It’s an absolutely fantastic way to gain weight and slow your metabolism. I couldn’t recommend it more highly. As long as you can tolerate losing a little bit of weight in the beginning, and not everyone can, there’s no better way I know of to train your body to hoard every available nutrient long term.
As in most people will be able to go back on a regular diet fairly soon and still kept the weight on, & many will even keep gaining. If you’re lucky, you might be in the group that will gain for a solid 2 years after only a few months fasting, but even just the freedom of being able to keep the weight on regardless of diet/activity levels for a few years? Even if it takes you longer, with fewer gains, its still a huge relief to not have to struggle to keep weight on all the time! Ectomorphs will generally need longer to slow their metabolism than Mesomorphs; meanwhile people with histories of eating disorders, starvation, or yo-yo dieting will need hardly any time at all (their bodies are well practiced at assuming food scarcity to famine conditions at the drop of a hat... this is the crowd I’m in, used to drive my ex nuts, because as a body builder and an ectomorph he had to work really hard to bulk up, but all I had to do was forget to eat a couple times a week.).
Clearly, people on medications that increase metabolism and those with naturally high metabolisms will probably struggle more than most to balance it back downward. But if you’re chronically underweight, dealing with osteoporosis and/or other absorption issues, or are looking to bulk up (body building) before cutting? It’s really really worth it. As long as you can stand to lose the initial few pounds for the big gains later. If your nutritionist is recommending it though, they probably think it’s safe. Becoming an efficient metaboliser isn’t permenant for most people, though. So you’ll probably have to plan on cycling this into your lifestyle every few years, if you want to keep hoarding nutrients.
Don’t be discouraged if you don’t start seeing gains right away (after the initial weight drop), especially if you have an eye towards increasing your bone density. Fat doesn’t weigh that much, and it’s the first place nutrients get sent to / most people get a nice good layer of fat first. It can take a little bit of time for your body to feel safe sending bulk to muscles, organs, and making your bones more dense.