I got some intricate questions on my first post on this subject, requiring more intricate answers. :)
How does the quantity of ingested pesticides compare to the total quantity of toxins absorbed by a human being in realistic quantities?
First of all, toxicity as a phenomena is not an either/or issue. A given substance's toxicity is a function of dose and exposure time relative to the organism's capacity to handle that particular profile of exposure without compromising health. This means that quantifying the toxicity of a given substance in a generalizable way is often difficult. This is particularly so when we are considering substances that are not toxic enough to reliably kill in a small acute quantity, but that with some probability may cause harm with chronic exposures.
I think that the above problem of context and quantification is one of the fundamental reasons why there will never be complete consensus about the toxicity of substances that are not positioned at extremes of the toxicity spectrum (such as botulinum toxin and water).
Those who expose us to pesticides and other synthetic, or "evolutionary novel", substances (with or against our will), tend to declare these substances as universally safe. And they do so with reference to relatively high "LD50s" (median lethal doses), while downplaying the potential harm from chronic low grade exposure.
Those, however, who take the opposite view will point to subtle effects observed with lower dose exposures on factors such as cancer risk and various health markers. The doses that are used in such studies are typically quite high compared to typical "realistic" everyday exposure, as this approach may be the only way to achieve statistical significance in the presence of confounding variables. So, for example, to study the effects on toxicity in humans, investigators often chose to include people who, for instance, get occupational exposure to the substances on a frequent basis.
Each side of this debate can easily be accused by the other side of cherry picking data; to not account for various contextual factor; to use either too low or too high doses in experiments; to be biased for financial or ideological reasons, etc.
So (to get back to your question!) an attempt to make an inventory of all our exposures (through ingestion, inhalation and skin contact) and assign a universally applicable toxicity level to each of them is a difficult task to say the least, and probably not meaningful as individual variability in terms of both specific exposures and "total body burden" is so high.
Another problem related to both exposure and individual propensities is the compounding effect of all types of environmental stress including toxicity. The presence above a certain threshold level in an individual of a given toxin could make that individual more vulnerable to any, or all, classes of toxins than would otherwise be the case. For example, if a person is exposed to mercury from leaking fillings his antioxidant reserves may be depleted and not be sufficient to neutralize the load of other potentially toxic exposures.
So, should you worry more about the pesticides in your food than, for instance, the air pollutants that you inhale?
Generally speaking, I think that you must come to some reasonably accurate estimates of your specific exposures and your vulnerabilities, and act accordingly.
Identify and take care of the obvious problems first.
If you, for instance, smoke a daily pack of cigarettes, jog frequently in a high traffic zone, live in a mold infested home, drink contaminated well water, eat junk foods on a daily basis, or expose yourself to any other established health risk, you should probably change those habits before you even think about pesticides in your vegetables, or low grade air pollution for that matter.
In other words, prioritize getting rid of the big and sustained exposures, and then survey your entire "perimeter" for potential lower risk threats and eliminate them as far as possible and desirable. (Context beyond pure health considerations, such as time, money, etc, obviously plays in strongly here.)
Remember that if the totality of what you are doing for your health (including diet, exercise, sleep, stress reduction, supplementation) is enough, you should be free of all persistent symptoms of any kind; or, if you are not there yet, you should see a gradual improvement of any persistent symptoms that you do experience. You should see good or improving results on functional lab tests. You should feel good, or gradually feel better. You should get stronger in the gym, or at least not move backwards.
What is the basic evidence that pesticides ingested in realistic quantities are harmful for a normal person?
Aside from the fact that there is no typical person relative to low potency toxins, and that harm is a very broad concept, the first types of evidence to consider are studies on health markers in animals exposed to pesticides, and studies on populations with occupational exposure.
You will probably find that you can look at such studies and come to very different assessments depending on your perception of risk and of harm, but that there are a lot of indicative findings.
I think that from a biological perspective it makes sense to believe that the toxic burden from manmade chemicals that have been added to the the background burden of natural toxins over just, say, a hundred years, is likely to be a challenge to the human body. This is because our genes have not had sufficient time to develop adaptations to the new level of environmental toxicity and/or to specific types of new toxins.
It it also reasonable to believe that we are even more vulnerable to toxins now (vs. pre-agricultural eras) because of a higher load of chronic stress which degrade the body's overall resistance to challenges, including chemical challenges. Incidentally, a crude measure of detoxification capacity is the presence of bile acids and lipid peroxides in the urine, and these markers tend to improve with reducing psychological-, dietary- and other types of chronic stress.
(I'm not saying that early eras were free of stress - quite the contrary, but that chronic stress was less common, and I think that the basic evidence in support of this idea lies in the fact that the human body is very vulnerable to chronic stress.)
So, the core issue is not that our bodies are inherently unable to deal with a variety of toxins. The issue is how to not exceed its capacity to do so.
Further regarding capacity, one needs to also take into account that even if one's body can detox and eliminate a certain quantity of toxins, such "processing" occurs at the expense of other processes in the body such as repair and regeneration. The body can only do so much given the resources that it has at its disposal.
The same Bruce Ames as mentioned above has proposed a compelling "triage theory of aging", essentially pointing out that modest shortages of micronutrients (vitamins and minerals) are likely one important factor behind the rise in chronic illness and premature aging. (The idea is that when the body is low on micronutrients, it will prioritize their use for processes needed for survival in the moment above repair processes that keep us functionally young).
If we integrate Ames' theory with the biochemical observation that micronutrients are used up in detoxification processes in the body, we arrive at the conclusion that the greater our toxic exposure (regardless of source), the greater our need for micronutrients for keeping ourselves youthful and healthy.
A problem however with adding micronutrients (or other compounds) as one's sole strategy to deal with toxicity is that micronutrients also become harmful above certain thresholds that are dependent on factors in each individual (each person essentially has an "RDA" at any given time for all nutrients), so reducing one's total toxic load seems to be the prudent choice over (only) trying to throw supplements at the problem. (I'm all for supplementation under reasonably controlled and monitored circumstances.)
Now, aside from these more theoretical types of arguments, it is frequently reported by health practitioners that helping patients discover and eliminate various toxins (which may be current exposures, or toxins accumulated in the body) improves symptoms and health outcomes. Perhaps these patients represent a subclass of particularly vulnerable people, but, how do you know for sure that you are not one of them now, or will become one of them later?
[Also read part III in this series!]