Because natural selection did not design us to thrive on carbohydrates, doing so creates myriad problems. The one that most low-carb people focus on is obesity and related symptoms of Metabolic Syndrome. But there are plenty of other reasons to restrict your carb intake. Spreading the word about these other effects is especially important when you're talking to people who clearly don't have a weight control problem.
(And I should know -- at 5'8, I've never weighed more than 140 lbs in my whole life, nor have I ever packed on much body fat at all. I certainly didn't get interested in carb restriction in order to lose weight, but rather to have more even more energy than I normally did.)
Just as no one wants to be fat, no ones wants to age physically. And the clearest sign of aging is not how overweight you are, how tall you are, or even how much hair you have -- it's the quality of your skin. You could take a short, heavyset high school boy and shave him bald, but no one would mistake him for a 30-something. His face would give him away. The same goes for females, of course: even with a pudgy belly and a shaved head, a young girl could not be mistaken for a 30-something woman once you saw the babyfat around her mouth and cheek area and noted the absence of crow's feet wrinkles around the eyes.
There are two proteins that are primarily responsible for giving skin its youthful bounce and elasticity -- collagen and elastin. Through a process called glycation, these proteins can become damaged when a sugar smacks into them. Sometimes a protein and a sugar are designed to fit together, but in this case the process is regulated by an enzyme -- like how an electrician knows which wires are supposed to connect with which doo-hickies, and which tab is supposed to go into which slot. Glycation is when the sugar and protein smack into each other at random, as though you placed all the wires and devices for your entire house into a single box and shook it up -- probably you'd get the wrong wires plugged into a certain device, or the right wire for the device but plugged into the wrong slot. Their function would be compromised, to say the least.
Rather than describe how glycation happens at length, I'll simply direct you to a good and brief review of the process from Skin Inc -- part 1 and part 2 (includes helpful pictures). There are several steps in the process, but what you get in the end are Advanced Glycation End-products -- AGEs (the acronym is not a coincidence).
The total amount of glycation that a protein suffers is just the rate of glycation times the length of time that it's assaulted by sugars. So, proteins with high turnover rates won't hang around long enough to get really screwed up, but one's that have long lives will be hit the hardest. Foremost among them is the collagen in the cardiovascular system. But that also includes the collagen and elastin in your skin. And once they're damaged, they can't perform their function of snapping the skin back into place when you stretch it. The result is wrinkles and slackened skin.
Although you can ingest these freakish protein-sugar combinations, you have to worry most about them forming inside your body. It may sound obvious, but what raises your blood sugar levels? Why, carbohydrates in the diet, especially the ones from sweets and starches that really flood your body with glucose and fructose. The preventative solution is obvious: restrict foods that raise your blood sugar. No Wonder bread, spaghetti, Chinese take-out, french fries, donuts -- or baguettes, penne pasta, basmati rice, artisanal kettle-cooked chips, or almond pastries sweetened with organic free-range agave nectar.
Does that actually work? This doesn't seem to be a very well researched topic, but I did find two studies (one for each extreme) that suggest that it does.
First, there is "Short-Term Low Calorie Diet Intervention Reduces Serum Advanced Glycation End Products in Healthy Overweight or Obese Adults". They split overweight people into a control group and a low-calorie group, and the low-calorie diet resulted in lower concentrations of AGEs. Of course, we know that calories don't have anything to do with it -- did the authors forget what the root word of "glycation" is? -- so let's see what they were fed.
The paper doesn't say exactly what they ate, just that the percent of daily calories that came from protein, fat, and carbohydrates were 47, 7, and 40 (I know, where's the extra 6%?). It's pretty tough to eat more than 30 - 35% of your calories from sheer protein, and 40% energy from carbs is still significantly less than the 45 - 65% that our government recommends. So, this is a lower-carb, low-fat, super-high protein diet. Clearly no one could follow it for long -- they should have reduced protein to about 35, carbs to 5, and fat to 60, and they would've seen an even bigger effect. Still, even this moderate restriction of carbs had an effect.
How do we know it was carb restriction rather than fat restriction that did the trick? The glucose levels of the experimental group dropped by about 5% (other health markers like BMI, blood pressure, etc., also improved by a similar amount). So we know it was the lowered carbs that lowered glucose, and that this lowered the formation of AGEs. That's the most straightforward interpretation based on how AGEs are formed, so that's what we go with.
Second, there is "Plasma levels of advanced glycation end products in healthy, long-term vegetarians and subjects on a western mixed diet". This post is already long enough, so I'll make a new one soon with all the cool charts and tables from this paper, but the up-shot is that all varieties of vegetarians -- semi-vegetarians, ovo-lacto vegetarians, and vegans -- had a higher proportion of the proteins in their blood that were glycated, compared to their omnivorous counterparts.
They were able to rule out a bunch of explanations such as smoking and age, but they couldn't point conclusively to what caused the difference. Glucose levels and overall carb intake were similar. The largest difference was that the vegetarians ate a lot more legumes and pulses, as well as whole grain products and processed cereal products (like muesli). This was especially true for vegans. The authors suggest that there could be some extra factor in these products, or perhaps just the fact that they were more processed. So, even in a dream scenario where you still don't eat more carbs or have higher glucose levels than an omnivore, vegetarians still get zapped with more glycation.
Since, according to the articles from Skin Inc, glycation of collagen and elastin doesn't become obvious until the mid 30s, the massive switch in our diet from fat to carbs may not be so visible among the young. But consider Hollywood celebrities who are in their 30s or beyond -- don't they look much more like mummies these days than they did decades ago? Apparently, there's no effective way to undo the damage done to glycated collagen or elastin, so the present-day luxury of plastic surgery is of no help to them.
On the bright side, since the effects don't immediately set in, if you're well under 35, you can take preventative measures now to make sure its effects will be minimal. And if you've got a son, or any other young dude who you have some influence over, and you want him to still be able to find a wife 10 or 15 years younger when he's 35 years old -- or if you've got a daughter who you want to protect from ever getting divorced -- tell them to throw away their bagels, Ramen noodles, blueberry smoothies from Jamba Juice, and Frappuccino ice cream, and to have some lamb, eggs, and avocados instead. And whatever you do, don't let them experiment with vegetarianism for very long -- persuade them to take up something that will make them more productive, like cocaine.