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What are Phenylalanine Side Effects?

By Matt Brady
Updated Mar 03, 2024
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Though phenylalanine side effects are generally beneficial to the body, in individuals who intake too much or can't properly metabolize it, side effects can be severely negative. In these cases, side effects can include hypertension, seizures and brain damage. As an essential amino acid, phenylalanine’s most apparent and beneficial side effect is its function as a necessary building block for protein; it also may help create chemicals that work to regulate appetite and mood.

In spite of it being a necessary part of a healthy diet, the body doesn’t create phenylalanine naturally. Fortunately, it can be found in a variety of food products, particularly items high in protein, such as beef, poultry, pork and fish. It's also naturally found in milk and bananas as well as a variety of nuts. Other food products have artificially added phenylalanine, such as certain chewing gums and soft drinks that use the sweetener aspartame. There are three major types of phenylalanine, each with different side effects: L-phenylalanine, the amino acid's natural form; D-phenylalanine, the same as L-phenylalanine but made in a lab; and DL-phenylalanine, a mixture of the L and D types.

A deficiency in L-phenylalanine is often evident when one exhibits signs of depression and fatigue. This may be due to the body's use of phenylalanine to produce the amino acid tyrosine, which in turn produces dopamine, norepinephrine and other chemicals that regulate mood and adrenaline. Thus, many believe that one of the most significant phenylalanine side effects is its ability to regulate mood and help combat depression.

Studies have also found that L-phenylalanine may be helpful in the treatment of vitiligo, a condition that causes the skin to depigment, resulting in splotchy patches of white skin. One of the main treatments for vitiligo is UVA radiation. Some studies suggest that topical or oral ingestion of L-phenylalanine enhances the effect of the radiation treatment.

Some researchers also believe that beneficial phenylalanine side effects include alleviating symptoms of Parkinson's disease. Parkinson's disease attacks the central nervous system, negatively impacting an array of motor skills. It’s been proposed that D-phenylalanine improves muscle rigidity, and helps with walking and speech difficulties, thus providing a helpful supplement to the treatment of Parkinson's disease.

Some researchers also suggest that phenylalanine may act as a natural pain killer, or analgesic. As such, it is sometimes use to treat chronic pain, including back pain, migraines and toothaches. It also is sometimes used as a natural appetite suppressant.

DL-phenylalanine is used by some to treat symptoms of premenstrual syndrome, better known as PMS. The science behind treating PMS with this form of phenylalanine is the same as using L-phenylalanine to treat depression. The theory is that DL-phenylalanine spurs on chemical processes within the body to better regulate depression and anxiety.

Phenylalanine side effects can be as damaging as they are often beneficial. One of the most frightening side effects occurs in people who suffer from phenylketonuria (PKU), a disorder in which the afflicted individual is unable to produce the enzyme needed to metabolize phenylalanine. Left untreated, PKU can cause seizures and brain damage. Abnormalities from PKU are so feared that it is standard for most babies to be checked for this disorder within the first few days of life.

Although there is no cure, PKU can be treated with a carefully regulated diet, as well as with close monitoring of phenylalanine blood levels. With such treatment, it’s not uncommon for individuals afflicted with PKU to avoid negative phenylalanine side effects and live normal lives.

It isn’t just people with PKU who can suffer from a diet too rich in phenylalanine; anyone is capable of overdosing, with damaging results. Some children, for example, may experience hypertension as a result of phenylalanine being an ingredient in some medications for Attention Deficit Disorder (ADD).

High doses of the amino acid may also cause nerve damage; phenylalanine has been found in some studies to cause cellular death in nerves. This can be a result of a diet high in aspartame, an artificial sweetener loaded with phenylalanine that's found in many packaged foods, sweeteners and soft drinks. Individuals can also experience headaches, nausea and heartburn as a side effect of overdosing on phenylalanine.

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Discussion Comments
By anon958747 — On Jun 29, 2014

Taking DL-Phenylalanine 500mg twice a day has reduced my chronic low back pain.

By anon951968 — On May 19, 2014

I was taking LPhenylalanine 500Mg per day for about a month and since last week I started experiencing headaches and migraines, so I stopped taking anything to figure out the cause. So now I'd like to know how much would it take to lower my lPhenylalanine leves on my system, since I'm still having headaches.

By anon292345 — On Sep 19, 2012

This past summer I was in and out of the hospital for panic attacks, nausea and rapid weight loss. The doctors just loaded me with anti depressants, ulcer meds, and anxiety meds. All it did was mask what was still going on.

I'm 29 and I had never been this ill, so I dug into my own diet and found that a powder mix I used after I worked out as well as the Jello I was eating to fill my stomach contained phenylalanine.

After removing all aspartame and phenylalanine from my diet, my mood completely switched. I went from fearing getting into the car and weighing 95 pounds to confident at 108 pounds in two months after omitting those things from my diet.

This week I started feeling anxious and threw up at school today and had no idea why. Coincidentally, I looked at the pack of gum I was chewing this week and it has phenylalanine.

Can you develop an intolerance to this stuff? I have never had a problem with my nervous system or stress until this year. This stuff is poison!

By anon281961 — On Jul 26, 2012

@sherlock: As a person who has PKU, I know a vegetarian can still get adequate amounts of phe through all types of protein-soy, eggs, dairy, etc. Just for your information.

By anon151838 — On Feb 11, 2011

A week or so ago, a study was released to the major network media as to a 60 percent increase in heart problems directly associated with diet soft drinks. They did not elaborate as to the aspartame but I am sure that this was the cause they were alluding to.

By anon146004 — On Jan 25, 2011

You are correct. Most products with artificial sweeteners are no more dangerous than other products as long as they are consumed in moderation.

I have only but one concern with the issue. Most people I know who consume diet sodas or beverages with artificial sweeteners (field dominated by aspartame), consume large amounts of the products in a day. I know multiple people who consume more than a twelve pack of diet sodas a day. They may be addicted to it. Or maybe they just abuse the fact that it has no calories and ignore the hazards.

By sherlock87 — On Nov 07, 2010

While I admit that I consume beverages sweetened with aspartame and containing phenylalanine almost daily, I am a vegetarian and don't really get phenylalanine any other way. While I know that artificial sweetener side effects can be potentially dangerous, now I wonder if aspartame dangers could be canceled out by phenylalanine benefits, provided one does not suffer from PKU.

By widget2010 — On Nov 07, 2010

@sherlock87, While I'm not sure one cancels out the other, I agree, this also makes me wonder. Artificial sweeteners are one of those products which are often labeled as being highly dangerous or the cause of so many problems, yet they probably are not any more harmful than most other things, if consumed in reason.

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