With reference to your request for information on how I reached the conclusion that Aspartame was responsible for the pain in my legs, I established this very simply.
I was guzzling between 1 and 2 litres (I live in Canada 1 litre is approximately 1 US Quart) of a pleasant tasting Key Lime flavoured “low calorie sparkling water beverage”. The ingredient list is as follows: Carbonated water, citric acid, natural flavour, potassium citrate, Aspartame (on the label it states in brackets “contains phenylalanine”), potassium benzoate.
Listed under Nutritional Information per 250 mL serving (1 Cup) Energy 4 calories or 20 KJ, protein 0.1g, fat 0.0g, carbohydrate 0.0g,Aspartame 132 mg, sodium 12 mg, potassium 82 mg.
At the rate of 2 litres/day and therefore 8 cups/day I was ingesting 1056 mg or a little over 1 gram per day of Aspartame.
I stopped all ingestion of this product completely with NO OTHER changes in my diet. Within a week the pain had almost entirely gone and feeling was returning in my feet. After a month I tried it again and within a day I was getting uncomfortable feelings in my legs again which again vanished on ceasing ingestion of this drink.
I believe that this in itself is adequate evidence that the problem was caused by the product and since the body does require levels of potassium and sodium they were unlikely the causative factor especially since my diet contains considerable quantities of sodium.
As a trained investigator I am well aware of procedures for establishing causative factors and have a lot more than a passing acquaintance with chemistry and physics and other scientific fields. I am not an ill-informed or uneducated adolescent, being 68 years old with many diverse fields of interest and have travelled widely.