In animal studies the researcher has conducted, the amount of Neuropeptide Y produced by cells in the PVN correlates directly, positively, with carbohydrate intake. The more Neuropeptide Y we produce, the more we eat carbohydrate.
"These Cells Tell Us To Eat"
"We can see these neurons and analyze the neuropeptides in them," says Leibowitz. "We know that these cells tell us to eat carbohydrate. In studies, we either give injections of a known amount of Neuropeptide Y, or measure the amount of Neuropeptide Y that's naturally there. Then we correlate it to what the animal ate in carbohydrate." Neuropeptide Y increases both the size and duration of carbohydrate-rich meals.
If production of Neuropeptide Y turns on the taste for carbohydrate, what sets production of Neuropeptide Y spinning? Probably signals from the burning of carbohydrates as fuel are the routine appetite stimulants. But Leibowitz has found that cortisol, a hormone produced by the body during stress, has a particular propensity to turn on the taste for carbohydrate by revving up production of Neuropeptide Y. High levels of Neuropeptide Y lead to weight gain by prompting overeating of carbohydrate.
Fat's Chance
The body also has a built-in appetite system for fat, the most concentrated form of energy, and it marches to a different neurochemical drumbeat, a neuropeptide called galanin, also produced in the paraventricular nucleus of the hypothalamus. Galanin is the second star player in Leibowitz' studies.
These have shown that the amount of galanin an animal produces correlates positively with what the animal eats in fat. And that correlates with what the animal's body weight will become. The more galanin produced, the heavier the animal will become later on. To add insult to injury, galanin not only turns on the taste for fat, it affects other hormones in such a way as to ensure that fat consumed is turned into stored fat.
What turns on the taste for galanin? When the body burns stored fats as fuel, the resulting metabolic byproducts signal the paraventricular nucleus for more fat--a case of nature safeguarding our energy storage. But hormones also turn galanin production on. To be specific, the sexual hormone estrogen activates galanin.
"Estrogen just increases the production of galanin and it makes us want to eat. It makes us want to deposit fat," says Leibowitz. The influence of estrogen on our taste for fat "is important in the menstrual cycle and in the developmental cycle, when we hit puberty."
Of Time and the Nibbler
The two neurohormones of nibbling are not uniformly active throughout the day. Each has its own built-in cycle of activity.
Neuropeptide Y has its greatest effect on appetite at the start of the feeding cycle--morning, when we're just waking up. It starts up the entire feeding cycle. After overnight fasting, we have an immediate need for energy intake. Neuropeptide Y is also switched on after any environmentally imposed period of food deprivation--such as dieting. And by stress. "If you have lots of Neuropeptide Y in the system at breakfast," says Leibowitz, "you're going to be doing lots of eating."
Necessary as a quick-energy start is to get going, man cannot live by carbohydrate alone. After carbohydrate turns on our engines, the desire for this nutrient begins a slow decline over the rest of the daily cycle.
Around lunchtime, we begin looking for a little more sustenance. An afternoon of sustained energy expenditure stretches before us; we can afford to take in the other major nutrients--fat to refill our fat cells and protein to rebuild muscle. Both of these are converted more slowly to fuel. Our interest in protein rises gradually toward midday, holds its own at lunch, and keeps a more or less steady course during the rest of the day.
A Clockwork Orange
After lunch, the taste for fat begins rising, supported by increasing sensitivity to galanin and increasing galanin production; it peaks with our heaviest meal, at the end of the daily cycle. That's when the body is looking to store energy in anticipation of overnight fasting.
Take a late-afternoon coffee--or tea--break and you're virtually programmed to dive for energy-rich pastry, as appetite, spearheaded by the drive for fat, is gaining. We might, however, be better off appeasing the chemicals of consumption with a banana, or an orange.
Leibowitz believes that circadian cycles of neurochemical activity play a major role in eating problems. A late-afternoon fat snack, for example, could prime us neurochemically to consume more fat later into the night. Galanin activity late in the day gives fat consumed at dinner a head start, as it were, on our thighs.
Silent Signals
What drives us from a carbohydrate-rich breakfast to a more nutrient-mixed lunch? The carbohydrate we take in at breakfast has a direct impact on more widely distributed neurotransmitters such as serotonin. Active in many systems of the brain, including learning and memory, serotonin is believed to play a general role of modulator; it is essentially an inhibitor of activity.
Eating carbohydrate leads straightaway to synthesis of serotonin. Under normal conditions, rising levels of serotonin are the feedback signals to the paraventricular nucleus to shut off production of Neuropeptide Y and put a stop to the desire for carbohydrate.
Behind the Binge
Tags:
battle of the bulge,
behavioral scientist,
brain chemicals,
breakfast lunch,
control appetite,
decisive victories,
dietary regime,
galanin,
healthy appetites,
high tea,
knife and fork,
leibowitz,
natural patterns,
neurochemical systems,
physiologic signals,
physiological function,
rockefeller university,
sitting ducks,
solemn vows,
waistlines
