According to Google, when appropriate, it will search not only for your search terms, but also for words that are similar to some or all of those terms. For example, if you search for "pet lemur dietary needs", Google will also search for "pet lemur diet needs", and other related variations of your terms. Any variants of your terms that were searched for will be highlighted in the summary or snippet of text accompanying each search result.
Unfortunately, Google is not always consistent. For some words it looks for common variants and for others, it does not. Words that can end in "ing" are a common Google variant. Be aware that sometimes you will need to scroll past the first ten results to notice variants highlighted in bold in the summary text.
What is the net result? Searches appear to be less precise on Google. While this can occasionally be a good thing, over-all it has caused Google's results to be less focused than in the past, and arguably less accurate. When so many terms already have millions of possible matches, is it really a good idea for Google to broaden its net to a wider array of terms?
In any case, you should consider common variants for your keywords. Try to incorporate them into your pages. If you mention both "diet" and "dietary" in your content, then you should increase the odds that Google will find your page relevant.
Consider comparing your page to the top 10 ranking pages for the variations of your keyword. In other words, optimize your page for the term diet, dietary and even dieting. Google may now be looking for a certain quantity of each variation to establish a "theme." By doing this, you'll be more accurately mimicking the top ranking pages for your keyword topic. It's a little more work, but with keywords on Google producing so much traffic today, it can be well worth the extra effort.