Alternative "medicine" is mostly cons and frauds. Why are you talking about "alternative medicine" in reference to basic nutrition?
Because (whether through luck or being a broken clock that's right twice a day), it just so happens to shake out that in many cases, the nutritional/dietary advice they give happens to be better than that of most GPs -- even if it's outside of what their supposed "expertise" is supposed to be.
I've shared before that I grew up in a family that was into all that stuff... I'm the outlier in that I'm one of the few in the family that doesn't go to those sorts of "practitioners"
But for example:
Chiropractors believe that all disease its caused by neurological interference cause by "subluxation" and the the removal the "subluxation" will unlock some sort of magical ability for the body to heal itself. -- and they believe that's true for everything from ear infections, to digestive disorders...up to more serious things like cancer. And the whole homeopathy thing is even more laughable.
We know that's hogwash... a "chiropractic adjustment" isn't going to heal an ear infection, and it's downright dangerous to trust that over real oncology care for cancer.
However, on the flip side.
When my uncle was dealing with work-related knee pain, the GP wanted to put him on a regular regiment of high dose Naproxen, and then potentially stronger stuff... whereas, his Chiropractor gave him exercises and stretches to do at home, and put together a little diet plan to help him drop some lbs. (which obviously losing weight can alleviate some knee pain for people)
When my mom found out her cholesterol was high when she was in her early 50's, her doctor wanted to get her on statins ASAP. When she went to a Naturopathic/Homeopathic person, they saw the numbers and said "yeah, it's a little high, but at your age, for women there's not a huge benefit to rushing to get on statins", and then gave a list of certain foods to cut out, and other foods to add, and it ended up helping. She's still in the normal reference range a decade later.
That doesn't change the fact that Chiropractic and Homeopathy are nonsense (in terms of their primary philosophies and what they claim to specialize in), but it just demonstrates that they can be right about a few indirectly related things in areas where actual doctors may have some blind spots.
Which, those blind spots are somewhat predictable when you consider that your average MD only got 15-20 hours of nutrition training across that whole time they spend in med school.