Types

A restricted diet is more often pursued by those wanting to lose weight. Some people follow a diet to gain weight (usually in the form of muscle). Diets can also be used to maintain a stable body weight and improve health.

Low-fatedit

Low-fat diets involve the reduction of the percentage of fat in one's diet. Calorie consumption is reduced because less fat is consumed. Diets of this type include NCEP Step I and II. A meta-analysis of 16 trials of 2–12 months' duration found that low-fat diets (without intentional restriction of caloric intake) resulted in average weight loss of 3.2 kg (7.1 lb) over habitual eating.

A low-fat, plant-based diet has been found to improve control of weight, blood sugar levels, and cardiovascular health.

Low-carbohydrateedit

Low-carbohydrate diets are relatively high in protein and fats. Low-carbohydrate diets are sometimes ketogenic (i.e., they restrict carbohydrate intake sufficiently to cause ketosis).

"The glycemic index (GI) factor is a ranking of foods based on their overall effect on blood sugar levels. The diet based around this research is called the Low GI diet. Low glycemic index foods, such as lentils, provide a slower, more consistent source of glucose to the bloodstream, thereby stimulating less insulin release than high glycemic index foods, such as white bread."

A randomized controlled trial comparing four diets concluded that the high-carbohydrate, low-glycemic index diet was the most favorable as it led to both high weight loss and a decline in low density lipoprotein.

The "glycemic load" is the glycemic index multiplied by the amount of carbohydrate. A meta-analysis by the Cochrane Collaboration concluded that low glycemic index or low glycemic load diets led to more weight loss and better lipid profiles but did not separate the effects of the load versus the index.

Low-calorieedit

Low-calorie diets usually produce an energy deficit of 500–1,000 calories per day, which can result in a 0.5 to 1 kilogram (1.1 to 2.2 pounds) weight loss per week. One of the most commonly used low-calorie diets is Weight Watchers. The National Institutes of Health reviewed 34 randomized controlled trials to determine the effectiveness of low-calorie diets. They found that these diets lowered total body mass by 8% in the short term, over 3–12 months. Women doing low-calorie diets should have at least 1,000 calories per day and men should have approximately 1,200 calories per day. These caloric intake values vary depending on additional factors, such as age and weight.

Very low-calorieedit

Very low calorie diets provide 200–800 calories per day, maintaining protein intake but limiting calories from both fat and carbohydrates. They subject the body to starvation and produce an average loss of 1.5–2.5 kg (3.3–5.5 lb) per week. "2-4-6-8", a popular diet of this variety, follows a four-day cycle in which only 200 calories are consumed the first day, 400 the second day, 600 the third day, 800 the fourth day, and then totally fasting, after which the cycle repeats. These is some evidence that these diets results in considerable weight loss. These diets are not recommended for general use and should be reserved for the management of obesity as they are associated with adverse side effects such as loss of lean muscle mass, increased risks of gout, and electrolyte imbalances. People attempting these diets must be monitored closely by a physician to prevent complications.

The concept of crash dieting is to drastically reduce calories, using a very-low-calorie diet. Crash dieting can be highly dangerous because it can cause various kind of issues for the human body. Crash dieting can produce weight loss but without professional supervision all along, the extreme reduction in calories and potential unbalance in the diet's composition can lead to detrimental effects, including sudden death.

Fastingedit

Fasting is when there is a long time interval between the meals. In dieting, long term (periodic) fasting is not recommended, instead, having small portions of food after small intervals is encouraged. Lengthy fasting can also be dangerous due to the risk of malnutrition and should be carried out only under medical supervision. During prolonged fasting or very low calorie diets the reduction of blood glucose, the preferred energy source of the brain, causes the body to deplete its glycogen stores. Once glycogen is depleted the body begins to fuel the brain using ketones, while also metabolizing body protein (including but not limited to skeletal muscle) to be used to synthesize sugars for use as energy by the rest of the body. Most experts believe that a prolonged fast can lead to muscle wasting, although some dispute this. The use of short-term fasting, or various forms of intermittent fasting, have been used as a form of dieting to circumvent the issues of long fasting.

Detoxedit

Detox diets are promoted with unsubstantiated claims that they can eliminate "toxins" from the human body. Many of these diets use herbs or celery and other juicy low-calorie vegetables.

Environmentally sustainableedit

Another kind of diet focuses not on the dieter's health effects, but on its environment. The One Blue Dot plan of the BDA offers recommendations towards reducing diets' environmental impacts, by:

1. Reducing meat to 70g per person per day.
2. Prioritising plant proteins.
3. Promoting fish from sustainable sources.
4. Moderate dairy consumption.
5. Focusing on wholegrain starchy foods.
6. Promoting seasonal locally sourced fruits and vegetables.
7. Reducing high fat, sugar and salty foods overconsumption.
8. Promoting tap water and unsweetened tea/coffee as the de facto choice for healthy hydration.
9. Reducing food waste.