.

Although migration patterns surely make the subject more complicated than I have laid it out here, it seems plausible that the difference in Black Death mortality between the cities around Flanders versus elsewhere relates to the percentage of the population suffering long-term effects of malnutrition during the famine.

But this pattern does not repeat in other years, in other plagues. This along with the lack of concern during the Famine's first year suggests, I think, that medieval people typically did quite well in terms of eating sufficient numbers of calories, whatever form they might have come in.

ETA Source upon request: William Chester Jordan (as noted in post), The Great Famine, see esp. pp. 87-107; 142-43; 185-87

ETA 2: For anyone less familiar with how historical interpretation works in the scarcity of sources that we have from the Middle Ages, [here's a post] (https://www.reddit.com/r/AskHistorians/comments/4dvzda/would_i_have_trouble_eating_enough_about_2000/d1v5q2u) I made further down the thread to clarify why I answered the question this way.

Follow up question:

To what extent do we know about how people used non-farmed or non-traditional sources of calories?

For some examples: picking and eating wild or "weed" greens -- such as dandelions, nettles, collard greens in some parts of the world, etc -- picking and eating wild or volunteer mushrooms, catching and eating things like rats and mice.

Essentially, do we know how routinely peasants did or did not turn to "bowl of brown" or its equivalent, as a meaningful part of their caloric intake? (Not just picking a few wild berries in the summer)

This is a huge question that depends on a whole lot of variables, not the least of which are time, location, and social class of the person in question. To carve off just one piece (Medieval, peasant, Western Europe, period of no war or famine), you would be surprisingly able to exceed your calorie target. (Remember that peasants led a very active life, so their caloric needs were higher than yours.) Most of your calories would come from grains. Depending your exact location and local weather, that would be some combination of rye, wheat, barley, and/or oats (bulking never changes). These grains would be consumed in a variety of ways that are familiar to us today -- stews, breads, and weak ales/beers. Protein, however, would be far more limited and would mostly come from crops like beans, peas, or lentils; that might be supplemented by fish if you lived close to a coast or body of water, or eggs if you owned poultry, but very rarely by meat like pork, chicken, or beef that would be prohibitively expensive. Fats too would be limited unless you had access to dairy, eggs, or oils. Vitamins and other nutrients would be hit or miss depending on the seasonal availability of produce.

Source: Singman, Daily Life in Medieval Europe

Would any of this be offset by the plague, could the difference be population density rather than/in addition to childhood malnutrition?

If you want to read up on this as well as what daily life was like in general (at least in England anyway), Ian Mortimer has a fantastic set of books called The Time Traveler's Guides. So far he's covered Elizabethan England and Medieval England. They do an amazing job of discussing what daily life was like in those time periods, including what people ate, be they peasant or king.