How confident am I in my knowledge of nutrition/exercise is accurate?
How sure can we be? (my risks and your risks)
For some types of knowledge, we trust implicitly. We act based upon that knowledge, with get immediately (or rather immediate) feedback of its truth, and it never fails us.
The sun rises everyday.
Apples fall down (near the surface of the earth) unless otherwise held up.
Air is all around us (unless we seek out a place where it is not).
If we combine 3 items and 4 items, we will have 7 items.
About 99.99999% sure
A human-species-threatening meteor impact does not occur within the next week.
That, if you flip a coin specified large number of times, you can be 99.9999% sure you will get at least one heads.
You personally are not going to discover a new mammalian species within the next month.
When you add two numbers on your calculator, it will give you an accurate result.
Many things we count on regularly, but know they could surprise us
When you push on your car's brake, the car will begin to slow.
When you pick up your child from school, the classes will have ended at the standard time.
When you open the water tap, water will come out.
Your bank will not make a mistake in calculating your balance (according to their records)
That this site becomes unavailable because the hosting and development company went out of business.
Other things involve greater risk, but we still rely upon them to be predictable to be one way and not the other
Your wife or husband will not file for divorce within the next year.
A very punctual and reliable friend does not show up for an appointment.
The treasurer will not embezzle funds from your non-profit.
That the airline will honor the ticket you bought and paid for when you show up at the check-in counter.
As we continue into things being less certain, things can get fuzzy
We like to feel safe. We like to be able to think we know what is so and what isn't, what will happen and what won't. Our desire and tendency is to think in terms of 100% or 0%, especially involving things that personally affect us (or affect the view we like to have of ourselves). Scientists have identified several cognitive biases (that we're all prone to) that actually lead us down the garden path to thinking we know more than do or that we can be more certain of things than we should be. Ask any newly wed (especially for their first time marriage) what they think the chances of their marriage ending in divorce. Almost all will give you a number of 0% to 1%, even though the national average is over 50% (in the USA). This is called the optimism bias.
Additionally, when the relationship between cause and effect that is longer term (the effect takes longer to become obvious) and many causes may be consorting to create one or more effects, in order to maintain cognitive integrity, we must then allow for the chances that the things we believe are true (whether directly from our own thinking and evidence) or as presented to us by others whom we have decided to "trust" (which brings into question another new level of uncertainly) cannot be stated with near 100% certainty. To be more accurate in our assessment of what we really know, we should put some percentage on our level of certainty.
Combine that with a claim like, "Here's an easy way to know whether or not the way you eat is the best." A diet can consist of many hundreds of different types of food, processed different ways, and prepared for eating different ways. And then there's how much you eat of each and how often you eat. Thousand of variables that can affect whether or not your died is terrible, bad, not too bad, okay, above average, good, and best. Which factor contributed towards it being a better diet and which did not.
And then how and when is "best" measured? And for whom? Are we going to measure energy level, HDL and LDL, certain brains functions, how long you actually live, what you're BMI is, and/or which life functions are maintained or improved...or deteriorate (like dizziness or lack thereof). And even if we do end up with "best," how do we know whether or not some aspect of our diet detracted from that instead on adding?
To further add to the fuzziness, we have to measure "best" only by what we know now (and there's often disagreement among very smart people about what is true or not in the area of nutrition), without even to look at what we may know in the future that either adds to (or subtracts from) what we know now.
And, even that's assuming that any one person could adequately and objectively review and make reasoned decisions about all the information on diet that is currently available in the world, which is impossible. Then it comes down to just you and me, just two non-diet scientists trying to get a handle on what might be the best diet for us individually.
Given, all this, it's seems hubristic for any one of us to claim that we know more than next to nothing.