When E171 is part of a food product, it passes through the digestive system without causing harm because E171 combines with the other ingredients. 

What Is Titanium Dioxide?
Titanium dioxide, also called titania, is an odorless white powder and naturally occurring mineral that is widely used as a pigment for its brightness and whitening effects on a variety of materials, such as paint, plastic, paper, cosmetics, sunscreens, toothpastes and foods.
It’s produced through the sulfate or chloride process, which both involve treating titanium ore with sulfuric or hydrochloric acid to produce titanium sulfate or titanium chloride. These materials are then further processed to remove impurities and produce titanium dioxide in its final form.
Food-grade titanium dioxide differs from what’s added to plastics and paints to enhance whiteness. However, there have been concerns about the environmental impact of titanium dioxide production and the potential health risks from exposure to its particles.
Although food-grade titanium dioxide must be 99 percent pure, there’s still a risk of it containing potential contaminants, such as mercury, lead and arsenic. Additionally, inhaling the mineral over time can possibly cause it to build up in your body, leading to adverse effects.
Uses

In a study published in the journal Environmental Toxicology and Pharmacology in 2020, researchers examined the effects of food additives titanium dioxide and silica on the intestinal tract by grouping and feeding mice three different food-grade particles — micro-TiO2, nano-TiO2, and nano-SiO2.  With all three groups, researchers observed changes in the gut microbiota, particularly mucus-associated bacteria. Furthermore, all three groups experienced inflammatory damage to the intestine, but the nano-TiO2 displayed the most pronounced changes. The researchers wrote: “Our results suggest that the toxic effects on the intestine were due to reduced intestinal mucus barrier function and an increase in metabolite lipopolysaccharides which activated the expression of inflammatory factors downstream. In mice exposed to nano-TiO2, the intestinal PKC/TLR4/NF-κB signaling pathway was activated. These findings will raise awareness of toxicities associated with the use of food-grade TiO2 and SiO2.”