Fructose

Fructose is a monosaccharide that Ray Peat identified as a uniquely protective sugar, primarily because it catalytically increases the overall metabolic rate of the organism, with experiments showing up to a 50% increase in whole-body metabolism. 1 Unlike glucose from starch, fructose does not provoke a sharp insulin spike; instead, it is absorbed more slowly from the intestine, which buffers the rapid rise in blood glucose that triggers a cascade of insulin and cortisol. 2 Peat argued that fructose enters cells easily and is converted in a way that promotes glycogen storage, thereby steadying blood sugar over time and acting as an anti-stress and anti-insulin agent. 5

Mechanistically, Peat explained that fructose and the pyruvic acid it turns into stimulate the enzymes that oxidize pyruvic acid, an action indistinguishable from insulin’s effect on the same enzyme, allowing fructose to fulfill insulin’s role in the relative absence of the hormone. 3 This means fructose accelerates the uptake of glucose by tissues including the brain, kidney, and heart, bypassing the need for insulin secretion and sparing the pancreas. 2 Furthermore, the high potassium content naturally accompanying fructose in fruit exerts its own insulin-like activity, helping cells absorb glucose without requiring insulin, which makes fruit a particularly safe sugar source for diabetics. 2 Peat also noted that fructose supports liver function by promoting cholesterol production, which, when thyroid function is adequate, provides the substrate for protective hormones like pregnenolone and progesterone. 2

Peat directly challenged the narrative that fructose causes fatty liver disease, attributing such findings to experimental designs that used diets high in polyunsaturated fats (typically 5% corn oil), excessive total calories, and sometimes magnesium-deficient chow, creating a setup biased to produce fatty acid poisoning of the liver rather than isolating a toxic effect of fructose itself. 2 He cited historical clinical evidence from 19th-century physicians who cured terminal diabetic patients by adding large amounts of sugar to a diet of milk, meat, and fruit, observing that the patients stopped wasting away, ceased excreting glucose in their urine, and rapidly rebuilt muscle. 4 In wild monkeys, a diet high in fruit correlates with low cortisol, and human studies show that sucrose consumption lowers ACTH, the primary pituitary stress hormone, suggesting that the body perceives a need for sugar under stress. 9

Danny Roddy has defended Peat’s view against the "paleosphere" critique that fructose causes insulin resistance and gut permeability, countering that those who consume the most fructose, such as raw fruitarians, exhibit none of the alleged pathology in real life. 6 Peat acknowledged that manufactured fructose can provoke allergic-type reactions in some people, but emphasized that fructose consumed in whole fruits is buffered by minerals and does not present this problem. 4 He also noted that while many doctors claim the brain and heart cannot use fructose, evidence shows these tissues do possess the transporter for it, and that fructose enters glycolysis at nearly the same point as glucose to be oxidized for energy. 8