The tuna diet is a short-term eating pattern in which you mainly eat tuna and water.

While it causes rapid weight loss, it is very restrictive and has several extreme downsides.

This article tells you everything you need to know about the tuna diet.

  • Overall score: 1.21
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BOTTOM LINE: The tuna diet promotes rapid weight loss at the expense of nutrient deficiencies, potential mercury poisoning, and severe calorie restriction.

The tuna diet is a low-calorie, low-carb, high-protein eating plan created by bodybuilder Dave Draper.

You’re meant to primarily consume water and tuna for three days.

Then, you can add low-fat dairy products, fruit, poultry, and vegetables for an unspecified period. During this phase, your macronutrient ratio should be 40% protein, 30% carbs, and 30% fat.

Although promoted as a way to break bad dietary habits and encourage quick weight loss, it’s a crash diet that isn’t supported by research.


The tuna diet is a low-calorie, high-protein diet that promotes rapid weight loss. However, it’s not supported by science.

To follow this diet, you should eat only tuna and water for three consecutive days.

The tuna must be plain — without oil, mayonnaise, vinegar, or spices — and enough to give you 1.5 grams of protein per kilogram of body weight (3.3 grams per lb.) each day.

You’re also supposed to drink 34 ounces (2 liters) of water daily, down a serving of Metamucil each night for fiber, and take vitamin, mineral, and branched-chain amino acid supplements.

After three days, you can add leafy green vegetables, steamed non-starchy vegetables, fruit, low-fat dairy products, and chicken.

Although there’s no set duration, you’re likely meant to follow it until you reach your target weight, then repeat it from time to time for rapid weight loss.

Other versions of the diet

While Draper’s plan is strict and regimented, various websites offer slightly different rules.

In fact, many of these adapted diets allow for additional foods, such as starchy vegetables, grains, unsweetened beverages like coffee and tea, and other protein sources like eggs.

Still, none of these plans are supported by scientific research.


The tuna diet allows only tuna and water for the first three days, then a few other foods — although some versions are slightly more flexible.

The tuna diet is an extremely restrictive plan that may cause rapid weight loss due to its low calorie count. Yet, diets that severely restrict calories can harm your health.

Notably, severe calorie restriction slows your metabolism and impairs muscle mass. Many studies show that regularly eating far less than your body needs reduces the number of calories your body burns at rest (1, 2, 3, 4, 5).

What’s more, severe calorie restriction can trigger severe hunger — and even result in weight gain after you go off your diet (3).

Overall, research suggests that very-low-calorie diets like the tuna diet are unsustainable and fail to improve body composition (4).


The tuna diet may result in rapid initial weight loss but, like many crash diets, is unsustainable, encourages severe calorie restriction, and may even lead to weight gain over time.

In moderation, tuna is a healthy, low-calorie protein source.

It’s rich in omega-3 fatty acids, which are essential nutrients that aid your heart, brain, and immune system (6).

Additionally, this fish is high in selenium, an essential micronutrient that provides anti-inflammatory and antioxidant effects, in addition to promoting thyroid function (7, 8).

Nonetheless, tuna doesn’t provide all the nutrients your body needs. As such, the risks of the tuna diet far outweigh its benefits.


Tuna is a healthy protein that can be part of a balanced diet. However, the tuna diet is far from balanced — as this fish is not meant to be your sole source of nutrients.

The tuna diet has several severe downsides, including its low calorie count, highly restrictive nature, and risk of mercury poisoning.

Fails to provide adequate calories

The tuna diet does not provide enough calories for most adults.

A 3-ounce (85-gram) can of tuna packed in water contains 73 calories, 16.5 grams of protein, 0.6 grams of fat, and 0 grams of carbs (9).

A 150-pound (68-kg) person would require 102 grams of protein daily on this diet, or 18.5 ounces (524 grams) of tuna per day (9).

