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How Undereating Can Affect Your Thyroid

Your thyroid gland needs nutrients to function properly. Find out how undereating can affect your thyroid health.
How Undereating Can Affect Your Thyroid
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Have you wondered if cutting back significantly on what you eat could help you drop a few pounds or more? Undereating is a popular way for many people to try shedding excess weight. But it’s not likely to yield the best results and can even lead to more weight gain in the long run while affecting your thyroid gland and overall health.

Your thyroid is sensitive to changes in your body, including diet, pregnancy, menopause, and toxic exposure. One factor that affects your thyroid -- as well as your overall health -- is eating too little.  In this article, we look at the link between your diet and your thyroid, exploring the effects undereating can have on your health.

What does it mean to undereat?

Undereating means eating fewer calories than your body needs to function properly. Everyone has an optimum amount to eat so their body has enough nutrients and energy for all its processes. Your daily caloric needs vary according to your age, weight, and sex. You will also need more calories if you are pregnant, breastfeeding, or engaged in rigorous activities.

On average, men need between 2600 to 2800 kcal per day, while women need about 2000 to 2200 kcal per day. Undereating is often called “dieting, “meaning your body will take in fewer calories than before. If you go too low on calories, your body will get less nutrients and energy and begin to burn energy stores, leading to weight loss. Undereating for a short period won’t have any health effects on people at a healthy weight. But when you reduce your calories drastically over a longer timeframe, it can have adverse effects.

In the early days of undereating, you may experience episodes of low blood sugar, also known as hypoglycemia). Low blood sugar can show up with symptoms that include: 

  • Headaches
  • Shakiness
  • Extreme hunger
  • Irritability
  • Nausea
  • Dizziness
  • Tiredness
  • Concentration problems
  • Clammy skin
  • Feeling overheated and breaking out in a sweat
  • Fast heartbeat

Fortunately, those symptoms can be quickly resolved if you eat a sugary snack.

Over time, undereating can lead to malnutrition. Depending on how low your calories and nutrients are, you can begin to experience symptoms of malnutrition, which may include

  • Hair loss
  • Frequent infections
  • Easy bruising
  • Sores that won’t heal
  • Inability to stay warm
  • Irregular or no menstrual periods
  • Loss of muscle tone and strength
  • An unhealthy appearance

How does undereating affect weight loss?

Most people undereat to lose weight. But undereating for a long time can impact your weight the wrong way. Here are a few ways undereating can affect weight loss:

  1. Initial Weight Loss: Undereating can lead to initial weight loss because you consume fewer calories than your body needs, creating a calorie deficit. This deficit causes you to lose glycogen from your liver and some muscle mass. Eventually, you will begin to burn stored fat for energy, resulting in weight loss.
  2. Metabolic Rate Reduction: However, prolonged undereating can cause your metabolism to slow down. When your body doesn’t get enough calories, it goes into “starvation mode” to conserve energy, slowing down metabolism to preserve energy stores. When you’re in starvation mode, hormones that control your appetite, such as leptin and ghrelin, are disrupted. The result is a slower metabolism than usual, and your body burns fewer and fewer calories. With fewer calories burned, you will find it harder to lose weight over time. 
  3. Loss of Lean Muscle Mass: Severely restricting calories can lead to the loss of not just fat but also lean muscle mass. This is detrimental because muscle helps burn calories even at rest, so losing muscle can further slow metabolism and make weight loss more challenging.
  4. Nutritional Deficiencies: Undereating can also lead to nutritional deficiencies if you’re not getting enough essential nutrients from your limited food. This can affect your overall health and well-being and may even hinder weight loss, as certain nutrients are necessary for proper metabolic function.
  5. Fatigue and Weakness: Not consuming enough calories can leave you feeling tired, weak, and lethargic, which can make it harder to engage in physical activity and exercise, further impacting weight loss efforts.
  6. Rebound binging: Some people who chronically overeat end up binge eating and fill up on high-calorie foods that are high in sugar and fat. This can quickly reverse whatever weight you lost by undereating.

How does undereating affect your thyroid?

Your thyroid needs nutrients to function properly, and the only way to get all the right nutrients is to have a sufficient and healthy diet. Here are some ways that undereating can affect your thyroid. 

Slowed metabolism

One of the primary ways undereating affects the thyroid is by slowing down metabolism. When you consume fewer calories than your body needs to function, your thyroid may go into a state of hypothyroidism, producing fewer thyroid hormones. This can result in a slower metabolic rate, making losing weight and maintaining energy levels even harder.

Nutrient deficiencies

Your thyroid needs macronutrients such as carbohydrates, proteins, and fats, as well as micronutrients such as vitamins. The macronutrients supply energy and materials for repair, while the micronutrients help your body’s enzymes, hormones, and cells function.

Undereating can lead to nutrient deficiencies, which are crucial for thyroid health. For example, iodine is an essential nutrient needed to produce thyroid hormones. Without enough iodine in your diet, your thyroid may not be able to produce enough hormones, leading to hypothyroidism. Additionally, undereating can result in deficiencies in essential nutrients like selenium, which plays a role in making thyroid hormones and controlling their function. Along with other nutrients, your thyroid needs vitamin A, zinc, and iron to function optimally.

Stress response and increased cortisol

Chronic stress can disrupt the balance of thyroid hormones in the body, leading to thyroid dysfunction. When you are undereating, your body is under stress, and more cortisol, a stress hormone that can interfere with thyroid hormone production and function, is produced. Increased cortisol can contribute to weight gain in several ways. 

