I've been eating a lot of homemade yogurt in the past year. This has been fermented using a starter culture of acidophilus. A result of the fermentation is that yogurt contains lactic acid [1].

A consequence of eating yogurt is that lactate levels in the blood increase:
"After ingestion of yogurt containing 1.06 mmol/kg body weight, D-lactic acid postprandial plasma D-lactate concentrations increased from 0.070 +/- 0.020 to a maximum of 0.200 +/- 0.010 mmol/l within 60 min." [2]

Lactate can get through the BBB:
"Specific, carrier-mediated transport systems facilitate transport of ... lactate".
"The concentration gradients for nutrients are generally in the direction from blood to brain." [3]

Exercise is another way to generate lactic acid in the body [4].

How do we react to Victorialou's reference?

Increased lactate levels in the brain prior to neurodegeneration does not necessarily mean that they cause the degeneration. They could be a response of the body to fight the cause of the degeneration.

Lactate: good or bad? Anyone got the answer?

References:
[1] http://en.wikipedia.org/wiki/Lactic_acid_fermentation
[2] "Postprandial plasma D-lactate concentrations after yogurt ingestion"
de Vrese M, Barth CA.
Z Ernahrungswiss. 1991 Jun;30(2):131-7.
http://www.ncbi.nlm.nih.gov/pubmed/1897274
[3] "The Blood-Brain Barrier in Health and Chronic Neurodegenerative Disorders"
Berislav V. Zlokovic
DOI 10.1016/j.neuron.2008.01.003
http://pdn.sciencedirect.com/science...00342-main.pdf
[4] http://en.wikipedia.org/wiki/Lactic_acid

John