How can scientist get grants in human nutrition field?

It depends on the proposal topic, if you need grab their attention to get suitable grant, you should focus deeply on a brilliant solution to mitigate a sickness due to food.

There are not many funding bodies specifically focussed on nutrition, but often, research projects on nutrition link diet to one of more health outcomes (e.g. cardiovascular, neuro, etc, etc). I would recommend exploring calls for grants on these diseases. 

The principles investigator should be prepared a research proposal and identify the government and industrial organizations to submit their proposal. In addition, the research proposal should be scientific with highlighting the background and objective of the study. The current trends including ready to eat drink & food, energy drinks according to the RDA will be highlights to solve the nutrients deficiency problems all groups of age.

You can check online the world renowned funders on human nutrition. Consider checking on open AI sources.

To get grants you need to have innovative ideas and scientists seem mucch more interested in publishing than thincking; that's why they find difficult to raise money!

Firstly identify organizations who give grants for this specific research. Secondarily identify a disease related to human nutrition. It can be related to vitamin deficiency or even excess. In the current epidemics of obesity, research on the effects of macronutrients  on weight gain is getting much attention.