Hoo Boy. When you say "Obviously these would be controversial statements." you are not, to add an extra Southern metaphor, whistling Dixie. (Albeit some felt that Fogel and Engerman might have been!) When Time on the Cross came out, it ignited a firestorm of debate and anger and sometimes rather vitriolic and rabid criticism.

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And here I need to explain WHY -- with a quick detour on two threads of historiography.

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1. Time on the Cross was seen by many as a rehashing of some very old proslavery arguments. The discussion of the condition of "wage slaves" versus southern slaves was often a way to engage by Southerners in a form of "me-too"ism. And the idea of "slaves had it better off in slavery" has been a painfully persistent thread that reaches into today. It has been famously invoked even in the last decade (even though this is a historiographical discussion, to avoid soapboxing or breaking the twenty year rule I can be messaged privately for citations in that regard if there is disbelief that this is persistent, but I suspect no one will need to as it is painfully familiar in racist discourse even today). Because of the use of these arguments, reviews by other historians even at the time of publication called out what seemed to be its focus on finding rationalizations about slavery.
2. Outside of its conclusiary callbacks, Time on the Cross was a broadside in a historiographical war about "what is the proper way to do history." Since the 1920s, there had been the rise of a particular school of historians under the banner of a very famous historian named Charles Beard. He and his wife Mary essentially founded what became known as the "Economic School of Interpretation." This most famously influenced generations of thinking with his book An Economic Interpretation of the United States. In many ways he was arguing against a previous, often romanticized view of American history. In his view, the Founders didn't come to create some greater document for the greater good; they created the Constitution for their own selfish interests. Or, to paraphrase a term coined during the Watergate Era, "Follow the money." Beardian scholars at their most extreme would scoff at a person's personal protestations of why they did things -- to a true Beardian (as they evolved), historians had often been vulnerable to gullibility in uncritically accepting the reasoning given by people in public and even in their private correspondence, and that real understandings of issues became unmuddled if you only look at hard, numerical facts. Numbers and money told the true story -- the rest was merely fluff.

Now, the Economic School was already taking some pretty heavy hits in the 1950s. As you might imagine, more ideological times influenced a number of historians, and the Beard School had already taken multiple big hits in the Era after World War 2. By the 1960s books like Bernard Bailyn's Ideological Origins of the American Revolution reentered the debate that ideological motives did actually drive people to do things.

Okay, so that background in mind, understand that Time on the Cross was like an Evangelical Revivalist preacher claiming that the Economic School had always been right and historians really didn't know all that much. Originally printed in two volumes, the first volume didn't even have standard footnotes but relied on you going to the second volume to understand how they derived their conclusions. And to get to their conclusions. . . well, they often walked a dog around the corner a LOT of times to get where they wanted to go.

Let me quote some passages about their methodology from a contemporaneous review by Michael Johnson, The History Teacher, Vol. 8, No. 4 (Aug., 1975), pp. 669-671:

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The most propitious combination of evidence, methods, and assumptions is in Chapter Three, which argues that slavery was profitable, an argument that has been widely accepted since the 1950's. Yet even this chapter displays disappointing flaws that recur throughout the book: flailing of straw men (what modern scholars support "the frequent contention that slaveowners preferred to work slaves to death at early ages, in order to avoid the burden of maintenance at late ages" [ I, p. 75]?) and dubious logic (does it follow that since slaveholders, on the average, "earned about 10 percent return on the market price of their bondsmen" and in general were economically "rational," they were capitalists?). The flaws in the other chapters are so great as to make one wonder whether Time on the Cross is a work of history at all. . . Rather than defining their subject as the economics of slavery and then proceding to tackle that subject with all their considerable ingenuity and skill-the standard historical approach-the authors confine their attention to problems they think their quantitative methods can handle, while grandly overlooking elements of the story which may be of central importance.

Mighty strong language, right? Johnson explains how they derive just ONE statistic on, for example, how much milk slaves drank (part of the calorie question you asked):

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Yet one should judge the book not on the basis of its occasional slips, but on the way it routinely combines evidence, methods, and assumptions. Take, for example, the statement, "The milk consumption [of slaves ] was low by free standards, but still amounted to about one glass per day for each slave" (I, p. 113). First, we learn from Volume II (p. 94) that the word "slave" means "blacks on plantations with 51 or more slaves that were at least 50 wagon miles from a city," too far away to sell their milk in an urban market. Thus, although the quoted passage makes the one-slave-one-glass-of-milk formula appear to apply to all slaves, the evidence applies only to slaves on an unspecified number of large, isolated plantations. Only about a third of the slaves in the Deep South and just over a tenth of those in the border states lived on holdings of fifty or more slaves in 1860. Presumably, the proportion of slaves on isolated large plantations in each area was even smaller. What, then, does the evidence tell us about the other two-thirds or more of slaves in the Deep South and in the and the other 90 percent or more in the border states?

. . . Second, we learn from Volume II (p. 97) that "one glass" of milk means 171 pounds of milk, the estimated yearly consumption of milk per slave, divided by 365 days, or .468 pounds per day, which is 7.5 ounces. Apparently on the large, isolated plantations the mean volume of drinking glasses was small, a little less than a standard cup measure. But how do we know that each slave consumed 171 pounds of milk each year? Well, we learn (II, p. 96) that (1) an estimate of the production of fluid milk based on the production of butter (which was reported in the census--this is the empirical base of the calculation), which was in turn derived from data for Northern farms, was modified, and (2) applied to the free farms in another sample of over 5000 farms in the cotton South, (3) to obtain a figure for the fluid milk production per cow, (4) which was then taken as the milk production per cow on slave farms, (5) which was then multiplied by the number of cows on the large, isolated plantations, (6) to obtain the total milk production of the plantation, (7) from which was subtracted a generous portion for white consumption, (8) leaving the remainder for slave consumption, the per-slave portion being determined by (9) dividing the remainder by the number of slaves. By this chain of assumptions, butter production on free farms in the North is linked to slaves drinking one glass of milk a day.

Note: Edited to get the quotes formatted properly