Weak acids are found in fruits and vegetables as well as whey protein and yogurt, honey, and some other foods. They can make you tired, cranky, etc. and make your skin dry. This is not natural and is largely a result of the industrialized way that we harvest crops.
Crops lose their acids as they ripen on the vine, but if you pick them before they are ripe they won't lose those acids as they ripen off the vine. In industrialized farming, crops (like apples) are harvested before they are ripe, then ripen off the vine while in storage (refrigerated train cars, warehouses, etc.) waiting to be sold. So when you eat it, it looks ripe but still has all the acids it did when it was picked unripe.
Solution? Either search high and low for crops picked recently or for a farm where you can "pick your own", or eat the few fruits that are always low in acids like bananas, dates, and pears (there are many lists online of food acid content).
Strong Acids
Strong acids are created by our digestive systems. If you eat a steak or a bunch of fatty foods (keeping in mind that fats are not bad) your body creates acids to break the food down. Some people process this acid efficiently (especially when they drink plenty of water), but many people are slow in processing strong acids and get acidosis, which can make you really tired, give you an ulcer, etc. Just ask last year's NBA MVP Derrick Rose, who went to the hospital with 2 ulcers the same year due to a love of spicy food and candy.
From ESPN Chicago:"'Everybody's got their poison, and mine is sugar... I'll be having binges where I'll just eat tons and tons of candy,' he said. 'I could go without eating candy for like two weeks, but right when I [have] the first taste of candy in a day, I'll go through like two pounds of candy. For real, like two pounds.'
A sugar high isn't a problem for a grizzled candy veteran like Rose.
'I'm so used to it,' he said. 'I could eat a whole bunch of candy and then go to sleep. Most people would be up, [saying] 'I can't go sleep.' I'd go right to sleep, man. That's how bad it is. I wish I could [stop].'
While Skittles and Gummy Bears are some of his favorites, Rose is an equal opportunity candy-a-holic. Everything but chocolate, that is."
Funny, chocolate is the only candy I do eat... anyway.
Weak vs. Strong Acids
These fruit and vegetable acids are weak acids, which the body must get rid of through oxidation (i.e. breathing). They are in the fruit itself, unlike strong acids (digestive acids) which are produced by us to break down foods and must be expelled through water i.e. sweat and urine.
Some people have difficulty expelling acids
The process of getting rid of acids is very fast for some people and very slow for others. So what happens for those who get rid of them slowly? The acids build up in the bloodstream and the are stored off in the tissues. Not good. As a countermeasure, the body releases alkaline minerals from the muscles to neutralize the acids--calcium and magnesium. So many people with acidosis end up with lowered minerals as well as losing muscle mass.
How to get rid of acids
Getting rid of acids is not hard once you know what to do; the bigger problem is to stop eating foods that bring/create them.
A quick way to get rid of acids is to neutralize them by taking common alkaline minerals. (Alkaline is the opposite of acidic.) The easiest are Calcium and Magnesium. You may actually feeler lighter in 20 minutes after a handful of magnesium tablets make their way into your bloodstream; they are neutralizing those acids (both strong and weak acids)in your tissues so that they literally disappear.
You can also speed up the process of getting rid of weak acids by doing aerobic exercise, the increase in respiration/oxygen intake will speed up the body's natural oxidation process.
For strong acids, you can help the body by drinking more water, which is needed for the kidneys to process these acids.
Christopher Vasey wrote one of the best books on Acids
The Acid-Alkaline Diet for Optimum Health: Restore Your Health by Creating pH Balance in Your Diet
