I got the impression, possibly from Ben Goldacre's book "Bad Medicine", that nutritionists are alternative medicine practitioners. Is that the case?
In the US, anyone can claim to be a nutritionist. Registered dietitian nutritionists (RDN) undergo pretty extensive training.
An RD may suggest herbal remedies, which would be alternative medicine, but typically they are helping people manage chronic conditions like diabetes, crohn's disease, or cancer.
About nutritionists, at least in France (I suppose it varies by country) it is a medical specialty that comes after having completed six years of medical studies.
Alternative practitioners who did not complete medical studies are called dietitians (in France); which doesn't mean they are less competent.
In medicine we aim to base everything on solid scientific results. We call this practice "evidence based medicine" (EBM). What we call "alternative medicine" are all the alternative approaches where such standards are not or insufficiently implemented. The way medicine used to be practiced centuries ago is a good example of alternative medicine, we usually refer to this as traditional medicine:
Traditional medicine (also known as indigenous or folk medicine) comprises knowledge systems that developed over generations within various societies before the era of modern medicine. The World Health Organization (WHO) defines traditional medicine as "the sum total of the knowledge, skills, and practices based on the theories, beliefs, and experiences indigenous to different cultures, whether explicable or not, used in the maintenance of health as well as in the prevention, diagnosis, improvement or treatment of physical and mental illness."1
Now nutrition, while an integral part of the regular medical discipline (if you get diagnosed with diabetes, your endocrinologist may well refer you to a dietitian) fits in well within traditional medicine in the following sense. The knowledge we have about the food we eat has been passed on from generation to generation long before modern science came into existence. Basically, you've been told to eat your vegetables by your mother who was told the same by her mother etc. etc, you could probably trace this back to the time before modern humans even existed. When applying rigorous scientific standards to nutrition we then face a problem. How do we even define the problem when evaluating foods we eat? E.g. broccoli is not some drug designed to solve some specific medical problem, so you don't have a well defined end point that you can test.
It's of course, not a problem in principle to study some particular health effects of certain foods. In practice this can already be difficult, you can't perform double blind tests, so you need to resort to observational studies or controlled experiments that typically won't last very long. But the real problem is then the choice of the end point for health outcomes and the choice for what to compare the food item to. You can't e.g. simply look at all cause mortally. Suppose e.g. that calorie restriction would prevent prevent cancer and heart disease, if you stick to that diet you'll live a lot longer (assuming it would really work). But then it turns out that you'll have a lack of energy, you'll end up sleeping 12 hours a day. So, the end point of all cause mortality which looks to be a good criterion, could well lead to totally worthless result.
But to even get such results, you must make a decision to compare one type of diet to another diet. In older statistical studies dating back from the 1980s and earlier the mistake was made to just correlate intake of nutrients to health. As explained in this article this often leads to spurious results, as total energy intake is a huge confounding factor. Modern studies do the statistical analysis in such a way that total energy intake is taken into account. But there may be other confounding factors lurking which may not be as easily identified and dealt with.
Now, the fact remains that if you today stick to what passes for a healthy diet, then you would be eating a diet that your great great great grandmother would probably also consider to be a healthy diet. So, even if we today know a bit more than your great great great grandmother (take e.g. vitamins), the information that describes what you would be eating still largely derives from ancient knowledge. Modern science has had only a limited impact here. E.g. we now know that saturated fats may not be very healthy. On the other hand, a mistake was made when we wrongly thought we could produce healthier fats, trans fats were made as a healthier alternative to saturated fats, but this turned out to be much more damaging to our health.
So, at the end of the day, the patient who gets referred to a dietitian will end up being put on a diet that has its origins in what we could call traditional medicine. The healthy foods the patient would be eating like carrots cauliflowers, broccoli were created not as a result of moderns scientific efforts but they were cultivated more than 3000 years ago.
A nutritionist may practice alternative medicine, but being a nutritionist does not inherently mean that you practice holistic medicine or are an alternative medicine practitioner. It totally depends on your certifications/education, and the kind of services you choose to provide as a nutrition professional! If you provide alternative therapies as a nutritionist, then you may consider yourself an alternative medicine practitioner.
Also, the kind of certification/degrees you choose to get as a nutrition professional really depend on the kind of practice you are looking to build for yourself. I work with thousands of nutritionists, dietitians, health coaches, alternative medicine practitioners, as well as other health and fitness professionals who hold various degrees and certifications.