Shoot the messenger

The blogosphere is buzzing with new theories, discussions, disagreements and downright squabbles. What better time to look at some of the ways to make your case. There are many ways to lose an argument, these are just a few examples.
Argumentum ad hominem (to the man)
When you have no basis for an argument, abuse the plaintiff. 
Cicero 
It can be surprisingly easy to attack a person or a person’s traits rather than that person’s arguments. When arguing ad hominem you are linking a negative trait, characteristic or belief to the truthfulness of that persons claim. Contrary to what many people think, calling someone an asshat is not argumentum ad hominem. That’s just a good old insult. If however, you were to say that your opponent cannot be takes seriously because he smokes joint now and then, you are making a logical fallacy.
In the nutrition blogosphere one ad hominem argument is often repeated.
Remember Matt Stone’s “Poor Poor Jimmy Moore “ post where he linked Jimmy’s weight to the truthfulness of his weight loss claims? Although he never directly said, «Jimmy is fat, therefore his low carbohydrate diet advice does not work» – it was strongly implied.
After the death of Robert Atkins many have tried to undermine his message by referring to images showing he was not very lean.
Anthony Colpo wrote, amongst much other silliness, this about Richard Nikoley:
…apparently there is an angry post written by some overweight joker who runs what would appear from its URL to be an animal liberation blog …
Once again, Colpo did not say Richard is fat therefore he is wrong, but it was implied and could thus be filed under the ad hominem label.
A fat dietician or nutritionist is actually not any less trustworthy than a lean one, although we easily feel it is so. If you reject a person’s claim due his weight it is a logical fallacy. Sadly, when you work with nutrition, you gain more acceptance if you yourself look fit. 
Calling another person fat is only a logical fallacy if it is formulated such as: “You are fat therefore you must be wrong.” But the “therefore” can often be implied.
The above examples are forms of ad hominem arguments called “Tu quoque”, meaning “you to”.
The uncrowned king of logical fallacies and ad hominem arguments in the nutrition blogosphere is Durianrider 
Here are a few examples from his blog: 
Durianrider is the type of person who makes me want to commit logical fallacies myself. Oh, what the hell: Durianrider is an asshat, so you cannot take anything he says seriously.
Here’s another example of Tu quoque:
Bob: «Smoking is a highly addictive habit and causes health problems. You should not smoke.»
Alice: «But you yourself smoke!»
The fact that Bob smokes doesn’t mean that he is wrong about the effects of smoking.
Imagine for example that you move to a new town that has two dentists, of which one has poor teeth and one has good teeth. Who do you choose as your dentist? The one with the poor teeth of course, because chances are that he does the teeth of the dentist with the fine smile.
Similarly if you are joining a gym, join the one with the lean customers and the fat instructor rather than the one with fat customers and a lean instructor.
Overweight has a large range of causes, nutrition being but one. We all know that knowing what is smart to do is not enough to do what is smart. Especially we need to remember psychological issues that are usually at play. Overweight people can give just as good nutritional advice as lean people.
There are other forms of ad hominem arguments. If you claim that an argument is incorrect due to its source; for example saying your trainers claim that “proteins are good for you” must be wrong because the trainer sells protein supplements, it is a logical fallacy called Ad hominem circumstantial. You might want be skeptic, but scepticism it is no ground to dismiss his argument. We cannot dismiss a scientific article because it is funded by Pepsi because even Pepsi might be right.
Guilt by association can also be a type of ad hominem fallacy. An example:
Person P makes claim C to paleo blogger.
Vegans, a group which is viewed negatively (nutjobs) by the paleo blogger, also makes claim C.
Therefore, person P is also viewed as a nutjob and claim C is dismissed.
Argumentum ad ignorantiam
Appeal to ignorance – the claim that whatever has not been proved false must be true, and vice versa (e.g., there is no compelling evidence that UFOs are not visiting the Earth; therefore UFOs exist – and there is intelligent life elsewhere in the Universe. Or: there may be seventy kazillion other worlds, but not one is known to have the moral advancement of the Earth, so we’re still central to the Universe.) This impatience with ambiguity can be criticized in the phrase: absence of evidence is not evidence of absence.
