From what I understand, of the major belligerents, Japan had the lowest calorie per day ration for its soldiers (perhaps civilians as well). Brown rice, of course, has more nutritional value than white rice, which has had the bran and germ removed. Given that white rice production began in the 19th century, to what degree was Japan already consuming white rice by World War II, and did they switch back, or consider switching back, to brown rice during the war to improve nutrition for the populace?
Hello ! I am not a specialist of this subject, but I have a book about it (Food and War in Mid-Twentieth-Century East Asia, dir. K.J. Cwiertka, Ashgate, 2013), so I will mostly use that source. also, this article is about the nutritional value of different types of rice + barley, not historical but a reminder.
At the end of WWI, there were "rice riots" which led to the creation of of the Imperial Government Institute of Nutrition ; apparently at the time the rice sent and sold to the cities was in great majority white rice, while in the country, people ate more and more white rice but mainly had cheaper brown rice and other cereals.
The concern for nutritional patterns in Japan began before WWII : in 1926 Sakai Tadasu wrote Progress of the Science of Nutrition in Japan for the Health Organization of the League of Nations. However, this report dealt with the problem of overabundance of starchy food (rice, barley, potatoes) compared to animal protein and fat intake in Japanese alimentation (he did advocate for mixing white rice and barley in the army, and succeeded, but for medical purposes : the reduction of beriberi disease -- but barley is less nutritious than brown rice).
Between WWI and 1942, consumption of animal proteins had doubled, mainly from fish and other marine products, but the main part of the diet was still rice (Reforming Nutrition in Occupied Japan (1945-1952), Christopher Aldous).
For the rice itself, however, the same article states that "[the drop in beriberi disease in 1945] was due to improvements in nutrition during the interwar period, and, ironically, the wartime pressure on rice supplies, which necessitated restrictions in the degree of rice polishing (whereby the thiamine was removed) and adulteration of the rice ration with wheat, barley and other cereals rich in vitamin B1" ; however, Japanese propaganda stated that the population had to be rationed in order to provide Japanese white rice for fighters.
For the World War II Japanese fighters, in An Insatiable Parasite : Eating and Drinking in WWII Armies of the Asia-Pacific Theatre (1937-1945), A.W. Moore states that Japanese soldiers in China and Korea did not encounter a lot of problems feeding themselves ('requisitioning' from the local population and receiving easily enough supplies by ports), meaning they mostly didn't choose the type of rice they ate.
However, "away from the teahouses and comfortable billets of rear areas in China, combat units [...] had to be very extremely flexible when it came to comestibles [...]. Japanese troops carried rice with them, but very little (as they were expected to seize provisions on the field), and well supplied units often had pickled vegetables and miso paste for soup [...]. In other [cases], Japanese troops were having for their meals just a portion of boiled sweet potatoes and rice -- or nothing".
Moore later states that units out of mainland had to use foreign, non white rice provided by the locals (like in the Philippines), and that white rice was indeed supplied only to the front line soldiers, whenever possible, the soldiers preferring the familiar taste of Japanese white rice.
So, no change of diet whenever possible for the troops, and apparently no obvious concern for the nutritional value of white rice.
As a side note, apparently the main problem was for the (town but not only) population to have access to food, like brown rice, for the front-line troops in the continent to have enough food to steal where they arrived (they couldn't choose what they had access to), and for the other troops to receive food at all. There was a gruesome moment when author explained cannibalism occurrences, stealing between soldiers and begging Australian and American troops for food towards the end of the Pacific war, so definitely no rice there either.
Edits : trying to understand formatting
Brown rice, of course, has more nutritional value than white rice, which has had the bran and germ removed.
Brown rice also spoils a lot faster than white rice and so cannot be stored for long unless there is refrigeration or special packaging is used. So it has to be consumed soon after it is milled. That makes it problematic for provisioning people distant in time or place from the source of the rice.