1. Sugar = usually means sucrose, one specific type of carbohydrate and you can absolutely live on a diet with no sucrose whatsoever.
2. You can live even without carbohydrates as such, only some fats and proteins are essential. Traditional Inuit diet consists of fat and protein; they are fairly healthy as long as they do not adopt modern Western diet.
“Traditional Inuit diets derive approximately 50% of their calories from fat, 30–35% from protein and 15–20% of their calories from carbohydrates, largely in the form of glycogen from the raw meat they consumed.
This high fat content provides valuable energy and prevents protein poisoning, which historically was sometimes a problem in late winter when game animals grew lean through winter starvation.
It has been suggested that because the fats of the Inuit's wild-caught game are largely monounsaturated and rich in omega-3 fatty acids, the diet does not pose the same health risks as a typical Western high-fat diet.
However, actual evidence has shown that Inuit have a similar prevalence of coronary artery disease as non-Inuit populations and they have excessive mortality due to cerebrovascular strokes, with twice the risk to that of the North American population.
Indeed, the cardiovascular risk of this diet is so severe that the addition of a more standard American diet has reduced the incidence of mortality in the Inuit population.
The liver will start to adapt by performing gluconeogenesis from fat, which has plenty of calories (almost 2x as much per gram as carbohydrates). A human can easily thrive on just fats and protein. See Inuit people who have lived on mostly blubber and meat for centuries.
The ‘everything is sugar’ meme really frustrates the conversation about nutrition and does lead to people making bad nutrition choices, as I’ve personally observed IRL.
Your body can get energy/calories from carbohydrates, fat or protein. Whatever is available. You absolutely need proteins. Between fat and carbohydrates, the body can compensate. There is a clear limit on the ability to process carbohydrates.
Minor nit: essential fatty acids must come from diet as they cannot be synthesized directly. These are alpha-linolenic acid and linoleic acid, with docosahexaenoic acid and gamma-linolenic acid as conditionally essential under some conditions. Though required quantities are minute.
Fat and protein. Humans have several metabolic pathways to convert these into energy, including the production of ketone bodies and gluconeogensis. These are the same metabolic pathways that allow humans to survive off of body fat during a fast. Without these pathways we would literally die within a day or two of not eating[0], since we’re incapable of storing large amounts of glucose from carbohydrates[1].
One can argue that you might be healthier in the long run with carbohydrates, I’m inclined to disagree, but to argue that you can’t get energy without them is just objectively incorrect.
0 - There is a tiny percentage (10 cases) of the population with the inability to build up body fat thanks to the Mardanoid-progeroid-lipodystrophy syndrome. Those with the syndrome can’t store excess calories in the form of body fat, and must eat continuous small meals to meet their metabolic needs. Curiously it seems like the lack of body fat makes their metabolism incredibly inefficient, as they are reported to average 5,000 to 8,000 calories a day. I presume this is because every calorie they eat is “use it or lose it”, resulting in a lot of waste.
1 - Humans can store some glucose in their liver and muscles for immediate use. But strenuous activity can easily exhaust this supply, so it’s not a reasonable source of energy for even moderate fasts.
Yes it’s about added sugar which seems to be in almost any food you can buy in the US. Some years ago I developed something like an allergy against sugar. It gives me headaches, joint pain and almost fever like symptoms if I get too much. It’s mind boggling to go shopping and read nutrition labels and realizing that almost every good is sweetened in some way.
2. You can live even without carbohydrates as such, only some fats and proteins are essential. Traditional Inuit diet consists of fat and protein; they are fairly healthy as long as they do not adopt modern Western diet.
https://theiflife.com/the-inuit-paradox-high-fat-lower-heart...