Carl Sagan 
People often arrive at the wrong conclusions simply because of ignorance or lack of knowledge. A creationist may argue that evolution must be wrong based on the complexity of nature, thus illustrating his lack of understanding.
I feel that the oft-used “we don’t know the long term effects of carbohydrate restriction, therefore we consider it unhealthy and recommend against it” falls under the above category. If however carbohydrate restriction was always found to be unhealthy in short term trials, claiming it is unhealthy long term, is not a fallacy.
The argumentum ad ignorantiam can go two ways. Arguing that a central control of obesity is nonexistent because no one has proven it beyond a doubt is a fallacy. It is also a fallacy to say that a central control of obesity exist because its non-existence has not been proved.
Post-hoc ergo propter hoc
The human brain is in many ways designed specifically to se correlations and to assess causality. When the stone age man learned that bear cubs alone in the forest meant get the hell out of there before mother bear comes, it saved his life. It can also be life saving to associate the moldiness of food with abdominal pain. We look for associations everywhere and all the time, but we also often fail miserably. Correlation does not prove causation.
Many people will argue from their own experience that homeopathic medicine works, even though most studies show that they don’t. It you get well after doing or eating something out of the ordinary you are likely to believe that particular action caused the improvement rather than thinking you would have gotten better anyway. This illustrates the dangers of advise based on n=1 experimenting.
If you make the observation that fat people are more sedentary than lean people and that people are thus fat because they are sedentary, you are making a logical fallacy. There is always the chance that obesity makes people sedentary and that leanness makes people move.
When the low carber finds his symptoms of poor health disappear, he argues that carbohydrates caused the poor health. This too can easily be a fallacy.
Straw men
By reducing an opponent’s arguments into simplified versions easier to dismantle you are building straw men. Nutritional researchers are exceptionally good at making straw men. You can find several studies claiming dietary fat reduction cause all sorts of good things, even though the studies did much more than just reduce dietary fat.
Some studies claim to have disproven the effectiveness of low carbohydrate diets by using a diet with quite a lot of carbohydrates but labeling it low carb. They have then created an illusion of having refuted a proposition by replacing it with a superficially similar yet unequivalent proposition (the «straw man»), and refuting it, without ever having actually refuted the original position.
Tautology 
If you say that obesity is caused by an excessive calorie intake, this can be considered a tautology. Obesity by definition means calorie intake has been high, at least higher that calorie expenditure, so the claim is meaningless and nonsensical.
Argumentum ad verecundiam (Argument from authority)
In any decent discussion the discussing parts needs to have a certain degree of knowledge. James Krieger says Gary Taubes is wrong. As a consequence less knowledgeable people appear and say that Taubes is wrong because Krieger says so. This is a logical fallacy. Whether Taubes is right or not is beside the point.
How often do you think a politician commits a logical fallacy by arguing from authority? Whatever your view is, it is never hard to find an “expert” to lean on. In nutritional politics this logical fallacy is common. When the politician says you should eat more grain because ADA says so, this is a logical fallacy, albeit a rather necessary fallacy.
The Golden Mean
The golden mean argument bugs the hell out of me. The golden mean fallacy says that the truth must be somewhere between the extremes, often right in the middle. Most people have a tendency not to believe in anything extreme. But the truth is the truth, extreme or not.
Political correctness often ends up in a golden mean argument. Official dietary guidelines are pretty much based on the golden mean fallacy.
Argumentum ad populum (appeal to the people)
Even bad ideas and untruths can be accepted by the majority, this however does not make the idea any more valid.
All your friends say cholonics are a good idea and very healthy so it must be true, is a prime example.
I Norway, leading nutritional experts will say that almost every government in the world agrees that the human diet should be grain based, implying that it must thus be true.
In health and nutrition ad populum arguments are very common so watch out.
False Dichotomy
Saying fat is either good or bad for you is a false dichotomy, and do not believe the answer to overweight is either food reward or macronutrients.
Anyone who has worked with exercise or nutrition knows how people will make false dichotomies all the time. “Is this food healthy or unhealthy? Is this exercise effective or ineffective?”
As always, there are links in the text even though they are not highlighted by a different color. If you’d like to watch the full range of logical fallacies live, watch Fox News.