When you switch to Paleo you'll be cutting out a lot of carbohydrates from grains (cereals, bread, pasta, rice, oats, corn etc), legumes (beans, peas, peanuts) and processed foods (chips, snack bars, baked goods, soda, flavored yoghurt and all other things in colorful packages which are nearly always laden with sugar).
While you can find adequate carbohydrates in vegetables and fruits, given that you aren't eating ridiculous amounts of fruit or starchy vegetables (which is not advised on a Paleo diet) it is unlikely that you will come even close to consuming as many carbohydrates as you were on a more standard Western diet - where people typically eat 60% or more of total calories as carbohydrates.
Consuming such vast quantities of carbohydrates, particularly the hyper-palatable, nutrient-poor refined sources like sugar, high-fructose corn syrup (HFCS), wheat and corn that are present in almost all processed foods is a sure-fire way to get fat, sick and eventually die from preventable illnesses like heart disease and diabetes.
Basically the human body does not cope well relying on (refined) carbohydrates as the primary fuel source, particularly when chronically over-consumed - that is, eating too much carbohydrates too often. Unfortunately this is practiced almost ubiquitously in Western culture these days.
There is not one developed nation in the world that does not suffer from diseases of civilization. Go to any country with a McDonald's and you will find metabolic disease like obesity, diabetes and heart disease. [But isn't McDonald's part of a healthy, balanced diet, when consumed in moderation, David? God help us!]
I cannot stress this point enough: the chronic overconsumption of processed foods that combine refined wheat or corn with refined vegetable and seed oils (or worse still, trans fats) with refined sugar or HFCS is at the root of most metabolic disease.
To reiterate, eating a crap diet (too much refined foods / not enough real foods) is the main culprit for the following conditions: overweight, obesity, diabetes, heart disease and metabolic syndrome. It also most definitely contributes to many other modern illnesses that are becoming epidemic these days such as autoimmune disease, depression, Alzheimer's, ADHD, rheumatoid arthritis, and possibly many cancers.
The science is quite clear on what happens when you eat these foods. Not being a scientist or doctor I can only describe my understanding of the physiological effects of chronic overconsumption of refined foods. Basically the body tries its best to maintain its metabolic health and endocrine (hormonal) function but after enough years of a bad diet the wheels will eventually fall off.
Arguably the most important hormone in the body - insulin, which controls a host of metabolic processes from maintaining a narrow range of blood glucose to promoting muscle growth and fat storage - becomes increasingly less effective as bodily tissues become resistant to it.
Crappy carbohydrates fit into the equation by spiking insulin levels. Every time you eat a donut, muesli bar, corn chips or cake, your body has to secrete insulin to get all that glucose out of your blood.
Too much glucose in the blood (hyperglycemia) is very harmful and dangerous to the body so the regulation of blood sugar is the first metabolic priority, even at the cost of burning out the beta-cells of your pancreas or storing excess fat. These are the consequences of screwing up insulin regulation. Turns out you can't have your cake and eat it too!
The downward spiral into metabolic disease often begins as impaired insulin sensitivity, which may eventually turn in to full blown insulin resistance. The causes of insulin resistance can be genetic, but given you aren't born with it, environment and lifestyle (diet!) are the key contributors.
In any case, having too much insulin floating around your body too much of the time will ultimately lead to some form of insulin resistance. Generally, being sedentary and eating crappy carbs will promote insulin resistance while doing exercise (particularly resistance training) and eating less crappy carbs will counter it.
With impaired insulin sensitivity the pancreas has to ramp up production of insulin as you require more of it to do the same job. This is when we start to see some serious adverse effects.
Under the influence of insulin, it is nearly impossible for the body to use stored fat as fuel, and fat storage is actually promoted. The liver and fat cells are signaled to retain glucose. If this fat-storing state of the body becomes the norm, it doesn't take much overeating to gain weight and become obese, particularly over a long period of time.
After years and years of eating donuts, corn chips and soda everyday (or even pasta, bread and fruit juice), you will inevitably become insulin resistant (pre-diabetic), and possibly become unable to produce insulin (diabetic). Most likely you will also become overweight or obese, and will be at a far higher risk of heart disease and many other illnesses mentioned above.
Don't take this lightly: eating a diet of crappy carbs and not real foods will catch up with you. How long it takes to get fat, sick and die depends on many other factors such as genetics, stress and activity levels, other hormones such as leptin, vitamin D, sleep, etc, etc.
If you are lucky you may live to be old, but the likelihood is your quality of life for your final decade or two will be fairly miserable and you will be heavily medicated and inactive - not to mention a huge burden to the grotesque health system.
The good news is it doesn't have to be like this! Just because this is the norm, thanks to the SAD (Standard American Diet), obesity, diabetes mellitus and even heart disease are largely preventable if you eat the right foods and follow a healthier lifestyle in general.
You'll have to check out my next post 'Eat More Fat!' or any of the my other posts under NUTRITION or FITNESS for more information on how to eat better, feel better, look better and be a better human.
"Eat Paleo, Train, Live Life!" - twitter.com/thepaleomodel
Gebel, Erika PhD, 'Understanding Insulin Resistance', Diabetes Forecast, June 2011, http://forecast.diabetes.org/magazine/diabetes-101/understanding-insulin-resistance [Accessed 08 May 2013]
[Image source: www.pricelessfunnypictures.com]