The invention relates to an extract of Ashwagandha that exhibit enhanced bioactivity and bioavailability comprising of enriched withanolide glycosides and saponins; with negligible amount of alkaloids, withanolide aglycones and oligosaccharides. The extract as disclosed prepared from root, stems, leaves and whole plant of Ashwagandha further shows improved immunomodulatory activity, anti-inflammatory activity, anti stress activity, antidiabetic activity and sleep quality. The disclosure also provides a method of improving bioactivity of withanolide glycosides even at lower doses, by the administration of an enteric coaled formulation of extract of Ashwagandha to humans. The enteric coaling protects the composition from hydrolysis in the acidic environment of the stomach to release the withanolide glycoside in neutral/alkaline pH in gastrointestinal tract (GIT) thus enhancing the absorption. Further the process of preparation of the extract of Ashwagandha enriched with withanolide glycosides and saponins are disclosed along with various formulations.