AGRC2001 Sugar and Fat Notes

The Western world is experiencing an obesity epidemic since the 1970s.
Coupled with Type II diabetes, it forms the 'metabolic syndrome'.
The similar population genetics compared to our grandparents suggests an environmental (e) rather than genetic (g) origin.
Likely environmental factors:
- Highly processed foods.
- Sugary foods and beverages.
- Calorie-rich, nutrient-deficient foods.
- Shift away from dietary fats.
- Sedentary lifestyles.
Stone age genes but space age lifestyles!
Chronically tips the caloric balance towards fat deposition, especially from dietary sugar.

Monogastrics 'run on sugar'.
Some tissues (e.g., brain) require sugar as the primary fuel.
Heart exclusively burns fat.
Skeletal muscle uses fat or sugar depending on availability.
The human body can handle a range of energy substrates.
ATP production from sugars, fats, and proteins is similar via Acetyl CoA, the mitochondrial TCA cycle, and the electron transport chain.
Steps before Acetyl CoA differ.

During low caloric intake (fasting, after sleeping, heavy exercise, low-carb diets), gluconeogenesis ensures the brain gets sugar.
Gluconeogenesis produces just enough sugar to meet demand.
It is a liver-based pathway creating sugar from non-carbohydrate precursors.
Substrates include lactate, glycerol (from hydrolyzed TAG), and glucogenic amino acids such as alanine (from wasted skeletal muscle in extreme starvation).

During high caloric intake, especially sugary foods, the body stores energy, leading to fat accumulation.
Excess sugar circulates to adipocytes and the liver for de novo lipogenesis.

Glucose is broken down to pyruvate (glycolysis).
Pyruvate enters the mitochondria for partial processing in the TCA cycle.
Citrate is formed and exported to the cytoplasm.
Citrate breaks down into Acetyl CoA.
The rate-limiting step is the formation of malonyl CoA from Acetyl CoA, catalyzed by Acetyl CoA carboxylase (ACACA).
Lipogenesis requires energy (ATP hydrolysis), reducing power (NADPH from the pentose phosphate pathway), and building blocks (Acetyl CoA).

Separate pools of Acetyl CoA (mitochondrial and cytoplasmic) prevent futile cycles (making and immediately destroying).

De novo lipogenesis in the lactating mammary gland produces a lot of fat-rich milk.
Ruminants lack sugar because rumen microbes consume dietary glucose.
Cattle make Acetyl CoA directly from the VFA acetate (from the microbes) catalyzed by Acetyl CoA synthetase.

Sugar is a more significant culprit of modern ailments, because any loss of blood sugar homeostasis creates havoc with your health -- particularly your vascular system.
Sugar consumption has increased, while dietary fat consumption probably has not.

Type II diabetes often results from sustained, poor lifestyle choices that disrupt sugar homeostasis.
Insulin insensitivity occurs, preventing insulin from driving excess sugar into muscle/fat cells.
Physical exercise is prescribed to cure type II diabetes because contracting muscles use up excess sugar from the circulation, storing it as muscle glycogen.

Natural sugars in full cream milk (lactose) and fruit (fructose) are converted to glucose in the liver.
This conversion takes time and resources, reducing blood sugar spikes.
Fructose is absorbed more slowly than glucose in the small intestine, also reducing spikes.

Animal foods are fatty and proteinaceous (e.g., eggs and meat).
Plant-based foods are carbohydrate-rich (e.g., leaves and fruits).
Exceptions: milk (sugary animal product); seeds and avocado (fatty plant products).
Trends away from animal products will decrease protein and fat consumption, increasing carbohydrate consumption, the full implications of which remain to be seen.