Why does Maimonides list matzot as an unhealthy food?

Question

In Hilchot De'ot 4:9, Maimonides calls לֶחֶם מַצּוֹת (matzot) a harmful food. Does any one know another source that says this and explains it?

Pasted below is the Hebrew text (from Sefaria): יֵשׁ מַאֲכָלוֹת שֶׁהֵם רָעִים בְּיוֹתֵר עַד מְאֹד וְרָאוּי לָאָדָם שֶׁלֹּא לְאָכְלָן לְעוֹלָם(There are foods which are extremely harmful and it is proper that one should never eat them, for example). כְּגוֹן הַדָּגִים הַגְּדוֹלִים הַמְּלוּחִים הַיְשָׁנִים. וְהַגְּבִינָה הַמְּלוּחָה הַיְשָׁנָה. וְהַכְּמֵהִין וּפִטְרִיּוֹת וְהַבָּשָׂר הַמָּלִיחַ הַיָּשָׁן. וְיַיִן מִגִּתּוֹ. וְתַבְשִׁיל שֶׁשָּׁהָא עַד שֶׁנָּדַף רֵיחוֹ. וְכֵן כָּל מַאֲכָל שֶׁרֵיחוֹ רַע אוֹ מַר בְּיוֹתֵר. הֲרֵי אֵלּוּ לַגּוּף כְּמוֹ סַם הַמָּוֶת. וְיֵשׁ מַאֲכָלוֹת שֶׁהֵן רָעִים אֲבָל אֵינָן כְּמוֹ הָרִאשׁוֹנִים לְרֹעַ(There are [other] foods which are harmful, but their harmful effects do not compare to those first [mentioned]. Therefore, a person ought to eat them only sparingly and after intervals of many days.) . לְפִיכָךְ רָאוּי לָאָדָם שֶׁלֹּא לֶאֱכל מֵהֶן אֶלָּא מְעַט וְאַחַר יָמִים הַרְבֵּה. וְלֹא יַרְגִּיל עַצְמוֹ לִהְיוֹת מְזוֹנוֹ מֵהֶם אוֹ לְאָכְלָן עִם מְזוֹנוֹ תָּמִיד. כְּגוֹן דָּגִים גְּדוֹלִים וּגְבִינָה וְחָלָב שֶׁשָּׁהָא אַחַר שֶׁנֶּחְלַב כ''ד שָׁעוֹת. וּבְשַׂר שְׁוָרִים גְּדוֹלִים וּתְיָשִׁים גְּדוֹלִים וְהַפּוֹל וְהָעֲדָשִׁים וְהַסַּפִּיר וְלֶחֶם שְׂעוֹרִים וְלֶחֶם מַצּוֹת וְהַכְּרוּב וְהֶחָצִיר וְהַבְּצָלִים וְהַשּׁוּמִים וְהַחַרְדָּל וְהַצְּנוֹן. כָּל אֵלּוּ מַאֲכָלִים רָעִים הֵם אֵין רָאוּי לָאָדָם לֶאֱכל מֵאֵלּוּ אֶלָּא מְעַט עַד מְאֹד וּבִימוֹת הַגְּשָׁמִים. אֲבָל בִּימוֹת הַחַמָּה לֹא יֹאכַל מֵהֶן כְּלָל. וְהַפּוֹל וְהָעֲדָשִׁים בִּלְבַד אֵין רָאוּי לְאָכְלָן לֹא בִּימוֹת הַחַמָּה וְלֹא בִּימוֹת הַגְּשָׁמִים. וְהַדְּלוּעִין אוֹכְלִין מֵהֶן בִּימוֹת הַחַמָּה:

Answer 1

The same may be asked of many of his health prescriptions in De'oth. That was the received wisdom in the medical field of his day. The writings of Galen (the Greek physician of the 2nd c. CE) were widely authoritative in the medieval era. Though not altogether uncritically, the Rambam relied heavily on Galen in areas of medicine. If you are interested in this topic see Y. Tzvi Langermann's chapter "Maimonides and Galen" in Brill's Companion to the Reception of Galen (short excerpt). Galen wrote the following expressing his ideas about bread and the degree to which they are leavened (On the Properties of Foodstuffs, p. 41):

The best-concocted breads are those that have been most leavened and very well kneaded, and baked in an oven with moderate heat. Greater heat scorches at once when first applied, and produces a pottery-like appearance on the outside; and the loaf turns out to be of poor quality on two counts, with its inside raw and inadequately baked, and its crust overbaked, dry and like pottery. With heat that is less than moderate the bread is not well baked, but the whole loaf is left rather raw, the inside most of all. Those that are baked uniformly throughout in moderate heat for a longer time are also very well concocted in the stomach and are most suited to the sequence of activities that occurs following concoction. Clearly, the worst breads are those to which none of the above applies.

Now that I have distinguished the extremes among them in both excellence and badness, it is no longer difficult for anyone on his own, without assistance from me, to grasp that some breads are close either to the best or to the worst extreme, that some are further away; and that others, as I said, are placed midway between both extremes. It is just as I was saying earlier about honey, namely that one should not say simply that it is good or bad for health, but rather that it is good for a phlegmatic nature, one that is moister or colder than a well-mixed nature, even if it is only colder without much moistness, or moister without much coldness; and that it is unsuited for warm mixtures, and even more so for warm, dry ones. So among breads too, while one that has not been very well baked nor has much leaven is suitable for an athlete, and one that has been very well baked in the oven and has much leaven is suitable for an ordinary individual or an old person, one which is absolutely unleavened is not fit for anybody.