It is said that on ekadashi days one should not eat Grains. So going by strict definitions avoiding rice, wheat, lentils, other pulses etc.
By that definition we can eat Tapioca aka Sabudana , so is it suitable to eat Sabudana on Ekadashi?
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For Vratas, usually grains are avoided throughout the day. Carbohydrate rich diet, fruits, and milk etc. are to be consumed by the devotee. As Sago is a starch hence it is allowed to be consumed for vratas like Ekadashi.
Sago (Metroxylon sagu) starch accumulates in the pith of the sago palm stem from the base upwards. At maturity, the trunk is fully saturated with starch almost to the crown. Apart from sago starch granules, the pith also contained other associated components.
I am quoting some references and posts from various ISCKON website on this topic,
Srila Prabhupada usually observed the Ekadasi fast in the simplest manner prescribed in scripture, by refraining from eating grains, beans, peas and dahl. Sometimes he observed a half-day fast. Some devotees eat only fruit on Ekadasi, some take only water, and some fully fast, even from water (this latter is called nirjala vrata). Non-grain high carbohydrate foods are often prepared on Ekadasi. Sago (tapioca) is a common example. But sago is sometimes mixed with rice flour, and apparently sometimes even with crushed insects. So unless one is sure about the source, sago should be avoided. In India, preparations resembling capatis and rice are made on Ekadasi from non-grain ingredients.
ekadasyam samayantyam prapunantyam jagattrayam
sthatavyam annam asritya bhavata papa purusa
sansare yani papani tany eva ekadasi dine
annam asritya tisthanti sriman narayana ajnaya
(Padma Purana, Kriya Yogasara 22.46,50)
When Ekadasi arrives, at that time, all of the sins from the three worlds reside in grains in the form of a papapurusa.
Whatever sins are available in this world, they all together reside in grains on Ekadasi day by the order of Lord Sri Narayana.
By eating on Ekadasi one eats sin alone (HBV 12.23) and verily he eats the flesh of a cow (HBV 12.25). By indulging the tongue, a terrible result is attained (HBV 12.21 from Skanda Purana):
matrha pitrha caiva bhratrha guruha tatha
ekadasyam tu yo bhunkte visnu-loka-cyuto bhavet
He is a killer of his mother and father, and indeed he is a killer of his brother and his guru. He, who eats on Ekadasi is barred from the abode of Visnu. Gautamiya-tantra cited in Bhakti-sandarbha, Raga-vartma-candrika and HBV 12.32:
vaisnavo yadi bhunjita ekadasyam pramadatah
visnv arcanam vrtha tasya narakam ghoram apnuyat
If a Vaisnava becomes so insane as to eat on Ekadasi, his worship of Visnu becomes worthless and he attains a terrible hell.
The 9th anga of sadhana is harivasara-sammana, "respecting the days of Hari". In illustration, Rupa Goswami quotes in BRS 1.2.109:
hari-vasara-sammano yatha brahma-vaivarte -
sarva-papa-prasamanam punyam atyantikam tatha
govinda-smaranam nrnam ekadasy amuposanam
Respecting the holy days of Hari, from Brahma-vaivarta Purana: "Ekadasi destroys all a person's sin, bestows limitless piety, and causes them to remember Govinda."
Thus, there are three aspects mentioned here to following Ekadasi (1) removal of bad karma, (2) gain of good karma, and (3) remembrance of Govinda.
ekadasi vrata samam vratam nasti jagattraye
anicchaya'pi yat krtva gatir evam vidha 'vayoh
ekadasi vratam ye tu bhakti bhavena kurvate
na jane kim bhavet tesam vasudeva anukampaya
(Padma Purana, Kriya Yogasara 23.172-174)
In the three worlds, there is no kind of fasting which is even comparable to the Ekadasi fast. Even if one performs this fast without a proper attitude, he achieves the Supreme Abode. Anyone who fasts on Ekadasi day, with full devotion, what happens to him by the mercy of the Supreme Lord, I can not say (how much merit gets cannot be described).