My recent blood test showed complement C3 was below the limit, while C4 was normal. I read that "complement deficiency may also be a factor in the development of autoimmune diseases. ", in relation to C3 and C4. Also "Though genetic deficiencies of C3 are exceedingly rare, deficiencies in other components which are more common (though still very rare) can result in low C3."

In what way can we influence that complement with diet or supplements?

What other deficiencies could cause low C3?

Ah well, maybe it was bad food in the Greek public hospital. I also read levels fall in malnutrition.

Low levels of some of the complement proteins, including C3, are risk factors for some autoimmune diseases. Generally these low levels reflect genetic rather than environmental (diet, etc) effects.

To my knowledge, backed up by a PubMed search (PubMed is the primary source for peer-reviewed biomedical papers) there are no reported associations between low C3 levels and Multifocal Motor Neuropathy.