Obesity and diabetes pose significant global health challenges, driving the need for effective interventions to promote weight loss and improve metabolic health. Dietary modifications, including methionine restriction, have garnered attention for their potential to address these issues. This article explores the benefits and risks associated with low-methionine diets, drawing upon scientific research and expert insights.
Introduction to Methionine
Methionine is an essential amino acid, meaning it cannot be synthesized by the human body and must be obtained through dietary sources. It plays a crucial role in various bodily functions, including protein synthesis, DNA repair, and the production of essential molecules like glutathione and S-adenosylmethionine (SAM). Methionine is found in many protein-containing foods, with higher concentrations typically present in animal products such as meat, poultry, fish, and eggs.
Potential Benefits of Methionine Restriction
Metabolic Health Improvements
Research suggests that methionine restriction (MR) can promote leanness and improve metabolic health in both mice and humans. Studies have demonstrated that short-term methionine deprivation (MD) can preferentially reduce fat mass, restoring normal body weight and glycemic control in diet-induced obese mice. These benefits are not solely attributed to calorie restriction but rather result from increased energy expenditure.
One study found that MD increased energy expenditure in a sex-specific manner, inducing the fibroblast growth factor (Fgf)-21-uncoupling protein (Ucp)-1 axis only in males. This suggests that dietary methionine regulates metabolic health through previously unrecognized sexually dimorphic effects.
Vegan Diets and Methionine
Vegan diets, which are typically lower in methionine compared to diets rich in animal products, have been associated with improved metabolic health. Clinical trials have shown that vegan diets can promote weight loss, reduce plasma lipid levels, and improve glycemic control in humans. Vegans have reduced plasma levels of methionine compared to humans who eat animal proteins.
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Longevity and Anti-Aging Effects
Chronic dietary restriction of methionine has been shown to extend the lifespan of both mice and rats. In mice, MR has been shown to promote leanness, insulin sensitivity, and improved lipid metabolism. The mechanisms underlying these effects are complex and not yet fully understood but may involve increased levels of the hormone fibroblast growth factor (Fgf)-21, which promotes hepatic insulin sensitivity and increases energy expenditure.
Cancer Treatment and Prevention
Some research suggests that methionine restriction may enhance the effectiveness of cancer treatments like chemotherapy and radiation therapy. Cancer cells have a higher demand for methionine due to their rapid growth and division, making them more vulnerable to the effects of methionine restriction.
A study supported by funding from AICR is investigating how dietary methionine influences lung cancer progression and response to chemotherapy. Preliminary findings suggest that methionine restriction could increase the sensitivity of lung cancer cells to carboplatin, a type of chemotherapy commonly used to treat lung cancer.
Other Potential Benefits
Methionine restriction has also been linked to:
- Improved cognitive function
- Reduced oxidative stress
- Enhanced stress resistance
- Improved cellular reproduction
Risks and Considerations of Methionine Restriction
Essential Nutrient
Methionine is an essential amino acid necessary for building, strengthening, and repairing body tissues, and for making antibodies, hormones, enzymes and other compounds that are critical to every body process.
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Homocysteine Levels
Methionine can be converted into homocysteine, an amino acid associated with heart disease. High intakes of methionine may lead to an increase in homocysteine, although some individuals are more susceptible to this process than others.
Potential Side Effects
While methionine is generally considered safe, high doses can cause minor side effects such as dizziness, sleepiness, and changes in blood pressure. In rare cases, excessive methionine intake has been linked to more severe adverse events.
Nutritional Deficiencies
Severely restricting methionine can lead to protein deficiency and other nutritional imbalances. Ensuring sufficient protein intake is essential for maintaining muscle mass, bone health, and overall well-being.
Long-Term Adherence
Long-term adherence to a methionine-restricted diet can be challenging. It is crucial to work with a healthcare professional or registered dietitian to ensure adequate nutrient intake and minimize potential risks.
MTHFR Variants
MTHFR gene mutations can lead to elevated homocysteine.
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Depression
There is some evidence that a lack of methionine could reduce levels of S-Adenosylmethionine or SAM-e increasing risk of depression.
Homocystinuria
If methionine-restriction may help kill cancer cells and increase longevity, shouldn’t everyone be on a methionine-restricted diet? We don’t know for sure, but it is an option for those who are interested in employing a novel dietary strategy for disease risk reduction.
Senile Graying of Hair
A lack of methionine has also been linked to senile graying of hair.
Dietary Sources of Methionine
Methionine is found in varying amounts in different foods. Animal products, such as meat, poultry, fish, and eggs, tend to be higher in methionine compared to plant-based foods. Vegans, who consume a purely plant-based diet, typically have the lowest methionine intake.
The most concentrated methionine sources are animal products such as meat, poultry and fish.
Recommendations and Guidelines
The daily recommended intake of methionine plus cysteine is 1.09 mg/lb (2.4 mg/kg) per day for adults, which is around 163.5 mg for someone weighing 150 pounds (68 kilograms). Some researchers have recommended consuming double this amount based on the limitations of the studies used to set the recommended intake.
The RDA (recommended dietary allowance) for methionine + cysteine (adults 19 yrs+) is 19 mg/kg/day, while the EAR (estimated average requirement) is 15 mg/kg/day. People should not dip too much below these levels as they represent the lower end of what is needed for human health. For methionine alone, 15 mg/kg is thought to be a reasonable lower limit. So, if a therapeutic, methionine-restricted diet is indicated for you, multiply your healthy body weight by 15 to find a level of methionine intake that is appropriate.
Plant-Based Diets
For most people, simply eating a plant-based diet is likely sufficient for reducing methionine intake. The most concentrated methionine sources are animal products such as meat, poultry and fish.
Future Research and Clinical Trials
To further explore the potential benefits of methionine reduction in cancer treatment, more human studies are needed. However, conducting clinical trials on methionine-restricted diets has proven challenging, due to the lack of people willing to sign up to participate.
Researchers are actively working on developing ways to make methionine-restricted diets more accessible and feasible. This includes investigating the use of meal replacement shakes or prepackaged meals to simplify the implementation of a methionine-restricted diet. Additionally, researchers are exploring other avenues to reduce body methionine levels, such as the development of enzyme-based pills that can break down methionine.