This equals 610 calories daily — substantially below the 2,000 calories your body likely needs (10).

Such drastic calorie restriction may result in slower metabolism, loss of muscle mass, inadequate nutrient intake, and extreme hunger (1, 2, 3, 4).

Eating too much tuna can cause mercury poisoning

Although tuna is a healthy fish, it harbors the heavy metal mercury.

According to the Environmental Protection Agency (EPA), you should limit your intake of canned light or skipjack tuna to 12 ounces (340 grams) per week (11).

Other tuna varieties, such as albacore, yellowfin, and bigeye, have higher levels of mercury and should be eaten less or not at all.

Keep in mind that a 150-pound (68-kg) person on the tuna diet would eat 18.5 ounces (524 grams) of tuna per day — or a whopping 55.5 ounces (1.6 kg) over 3 days.

The maximum safe dose of mercury is 0.045 mcg of mercury per pound of body weight (0.1 mcg per kg), meaning that a 150-pound (68-kg) person can consume up to 6.8 mcg of mercury per day (12).

However, the tuna diet packs so much of this fish that you can easily exceed your mercury limits.

Even eating only light tuna, a 150-pound (68-kg) person would take in 68 mcg of mercury daily — 10 times the recommended amount.

Mercury poisoning is associated with severe damage to your heart, kidneys, immune system, and nervous system (13).

Highly restrictive and short-term

The tuna diet is very restrictive in its food choices and nutrients.

Its first phase is only meant to be followed for three days, which discourages the changes in habits or lifestyle necessary to achieve long-term weight loss (14).

In fact, studies show that it’s difficult to stick to fad diets like the tuna diet — and their long-term effects are questionable (15, 16).

Focusing on short-term weight loss is unsustainable and likely hampers long-term success.

Other downsides

Other potential downsides of the tuna diet include:

  • Lack of individualization. The tuna diet is not tailored to meet your nutrient needs. Rigid eating patterns fail to account for individual differences.
  • No scientific research. Notably, this diet is not backed by any studies.
  • Unsustainable. The diet is not realistic or safe to follow for any length of time due to its restrictions and high mercury content.

The weight loss effects of the tuna diet are likely unsustainable. What’s more, it fails to provide adequate nutrients and increases your risk of mercury poisoning.

The foods allowed on the three-day phase of the tuna diet are:

  • Protein: canned tuna in water or tuna steak
  • Water: 34 ounces (2 liters) of water per day

After the initial stage, you can add the following foods:

  • Vegetables: green leafy vegetables and steamed non-starchy vegetables
  • Fruits: fresh fruit, such as apples, pears, berries, and melons
  • Low-fat dairy: cottage cheese and yogurt
  • Protein: plain chicken either baked, grilled, or boiled

The only permitted food in the three-day tuna diet is tuna, although a few more foods are allowed after the initial phase.

The tuna diet is quite restrictive. Here are some of the many banned foods:

  • Grains and starches: rice, wheat, bulgar wheat, quinoa, millet, potatoes, corn, etc.
  • Meat: Beef, lamb, pork, etc.
  • Legumes: chickpeas, pinto beans, black beans, kidney beans, etc.
  • Nuts and seeds: almonds, peanuts, cashews, walnuts, sunflower seeds, etc.
  • Full-fat dairy products: whole milk, butter, cheese, ice cream, etc.
  • Soda and other sugary beverages: soda, energy drinks, sports drinks, etc.

The tuna diet eliminates all foods except tuna in its first phase, meaning that it lacks several important nutrients and food groups required for optimal health.

While the tuna diet offers rapid weight loss, it’s not a sustainable, long-term solution.

In fact, it poses several risks, including slowed metabolism, loss of muscle mass, and mercury poisoning.

For lasting results, the best option is to follow a balanced meal plan with sufficient calories to meet your needs. Eat plenty of whole, unprocessed foods, and consider diet and lifestyle changes to support your weight goals.