In relation to overall caloric intake, carbohydrates comprise around 55% of the typical American diet, ranging from 200 to 350 g/day. The vast potential of refined carbohydrates to cause harmful effects were relatively neglected until recently. A greater intake of sugar-laden food is associated with a 44% increased prevalence of metabolic syndrome and obesity and a 26% increase in the risk of developing diabetes mellitus. In a 2012 study of all cardiometabolic deaths (heart disease, stroke, and type 2 diabetes) in the United States, an estimated 45.4% were associated with suboptimal intakes of 10 dietary factors. The largest estimated mortality was associated with high sodium intake (9.5%), followed by low intake of nuts and seeds (8.5%), high intake of processed meats (8.2%), low intake of omega-3 fats (7.8%), low intake of vegetables 7.6%), low intake of fruits (7.5%), and high intake of artificially sweetened beverages (7.4%). The lowest estimated mortality was associated with low polyunsaturated fats (2.3%) and unprocessed red meats (0.4%). In addition to this direct harm, excess consumption of low-quality carbohydrates may displace and leave no room in the diet for healthier foods like nuts, unprocessed grains,  fruits, and vegetables.
“Your liver produces ketones all the time, but the rate depends on carbohydrate and protein intake,” says Jeff Volek, Ph.D., R.D., a professor of human sciences at Ohio State University. When the majority of your diet is made up of of carbs and protein, ketogenesis slows. Replacing carbs and protein with fat will put your body into ketosis, thus ramping up ketone production. This takes about three days to induce.

The only draw back you will see is for those who do have high activities or are involved in lots of sprinting type exercise. Although a few will find they feel fine, even have more energy on a high fat/moderate protein diet, most of the time carbohydrates are the best source of fuel for these activities. That isn't that big of a problem though, it just means that that person should instead look into doing a TKD instead of a CKD.
I recall reading once that pink grapefruit and eggs for breakfast (or maybe it was the grapefruit 10-15 minutes before eggs) was supposed to launch you into some crazy fat burning mode. Something to do with hormone production, I seem to recall. Can’t say if it’s true or not. I tasted a grapefruit once when I was about 10, and have never desired to repeat that mistake in the 24 years since.

Aude, Y., A. S, Agatston, F. Lopez-Jimenez, et al. “The National Cholesterol Education Program Diet vs a Diet Lower in Carbohydrates and Higher in Protein and Monounsaturated Fat: A Randomized Trial.” JAMA Internal Medicine 164, no. 19 (2004): 2141–46. doi: 10.1001/archinte.164.19.2141. jamanetwork.com/journals/jamainternalmedicine/article-abstract/217514.
I was a Corpsman (not a corpse-man as some recent somewhat fanatical president would say), and I can tell you many stories of Marines and Sailors who maintained restrictive diets (aka picky eaters). Most obvious was lack of sustaining energy (hypoglycemia) at mile 15 (with 80lbs of gear including a 6.5lb rifle and 200 rnds of ammo, etc.) and depletion of essential vitamins, electrolyte imbalance. They were always the first to collapse and have to hear me scold “see I told you so.” An IV of D5W usually does the trick (D is for dextrose, OMG!)
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