There are many, many, more complicated areas of research, including most other areas inside biology. We have millenia of trial-and-error as a starting point for nutrition. The fundamentals haven't changed, although the problems are new and inventive.
I guess I mean an expanded view of nutrition science, where we fully understand the effects of what we eat. This would be beyond engineering.
One thing about engineering is it can succeed or fail fairly quickly, and the results can mostly be replicated. We build a reactor and we soon know whether it works or not. And reactors are something we understand enough that we can design them, and create them.
Meanwhile in nutrition we're making educated guesses and watching what happens over decades. What helps one person kills another. We're using statistics to tease out the truth, to get mere glimpses of the truth. We're on the outside making educated guesses about a system beyond our full comprehension.
A final thought: Which will happen first: commercially viable fusion energy, or the obesity rate in the US falling below 25%? Which would you bet on? (I am considering the challenge of how to improve the diet and nutrition of an entire nation under the umbrella of "nutrition science" here.)
You are conflating the state of the art with how difficult something is.
As other have noted in the comments, research into nutrition is often poorly distributed and implemented. It often over emphasizes child nutrition and neglects adult nutrition. And it is fraught with bad science and agendas.
This says nothing about how complex nutrition is compared to any other field of research.
