Diet Soda Has Zero Calories and Still Made People Gain Weight
Sweet taste alone can trigger insulin, hunger, and extra eating later in the day.
Andrew Huberman · How Sugar & Processed Foods Impact Your Health | Dr. Robert LustigDrinking a liter of diet soda a day for six months made people gain 2 kilos, even though it has zero calories. That result comes from a Copenhagen trial described by Dr. Robert Lustig, a hormone specialist who teaches children's hormone care in San Francisco.
In the study, 100 adults were split into four groups drinking a liter a day of sugary soda, diet soda, full-fat milk, or water. The sugary soda group gained 10 kilos, the water group lost 2 kilos, the milk group stayed the same, and the diet soda group gained 2 kilos.
Lustig ties the diet soda result to sweet taste itself. He describes a mechanism in which the tongue signals the brain that sugar is coming, the brain signals the pancreas, the gland that releases insulin, through the vagus nerve, the nerve linking gut and brain, and insulin then drives energy into fat and increases hunger.
A similar pattern appeared in a 2018 Singapore study in which people fasted and then had a sweetened drink, with meals afterward. People given aspartame, sucralose, or monk fruit in the morning ate more at lunch and dinner, and finished the day with as much insulin as people given sugar in the morning.
Questions people ask
If I want a fizzy drink, what should I drink instead?
None of these studies tested fizzy water. The Copenhagen trial tested still water, and the water drinkers lost weight. Whether unsweetened sparkling water would act the same way is untested in this research.
Do stevia and monk fruit do the same thing?
The Singapore data point that way for monk fruit. People given monk fruit in the morning also ate more later and ended the day with the same total insulin as people given sugar in the morning. Lustig groups zero-calorie sweeteners together because the issue is sweetness, not calories.
Does this prove diet soda causes diabetes?
No. The trials measured weight and insulin over a short period, not diabetes. Whether sweeteners raise long-term disease risk is a broader question beyond what these studies tested.