A paleo diet is an eating plan based on foods humans might have eaten during the Paleolithic Era. The Paleolithic Era dates from around 2.5 million to 10,000 years ago.

A modern paleo diet includes fruits, vegetables, lean meats, fish, eggs, nuts and seeds. These are foods that in the past people could get by hunting and gathering. It doesn't include foods that became more common when small-scale farming began about 10,000 years ago. These foods include grains, legumes and dairy products.

Other names for a paleo diet include Paleolithic diet, Stone Age diet, hunter-gatherer diet and cave man diet.

The purpose of a paleo diet is to eat foods likely eaten by early humans. The diet is based on the idea that our genes are not well adjusted for modern diets that grew out of farming.

Farming made foods such as grains and legumes more easily available. And it introduced dairy. Also, farming changed the diets of animals that people ate. The paleo diet idea is that these changes in diet outpaced the human body's ability to change, or adapt. This mismatch is believed to contribute to obesity, diabetes and heart disease today.

Why you might follow a paleo diet

You might choose to follow a paleo diet because you want to:

• Lose weight or keep a healthy weight
• Reduce heart disease, or cardiovascular, risk factors

Recommendations vary among paleo diets promoted in books and online. In general, paleo diets follow these rules.

What to eat

• Fruits
• Vegetables
• Nuts and seeds
• Eggs
• Lean meats, especially grass-fed animals or wild game
• Fish, especially those rich in omega-3 fatty acids, such as salmon, mackerel and albacore tuna
• Oils from fruits and nuts, such as olive oil or walnut oil

What to avoid

• Grains, such as wheat, oats and barley
• Legumes, such as beans, lentils, peanuts
• Dairy products, such as milk and cheese
• Refined and added sugar
• Added salt
• Starchy vegetables, such as corn, jicama, peas and white potatoes
• Highly processed foods, such as chips or cookies

A typical day's menu

Here's a look at what you might eat during a typical day following a paleo diet:

• Breakfast. Broiled salmon and cantaloupe.
• Lunch. Salad made with romaine, carrot, cucumber, tomatoes, avocado, walnuts and lemon juice dressing.
• Dinner. Lean beef sirloin tip roast; steamed broccoli; salad made with mixed greens, tomatoes, avocado, onions, almonds and lemon juice dressing; and strawberries for dessert.
• Snacks. An orange, carrot sticks or celery sticks.

In general, a paleo diet has many features of recommended healthy diets. Common features the paleo diet has include the emphasis on fruits, vegetables, lean meats and the avoidance of processed foods. But there is limited research on paleo diets compared with studies of balanced diets with more varied food groups.

Most studies of paleo diets included small numbers of people. Also, they only lasted from a few weeks to a few months. The definitions of the diet also vary from one study to another. So it's hard to say for sure what people can expect, especially over time.

In general, short-term, small studies suggest a paleo diet might help manage:

• Weight loss
• Blood pressure
• Cholesterol
• Triglycerides

One large study looked at the benefits of self-reported, long-term dietary patterns in young adults from Spain. The researchers found that the paleo diet was linked to lower heart disease, or cardiovascular, risk factors. The lower risk mostly came from avoiding highly processed foods, such as chips and candy, and eating lots of fruits and vegetables.

Longer trials with large groups of people randomly assigned to different diets are needed to understand the long-term, overall health benefits and possible risks of a paleo diet.

Some people doubt the idea that the human body didn't change, or adapt, to foods that came with farming. Some people are also worried about the foods the paleo diet cuts out.

Concerns about nutrition

The main concern about paleo diets is the lack of whole grains and legumes. These foods are considered good sources of fiber, vitamins, proteins and other nutrients. Also, low-fat dairy products are good sources of protein, calcium, vitamins and other nutrients. The potential risk of eating a paleo diet is that you may not get all recommended nutrients.

Whole grains, legumes and dairy also are generally more affordable and available than foods such as wild game, grass-fed animals and nuts. For some people, a paleo diet may be too costly. Or the cost of some paleo foods may lead to unintentionally getting less of certain essential nutrients.

The long-term risks of a paleo diet aren't known. Data from many studies of popular diets showed that a Mediterranean diet was the only one with many benefits without the risk of possible harmful effects. A Mediterranean diet includes fruits, vegetables, lean meats, fish, whole grains, legumes and low-fat dairy products.

Also, the potential benefits of a paleo diet may not outweigh the benefits of other healthy diets. One long-term study of self-reported diet patterns showed that closely following either a paleo diet or a Mediterranean diet led to similar drops in cardiovascular risk factors.

Questions about the paleo diet theory

Some experts have argued that the idea the paleo diet is based on isn't the full story. Arguments for a more complex understanding of how our dietary, or nutritional, needs have changed include:

• Many things — not only farming — shaped how human nutritional needs changed. Diets in early humans were varied because of differences in geography, climate and the availability of food.
• Archaeological researchers have found tools for grinding grains at 30,000-year-old sites — well before the introduction of farming. Researchers also have studied microfossils of plants found in the dental remains of Paleolithic humans and Neanderthals. These studies have shown that their diets included wild grains.
• Genetic research has shown that important evolutionary changes continued after the Paleolithic era. These include changes in the expression of genes related to the breakdown, or digestion, of starches in grains and lactose in milk.

A paleo diet may help you lose weight or keep a healthy weight. It also may have other helpful health effects. But there are no long-term clinical studies about the benefits and potential risks of the diet.

You might be able to achieve the same health benefits by getting enough exercise and eating a balanced, healthy diet. Be sure to eat a lot of fruits and vegetables as part of a healthy diet.