Are obese people obese because they eat too much, metabolize slowly, or both? Most studies show that overeating is the major factor, but what is the neuroscientific basis for this?

One of the leading hypotheses comes from studies on rats, and claims that blunted pleasure circuits in the brain result in overeating. Obesity-prone rats have a significantly lowered baseline level of dopamine and level of dopamine release. The practical result is that obesity-prone rats have to eat more to achieve the same pleasure from food as obesity-resistant rats can.

What about humans though? Do obese people have less dopamine release in their brain in response to food than thin people do? A study from the Oregon Research Institute conducted functional magnetic resonance imaging studies (brain scans) on young women, scanning their brains while they received chocolate milkshakes through syringe pumps. They found that the obese women showed blunted dorsal striatum responses to the food reward, likely prompting them to compensate for the blunted reward by overeating.

This sort of basic research into the pleasure circuits of the brain provides hope for conquering this enormous health problem that’s sweeping over the world, especially the United States. 

In preschool or elementary school you probably remember learning about the basic tastes, out of which every complex taste is constructed. The four most commonly known ones are sweetness, sourness, saltiness, and bitterness. The fifth, umami (or savoriness) is lesser known in the West, and is best described as a meaty or brothy taste with long-lasting, mouthwatering sesation on the tongue. Umami describes the tastes of glutamates and ribonucleotides - think MSG (monosodium glutamate).

The mechanism of taste perception is fascinating. It starts with the taste buds located on your tongue, soft palate, esophagus, and epiglottis. You might also have learned in school that different parts of your tongue are responsible for different tastes (sweetness on the top, saltiness and sourness on the sides, and bitterness way in the back) - this is a myth and is based on a mistranslation of a 1901 German study. All the taste qualities are found in all areas of the tongue, though some regions are more sensitive than others.

The different types of taste buds are activated by the various components of your food dissolved in your saliva, and these impulses travel up to your brainstem, where various structures control automatic eating-related behaviors like swallowing and salivation. The signals then travel up to the thalamus, the gateway structure to the cortex, and then fan out to higher-level primary gustatory cortex, which is responsible for the perception of taste. 

Finally, from the gustatory cortex, the signals travel back deeper into the brain, to limbic areas that associate the tastes with emotions, reward, and memories.

Now think about this in the context of an unborn baby. A fetus' tastebuds begin to mature in the second trimester of pregnancy, and she will begin her first automatic sucking and swallowing behaviors around this time as well, providing vital neural stimulation for the process of the taste buds becoming wired up to taste circuitry in the brain. 

The brainstem matures early, allowing the fetus to automatically salivate in response to sweets or protrude her tongue to expel bitter liquids. This happens even though her cortex hasn't finished developing yet, meaning she can't yet perceive the actual tastes.

By the third trimester, almost all of the taste circuitry has finished maturing, and the fetus will begin to develop lifelong taste preferences based on the eating habits of her mother. This also happens in rats, where studies have shown that if expecting mothers are fed high amounts of distinctive-tasting fluids like apple juice, their pups will show enhanced preference for the same taste.

Sure, innate preference for tastes is part of the story, but there is surprising potential for taste preferences resulting from what the fetus experienced in the womb. In fact, if a pregnant mother eats a wide variety of foods, exposing the fetus to many different tastes through the amniotic fluid, her baby will typically show increased acceptance of novel foods.

Expecting mothers, arm yourselves with this knowledge and give your child a leading edge and lifelong advantage over picky eaters who can’t eat as healthy!