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Dietitians are regulated by the Health and Care Professions Council. ("Nutrionist", by contrast, is a title which is not so regulated.)
Diet is hard to study - mostly relying on retrospective surveys, which are notoriously unreliable. This makes it difficult to be confident about advice on the relative benefits and harms of different dietary advice. Nevertheless, the lay press seem obsessed with transient ideas of benefit, to an extent which can obscure what is known, and what has been summarised succinctly by Michael Pollan.