A snack of warm milk and cookies, which you savored while your mother read Dr. Seuss to you at nightfall, is not the soporific we once thought.
According to modern nutrition research, whole milk with extra carbohydrate can make you drowsy, while skim milk standing alone is a better alertness enhancer than coffee.
Milk has two separate ingredients – fat and protein – which have competing effects. Change the fat content of milk, and you change the outcome.
Whole milk has 3.25% milk-fat. The fat content is sufficient to override any effects from the protein. Fat slows your digestion, drawing blood to the stomach and away from the brain. Drinking whole milk can create somnolence.
Skim milk has 0% milk-fat. All the fat is removed in manufacturing. When you drink skim milk, the outcome hinges entirely on how your body metabolizes the protein.
The research developed like a mystery story, one clue uncovered at a time. Years ago, researchers detected in milk a drowsiness chemical called tryptophan. Tryptophan is an amino acid, a building block of protein.
Then scientists uncovered a snag: The amount of tryptophan in milk is quite tiny, a mere 1.27% of its total protein content.
Actually, a different amino acid – called tyrosine – dominates the digestion of milk. Tyrosine happens to be an alertness chemical for the brain. When you drink a glass of milk, tyrosine floods your stomach and bloodstream, then wins the race across the blood-brain barrier. This discovery turned current wisdom on its ear.
What about the familiar ‘milk and cookies’ ritual? When does it work, when does it not? Follow along with the steps between swallowing a food, to seeing its impact on human mood and performance.
Each of our dietary proteins – meat, eggs, nuts, milk – has its own blueprint, its own unique arrangement of amino acids. An electron microscope reveals that a single protein molecule is actually hundreds of amino acids in length. It is like an elaborate edifice of bricks cemented together.
|Total Protein||8.26 grams||100 %|
|Tryptophan||0.105 gram||1.27 %|
|Tyrosine||0.417 gram||5.05 %|
The basic bricks get repeated many times, but the ordering is unique to, and defines, the given protein. Also crucial is the ratio of amino acids: how often does each type of brick appear, compared to the total number of bricks in the design? Such a fractionation of amino acids is published by the USDA Nutrient Laboratory. An excerpt for nonfat liquid milk appears in the table at left.
After a meal, the hydrochloric acid in your stomach dissolves the ‘cement’ binding a protein together, to allow the ‘bricks’ (amino acids) to flood your bloodstream. These individual amino acids are taken up by your muscles to build tissue, and by your brain to synthesize neurotransmitters (chemicals which influence how nerve cells behave).
In the brain, one of the amino acids – tryptophan – gets converted into serotonin; this precursor is required by the pineal gland to manufacture melatonin, a sleep-associated hormone. Another amino acid – tyrosine – gets converted into dopamine and norepinephrine; these two brain chemicals induce alertness. According to Dr. Richard Wurtman, pioneer in food-brain research from the Massachusetts Institute of Technology:
“Eating a food low in tryptophan, but rich in tyrosine, will perk up your brain.
Amino acids regularly move from your bloodstream into the fluid bathing the brain. They cross a filtering system called the blood-brain barrier. Not everything gets across this barrier. A cold virus, for example, is not chemically compatible (good thing!).
But nutrients such as amino acids are designed to pass. Richard Wurtman discovered that the major amino acids are all shuttled across the barrier by the same type of transport molecule. This makes their transport competitive. The amino acid which is highest in the blood will predominate in the brain.
The amount of tyrosine in the brain, then, can be amplified by eating foods rich in this amino acid. Don't worry about overdoing. There are physiologic limits – normal ranges – outside of which it's impossible to go. A feedback loop prevents levels of brain amino acids from going too high or too low.
Carbohydrates stimulate insulin secretion which decreases the amino acids floating in your bloodstream, with one exception: tryptophan stays level. Tryptophan then wins the race for transport into the brain where, at nerve synapses, it is converted to serotonin. Result: You feel calm and sleepy.
Carbohydrates vary in effectiveness. Fruit doesn't work well because its sugar, fructose, causes insulin to be released too slowly to trigger a serotonin boost. High-fat baked goods (muffins or carrot cake) make you sluggish because fats are slowly digested, but this does not cause serotonin to rise. Your best choice is starch.
What happens with the warm-milk and cookies ritual? Munching cookies (carbohydrate) spurs insulin secretion, which suppresses pre-existing amino acids in your blood. But this gets balanced out by the infusion of amino acids from the milk (protein) you just drank. While the brain's rise in tryptophan gets blocked, tyrosine doesn't rise much either. The result is neutral: You feel neither drowsy nor energized.
Pure protein raises blood levels of amino acids in direct proportion to what you eat. Insulin doesn't modify a protein meal, thus any amino acid that predominates in the bloodstream will also predominate in the brain. Which protein should you choose?
Amino acids fall into three categories. Essential amino acids cannot be synthesized in the body and must be consumed through food. Nonessential amino acids are produced by enzymes in the gastrointestinal tract. Conditional amino acids are usually not essential, except in times of illness and physical stress. See list, right.
Examples of complete proteins, which contain all essential amino acids, are milk, eggs, peanuts, beef, poultry, and fish. Among available proteins, skim milk stands out for two reasons:
Dr. Richard Wurtman fed single protein meals to lab rats, then dissected their brains. Then for each amino acid, he compared the level found in the bloodstream versus the level found in brain tissue. Wurtman did this to learn how molecules are transported across the delicate blood-brain barrier. The battles for transport are sometimes complex. The final result of this research:
“Skim milk elevates your brain tyrosine level by about 2½ times its pre-existing value. As well, due to complicated battles at the blood-brain barrier, your brain level of tryptophan actually drops, which means less serotonin – less of the drowsiness chemical. Result: Mental acuity improves. You think more quickly and accurately.
The milk must be digested without fat or carbohydrate, so the type of milk matters. Choose non-fat or skim milk (not whole milk, not 2% milk, not even 1% milk). You need at least half a cup, begun on an empty stomach. Drinking more won't enhance the effect, though periodic sipping will provide a steady supply to keep you alert.
Can't stand the thought of skim milk? Think it looks like thin blue paint-wash? Try using a china cup with a white interior. The milk will appear white and thick.
Diabetics and others with nutrition-related health concerns should consult a doctor before altering their diet. But for the majority: Start the morning or a work session with skim milk, balance protein and carbs through the day, and reach for cookies when you turn out the light. At night, if you drink milk with your delicious cookies, remember to start the carbs a shade ahead.
Essential amino acids cannot be synthesized by the body. Consume them all over the course of a day. The balance is important; you don't need all 9 at every meal.|
Nonessential amino acids are readily synthesized in the GI tract.
Conditional amino acids are needed at times of illness and stress.
Tyrosine is converted to the biologically important derivatives dopamine and norepinephrine. The final product of the metabolic chain is epinephrine (also called adrenalin). These neurotransmitters trigger alertness.
Tryptophan is converted to the biologically important derivative serotonin. The final product in the chain is melatonin. These neurotransmitters induce sleep.
Tuum Est - It Is Up To You
Position of solar system is labeled
Unknowingly, we plow the dust of stars, blown about us by the wind, and drink the universe in a glass of rain.
The US Department of Agriculture maintains a database with the nutrient content of 7,500 foods. The detailed charts include a fractionation of amino acids: For each food, the total protein content is weighed, and this value sub-divided to show the relative contribution of each amino acid.
Is it safe to adjust your diet based on neuro-transmitter physiology? Yes, provided you stick to ordinary food. Supplements may seem like a quick way to get specific amino acids, but researchers advise against it because the manufacturing process can introduce impurities or contaminants.
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