L-carnitine and your thyroid

A Health24 forum user has alerted me to the fact that L-carnitine, which is so often used as a slimming and exercise supplement, can interfere with thyroid hormone function and cause weight gain, instead of weight loss, in people with thyroid problems.

The user in question, who had her thyroid gland removed a few years ago and who were taking thyroid hormone replacement therapy, took L-carnitine to “boost metabolism and burn fat”, and help her to lose weight.

She was understandably upset when, instead of losing weight, she actually gained 5kg – despite the fact that she was following a low-fat, low-energy diet. Fortunately, she came across an article that explained how L-carnitine can affect thyroid hormone function and stopped taking the supplement.

Fascinated by her story, I did a literature survey on this subject and found some interesting information about the effect L-carnitine has on thyroid hormone function, potential weight gain instead of loss, and what nutrition and sports experts think about the use of L-carnitine for slimming and performance-enhancing purposes.

What is L-carnitine?
L-carnitine is an amine that's produced in the human body by the kidneys and the liver. It's found in biological fluids and most tissues. The muscle tissues store approximately 95% of the total of 20g of L-carnitine found in the body.

Diets that contain protein foods such as meat, fish, eggs, milk and dairy products should be able to satisfy the body’s daily requirement for L-carnitine (Benvenga, 2005).

What does L-carnitine do?
According to Heinonen (1996), carnitine plays a central role in fatty-acid metabolism. It transports long-chain fatty acids into mitochondria for beta-oxidation. Carnitine also modulates the metabolism of coenzyme-A.

It's therefore not surprising that the use of L-carnitine supplements to boost exercise performance and weight loss is increasing exponentially. However, the majority of review articles warn that most of the claims regarding L-carnitine haven't been substantiated.

a) Sportsmen and women
Research shows that carnitine supplementation:

  • Doesn't increase free fatty acid oxidation
  • Doesn't improve glycogen utilisation
  • Doesn't delay fatigue
  • Doesn't reduce buildup of lactate in the muscles
  • Doesn't increase maximum oxygen uptake (VO2 max)
(Heinonen, 1996)

Although there's some evidence that L-carnitine supplementation may enhance athletic performance when combined with intensive physical exercise, there's no evidence that using L-carnitine if you're a healthy, but otherwise sedentary person, will benefit you in any way.

b) Slimmers
Research shows that carnitine supplementation:

  • Doesn't reduce body fat
  • Doesn't assist in weight loss
(Heinonen, 1996; Brass, 2004; Benvenga, 2005)

L-carnitine and thyroid function
In addition to the findings that topping up your carnitine intake with supplements won't really help you lose weight, there's the danger that anyone who has an underactive thyroid may run into problems when taking carnitine to shed weight.

L-carnitine is actually used to counteract hyperthyroidism (overactive thyroid) by blocking the effects of thyroid hormones. It prevents thyroid hormones (T4) and (T3) from being taken up in body cells.

In a person with an overactive thyroid gland, who produces such large quantities of these thyroid hormones that he/she develops a variety of symptoms (including palpitations, increased body temperature, shakiness, irritability, insomnia and weight loss), the inhibiting effects of L-carnitine can be useful (Benvenga, 2005).

However, in patients who suffer from hypothyroidism or who have had their thyroid glands removed and are using thyroid hormone replacement therapy, having their thyroid hormone activity blocked or inhibited by L-carnitine is the last thing they need.

Consequently, anyone with an underactive thyroid shouldn't take L-carnitine for weight loss or any other purpose.

Avoid L-carnitine if you have thyroid problems
Ironically, people who suffer from hypothyroidism tend to gain weight and often start using a variety of slimming pills and products to help with weight loss. If you're tempted to use such slimming pills, do check on the labels to see if they contain L-carnitine. If they do, don’t take them because you may gain weight instead of losing it.

If you have an underactive thyroid and are taking thyroid hormone supplements, you should stick to taking your thyroid hormones as prescribed by your doctor. You should also try to manage your weight with the aid of a balanced, energy-reduced diet and daily exercise.

Exercise is particularly important, because by doing 30 or more minutes of physical exercise every day, you'll help to stimulate your sluggish metabolism and promote weight loss.

Athletes who use L-carnitine should have their thyroid function checked to ensure that if they happen to have a thyroid problem, they don't gain undesirable amounts of weight when using this supplement.

Slimmers who don't have an underactive thyroid should also keep in mind that there is as yet no hard proof that taking L-carnitine supplements will make any difference to your weight loss. Rather use the money you intend spending on these supplements to join a good gym.

(Dr I.V. van Heerden, DietDoc, July 2009)

Any questions? Ask DietDoc

(Benvenga S, 2005. Effects of L-carnitine on thyroid hormone metabolism & on physical exercise tolerance. Horm Metab Res, 37(9):566-71; Brass EP, 2004. Carnitine & sports medicine: use or abuse? Ann N Y Acad Sci, 1033:67-78; Heinonen OJ, 1996. Carnitine & physical exercise. Sports Med, 22(2):109-32.)

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