  1. Metabolism: Cortisol can increase glucose production, leading to higher blood sugar levels. This can result in insulin resistance and metabolic dysfunction, which may hinder weight loss efforts.
  2. Appetite: Cortisol can stimulate appetite, particularly for foods high in sugar and fat. This can lead to overeating and difficulty maintaining a calorie deficit, which is necessary for weight loss.
  3. Fat storage: Cortisol promotes the storage of fat, especially visceral fat, around the abdomen. High levels of cortisol can lead to an increase in abdominal obesity, which is associated with a higher risk of metabolic diseases.
  4. Muscle loss: Prolonged exposure to high cortisol levels can lead to muscle breakdown. Since muscle mass plays a crucial role in maintaining a high metabolism, muscle loss can make it harder to lose weight and easier to regain weight.
  5. Stress eating: Stress, which often accompanies high cortisol levels, can trigger emotional eating and cravings for comfort foods, typically high in calories and low in nutritional value.
  6. Sleep disturbances: Elevated cortisol levels can disrupt sleep patterns, leading to poor sleep quality or insomnia. Lack of adequate sleep is associated with weight gain and difficulty losing weight.

Managing stress levels through regular exercise, relaxation techniques, adequate sleep, and a balanced diet can help mitigate the effects of cortisol on weight loss.

Impaired hormone production and absorption

Undereating can affect thyroid hormone production in several ways.

  • Undereating can decrease the conversion of inactive T4 to T3, the active thyroid hormone. Studies show that even three weeks of calorie restriction can substantially reduce the conversion of the inactive thyroid hormone T4 into the active T3 hormone1. This disrupts the normal thyroid hormone balance.
  • Undereating can also increase levels of reverse T3 (RT3), an inactive form of thyroid hormone that competes with T3 for binding sites, thus blocking the effects of T3.
  • Chronic calorie restriction and undereating can decrease the overall production of thyroid hormones T4 and T3, leading to hypothyroidism.

Diets low in fat, carbohydrates, or protein can also affect your thyroid function.

  • Diets that are low in carbohydrates (also called keto or low-carb diets) have been found to reduce the amount of thyroid hormone T3 in your body.
  • Low-protein diets can suppress your thyroid function and cause you to experience symptoms of hypothyroidism, such as weight gain, anxiety, brain fog, cold hands and feet, and depression.
  • Finally, diets very low in fat can potentially affect thyroid function because fat plays a crucial role in hormone production and absorption, including thyroid hormones. Thyroid hormones rely on sufficient fat intake for their synthesis. When fat intake is severely restricted, it can disrupt the body’s ability to produce and regulate thyroid hormones. Additionally, fat-soluble vitamins like D, E, and K are essential for thyroid health and require fat for absorption. An inability to absorb these vitamins due to low-fat diets can further impact thyroid function.

How can you lose weight without hurting your thyroid?

Rather than undereating, you can lose weight without hurting your thyroid if you embrace a holistic weight loss plan that includes proper diet, physical activity, good sleep, and possibly professional support. Here are some tips to help you lose weight safely.

  • Proper diet: You will need to follow a healthy diet that supplies all the nutrients you need and still allows you to burn fat stores. Generally, your diet should be rich in proteins, fruits, vegetables, and fiber. You can also consider supplements to make sure you get enough micronutrients. Working with professionals to create a personalized diet plan is a good idea so you know what to eat and avoid.
  • Physical activity: Moving your body and weight training have numerous benefits for your health, mood, and longevity. They can also help you burn excess calories and lose weight. You can start gradually and build up to a recommended 130 to 300 minutes of vigorous weekly exercise. Great activities to begin with are walks, cycling, and swimming.
  • Good sleep: Sleep allows your body to rest and restore itself. Research shows that getting enough sleep is linked with eating less and better weight control. You also experience less stress and better moods. Consider committing to healthy sleep, beginning with a schedule of at least 7 to 9 hours daily. A dark, cool, quiet room and a sleeping routine can also help you get enough sleep.
  • Manage your stress: A normal stress level is healthy and necessary for you to be productive. But research shows that if your stress levels are persistently high, that can lead to unwanted weight gain. You can lower your stress levels by practicing self-care, meditation, yoga, being in nature, and spending time with pets and loved ones.
  • Professional help: It’s easier to lose weight with help. Trained healthcare providers can support your efforts to lose weight, helping you meet your weight goals faster and safer. A nutritionist can help you create a meal plan that works. And a thyroid-savvy physician can help check if your thyroid is functioning properly. You can also engage a physical trainer to help you get the right amount of exercise each week.

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A note from Paloma Health

When you have some weight to lose, undereating can appear to be an easy way to do so. But it’s unlikely to work well and can cause more harm than good.

To lose weight safely and effectively, as a Paloma member, you’ll have access to Paloma’s experienced team of thyroid care experts to get a thyroid treatment and weight loss plan tailored to your needs. In addition to virtual visits with top thyroid doctors, nutritionists, and health coaches, you’ll also have access to specialized diet help, recipes, a library of helpful resources, and a supportive community of fellow thyroid patients. If you have concerns about your thyroid health, Paloma’s convenient, at-home testing kit allows you to test your crucial thyroid biomarkers and evaluate your thyroid function. Once you get your results, our providers will be available to help you with the next steps.


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