Particularly for longevity... Please outline?
Dang that 50% protein chick sure came out of left field huh?
The answer to that question is actually very large, if you want a detailed outline. Hard to do it in two or three paragraphs to be honest, since there are so many components involved.
A brief answer would be that fat is the macronutrient that least promotes insulin and insulin like growth factors. At the cellular level, insulin activates mechanisms by which to increase glucose uptake and glucose oxidation and increase fatty acid synthesis, thereby making cells bigger. Growth factors signal cells synthesize proteins required for structural components as well as to to divide and replicate themselves, thereby making them more numerous and bigger. The net effects of both of these hormones are to signal the cells into an anabolic state, either by encouraging them to grow or to replicate, while discouraging them to die (apoptosis inhibition).
This might sound like a good thing on the surface, and it can be if you're looking to grow muscle tissue and become "bigger", or if you are a child who is growing. For purposes of longevity however, these anabolic processes have the net effect of aging, as aging is ultimately defined as the accumulation of mutations over the lifetime of an organism, due to cellular growth and replication. This is why cancer is more common in older populations, because mutations and errors are acquired over many years due to continuous cellular division. Therefore, speeding up the cellular proliferation processes can lead to early aging and in some cases cancer.
Protein has a very big say in the speed at which our cells engage in proliferation and therefore while it is indeed essential to keep us alive, a slight (not a large) restriction in protein, can actually lead to a longer lifespan (via slower aging) and a decreased likelihood of acquiring cancer. Carbohydrate restriction on the other hand also decreases glucose availability and consequently reduces insulin levels which on its own signals the body that it is in a fasted state.
Fat is the one nutrient that can be eaten that will not substantially raise insulin and insulin like growth factors and therefore will not increase cellular proliferation or fatty acid synthesis. In every experiment where animals have had their insulin and insulin like growth factor receptors decreased by genetic alterations, the animals have all lived longer than the control groups of animals, this implies that animals whose cells that don't react as strongly to insulin and insulin like growth factors, can eat similar amounts of carbohydrate and protein than the control animals and live longer because their cells are less influenced by insulin and insulin like growth factors.
So since we can't genetically modify ourselves, we can achieve very similar results simply by reducing our insulin and insulin like growth factor levels, which are directly proportional to our intake of carbohydrates and protein, respectively.
Please show us your references methodician. A big population study on dietary macros vs human longevity would kick us in to action. I think that eating certain macro ratios is a minor factor on longevity, compared to many other things you do in your life.
I'll propose a benefit for fat in general though. Underweight is more of a concern than overweight in otherwise healthy (not obese, not starving) people. Fat reserves are a hedge, your body hoards them, and they are the preferred metabolic fuel for low levels of activity. Generally women have a higher level of body fat than men, and higher longevity. [But is it due to fat, or just another correlation without causation....]
Rosedale's study may be of interest for the improvements seen on a high fat diet eaten ad libitum, but it's a 90 day study on a small population that lost weight. http://jrnlappliedresearch.com/articles/Vol9Iss4/Kohnilias.pdf The weight loss confounds the results, though the reduction in leptin is very interesting.
Both protein and carbs signal insulin, fat does not. CR/IF work by restricting insulin. Insulin is a very old hormone, even yeasts make it.
You can eat all the fat you want to the limit of satiation (make sure it's good quality fat), and you won't raise insulin, nor will you stop autophagy. If you do go over your satiation level, you'll find that your body will convert the glycerol backbones of fat molecules into glucose. (So at some point, you'll start to fall out of ketosis if you eat tons of fat, but usually this is a very small amount of glucose, and you're likely to experience not so fun trips to the bathroom every 3 minutes to get rid of all the excess because you won't have enough bile to process it all.)
Burning carbs produces more ROS than beta oxidation. The reason we have insulin is to prevent the toxic effects of high blood sugar which can destroy nerve tissues and harm kidneys when they start to filter the sugar out of the bloodstream. Additionally, most types of, but not all, cancer cells have broken mitochondria which can only process glucose. By limiting the intake of carbs, you limit their growth. By doing occasional fasts, you can causes them to go into apoptosis and prevent tons of problems later on.
We don't directly burn protein, we first have to convert it to carbs, this is an inefficient process that releases more toxins, such as ammonia, these have to be detoxed. In addition, after this process, you'll have to burn the newly generated carbs, or store them as fat/triglycerides. (This only applies to excess protein.)
The real issue isn't what macros you eat, in a species appropriate diet, humans will eat appropriate amounts. In the SAD, the game has been rigged to eat tons of carbs, mostly from grains, with meals that are devoid of micronutrients and full of toxins. Eating 50-200g of carbs/day is fine. Eating 600g of carbs per day is not (unless you're running marathons, and you shouldn't be doing that unless you want to deplete your heart of its stem cells, and cause it damage.)
From the standpoint of random guessing and shots in the dark::
Possibly that protein puts more wear and tear on the system by being harder to break down and motabolize and when it is promotes extra growth which again can provide more wear and tear to the system in the sense of burning its potential regeneration numbers. Carbs on the other hand are more likely to fuck with your hormones and confuse and break your bodys ability to regulate, site insulin. Fat on the other hand is arguably your bodies prefered method of energy as it is the way body actually chooses to store exess energy.
So I guess that's one negative each for carbs/protein and 1 positive for fat. That being said, that's probably a pretty ignorant and simplistic way of looking at the issue.
I'm still not quite sure what to make of dietary fats (more specifically, saturated fats and cholesterol) for apoE4 carriers. (Up to 20% of the population has 1 copy, a smaller percentage have the full double genes.)
http://www.longecity.org/forum/topic/56235-just-discovered-im-apoe4-now-what/ (Interesting that guy does weight training / HIIT on a low carb high fat diet, yet his total cholesterol gradually continued to rise.)
http://www.longecity.org/forum/topic/53493-my-blood-test-results-20-years-old-apoe-4-recommendations/ (Interesting that guy was 6ft tall and only 132lbs on a low carb high fat diet, yet tested for high LDL and low HDL.)
Longecity has a few E4 carriers who were eating high fat for longevity and noticed a huge spike in LDL / cholesterol (beyond what might be viewed as beneficial.)
Perhaps fat is the most metabolically benign macronutrient for those which have the genes that make it metabolically benign (e3/e3 carriers.) MUFA is looking a little better to me than SFA.
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