Parkinson’s and protein — what’s the connection?
We know that a high protein diet can affect levodopa absorption. But what does this mean for people with Parkinson’s?
Parkinson’s develops when cells in the brain stop working properly and are lost over time. These brain cells produce a chemical called dopamine. Symptoms start to appear when the brain can’t make enough dopamine to control movement properly.
Levodopa is an amino acid first discovered in the 1960s. Today, it is still one of the main drugs used to treat Parkinson’s symptoms at all stages of the condition. It works by replacing dopamine lost in the brain.
Why not just take dopamine?
In theory, the simplest way to treat Parkinson’s would be to directly replace the dopamine lost via supplements. But, there’s a complication. Dopamine itself is unable to cross the blood-brain barrier — a natural barrier that protects the brain from bacteria and viruses that may be in the blood. This means that taking dopamine directly is ineffective as it can’t get from the blood into the brain.
The problem with the blood-brain barrier
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Fortunately, levodopa can cross the blood-brain barrier, into the brain, where it’s turned into dopamine.
An indirect path
But, when a tablet of levodopa is swallowed, the drug doesn’t make its way directly to the brain where it’s needed.
Instead, it travels along the digestive system, reaching the small intestine. And it’s here that levodopa enters the bloodstream, where it’s then carried around to the brain.
A competitive exit
Levodopa moves from the small intestine into the bloodstream via a process called absorption.
There are a series of transporters that line the small intestine, each designed to transport amino acids, like levodopa, from the gut into the cell lining, where they then pass through into the bloodstream.
This is a very effective mechanism for getting levodopa into the blood. The problem is that other amino acids are also queuing up to use the transporters to leave the gut. And this creates competition to get through the exit before the drug gets broken down and can no longer work.
Dietary protein and Parkinson’s
This known interaction between levodopa and amino acids in the gut means that people with Parkinson’s need to be careful about their protein consumption around the time of taking their medication.
When you have a large amount of protein, they get broken down in the stomach into amino acids. And once these amino acids enter the small intestine, they start competing with levodopa to use the transporter system out into the blood. This competition can cause a reduction in the amount of levodopa reaching the brain, in turn reducing the effectiveness of the dose.
An early study, from 1987, found that on a low protein diet, 11 people with Parkinson’s were more sensitive to the effects of levodopa, and saw reduced fluctuations in their symptoms throughout the day compared with a high protein diet. However, we now know that a low-protein diet is not advisable for people with Parkinson’s. So, what’s the answer?
A practical approach
We spoke with Shelley Jones, a consultant pharmacist at King’s College Hospital, about how people with Parkinson’s should manage their protein intake:
“The most important thing to say is that we don’t recommend people with Parkinson’s follow a protein-free diet. We need protein to keep our muscles strong — this is particularly important for people living with dyskinesia, where fatigue is common. The best thing to do is to follow a healthy, well-balanced diet”
So, what can people with Parkinson’s do to make sure they are eating healthily but that their medication is still effective?
“As with most things with Parkinson’s, it’s not a one size fits all,” Shelley says. “For some, eating a protein-rich meal at a separate time of the day to when they take their medication works well. For others, eating smaller portions of protein throughout the day works better.”
“It about finding what’s best for you. For example, if your symptoms are worse in the morning, avoid protein-rich food at this time. This may help to increase your body’s response to medication and avoid unpredictable motor fluctuations. You could then eat your main protein meal in the evening, as a slower response to medication may not be as important at this time.”
You can find more practical advice about eating protein on our website:
Why protein is important for people with Parkinson's
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What about other medications?
It’s only levodopa that competes with amino acids to be absorbed in the small intestine. Dopamine agonists (drugs that mimic dopamine in the brain) are taken up in a different way.
But food can impact on how we absorb any medications. And this is particularly true in people with Parkinson’s.
In Parkinson’s, the digestive tract can slow right down, causing constipation. And this slowness increases the risk of medication being broken down in the gut before it reaches the brain. Many people with Parkinson’s find that delaying having a large meal until an hour or two after taking their medication can help with drug absorption and improve their symptoms.
However, it’s worth bearing in mind that some Parkinson’s drugs (particularly co-beneldopa and co-careldopa) can cause sickness if taken on an empty stomach. Eating a low protein snack (such as a cracker, biscuit or slice of toast) when you take your dose may help to reduce this side effect.
If you have any concerns about your medication, are considering changing your diet, or are experiencing constipation, it’s important you speak with your health care professional, who will be able to help.