Why Sugar Makes us Fat

Robert Lustig, MD explains this concept best in his lecture “The Trouble with Fructose: a Darwinian Perspective”. Essentially, this is why sugar makes us fat from an evolutionary standpoint:

It’s only available abundantly during a small window of time per year, usually right before food scarcity during the winter.
When we eat sugar, insulin increases, which also reduces leptin sensitivity (they share the same receptor in the brain).
- Leptin is a hormone that tells the brain how much fat is stored on the body.
- The brain uses that information to determine metabolic rate and energy levels.
  - More leptin signaling = more energy = a higher metabolism
  - Less leptin = signaling = less energy = a slower metabolism and hunger level
  - More leptin = less hunger, less leptin = more hunger
- When insulain is high, leptin sensitivity is drastically reduced, resultnig in a slower metabolism, reduced activity level, and increased hunger.
Now we’re stuck in a positive feedback loop:
- More sugar = more insulin (more fat gain) = less leptin signaling�
  - Metabolism is slowed to save energy for fat storage, activity level is decreased to save energy as well = more hunger = more sugar, repeat.
This is evolutionary beneficial so when fruit (and sugar sources) are available, we have a strong desire to eat as much as possible so we can store as much fat as possible right before the winter when we will be forced to live off a lot of that fat.

This is why sugar makes us fat, why we crave it when it’s available, why sugary foods are don’t make you full.

This is also why eliminating sugar has the opposite effect: makes you full, makes you have more energy and a higher metabolism, leads to more fat/weight loss, and eliminates carb cravings.