Finding yourself confused by the seemingly endless promotion of weight-loss strategies and diet plans? In this series, we take a look at some popular diets—and review the research behind them.
What Is It?
The Paleolithic or “Paleo” diet seeks to address 21st century ills by revisiting the way humans ate during the Paleolithic era more than 2 million years ago. Paleo proponents state that because our genetics and anatomy have changed very little since the Stone Age, we should eat foods available during that time to promote good health. Our predecessors used simple stone tools that were not advanced enough to grow and cultivate plants, so they hunted, fished, and gathered wild plants for food. If they lived long enough, they were believed to experience less modern-day diseases like diabetes, cancer, and heart disease because of a consistent diet of lean meats and plant foods along with a high level of physical activity from intensive hunting. However, the life expectancy of our predecessors was only a fraction of that of people today.
The popularity of the Paleo diet, which hit a peak in 2014, appealed to consumers’ increasing desire to eat more healthfully and to know where their food was coming from. 
How It Works
The Paleo diet, also referred to as the caveman or Stone-Age diet, includes lean meats, fish, fruits, vegetables, nuts, and seeds. Proponents of the diet emphasize choosing low-glycemic fruits and vegetables. There is debate about several aspects of the Paleo diet: what foods actually existed at the time, the variation in diets depending on region (e.g., tropical vs. Arctic), how modern-day fruits and vegetables bear little resemblance to prehistoric wild versions, and disagreement among Paleo diet enthusiasts on what is included/excluded from the diet. Because of these differences, there is not one “true” Paleo diet.
For example, although white potatoes were recorded as being available during the Paleolithic era, they are usually avoided on the Paleo diet because of their high glycemic index. Processed foods are also technically off limits due to an emphasis on fresh foods, but some Paleo diets allow frozen fruits and vegetables because the freezing process preserves most nutrients.
Overall, the diet is high in protein, moderate in fat (mainly from unsaturated fats), low-moderate in carbohydrate (specifically restricting high glycemic index carbohydrates), high in fiber, and low in sodium and refined sugars.  The monounsaturated and polyunsaturated fats (including the omega-3 fats EPA and DHA) come from marine fish, avocado, olive oil, and nuts and seeds.
Grass-fed beef is often highlighted on the diet, which is promoted to contain more omega-3 fats than conventional beef (due to being fed grass instead of grain). It does contain small amounts of alpha-linolenic acid (ALA), a precursor to EPA and DHA. However, only a small proportion of ALA can be converted in the body to long-chain omega-3 fatty acids (EPA and DHA). The amount of omega-3 is also highly variable depending on the exact feeding regimen and differences in fat metabolism among cattle breeds.  In general, the amount of omega-3 in grass-fed beef is much lower than that in oily marine fish.  Cooked salmon contains 1000-2000 mg of EPA/DHA per 3-ounce portion, whereas 3 ounces of grass-fed beef contains about 20-200 mg of ALA.
The following is a summary of foods generally permitted on the diet:
- Allowed: Fresh lean meats, fish, shellfish, eggs,nuts,seeds, fruits, vegetables, olive oil, coconut oil, and small amounts of honey. Certain root vegetables like sweet potatoes and cassava may be allowed in moderation because of their high nutrient content.
- Not Allowed: Whole grains, cereals, refined grains and sugars, dairy products, beans,white potatoes, legumes (peanuts, beans, lentils), alcohol, coffee, salt,refined vegetable oils such as canola, and most processed foods in general.
- Calorie counting and portion sizes are not emphasized. Some plans allow a few “cheat” non-Paleo meals a week, especially when first starting the diet, to improve overall compliance.
The Research So Far
Some randomized controlled trials have shown the Paleo diet to produce greater short-term benefits than diets based on national nutrition guidelines, including greater weight loss, reduced waist circumference, decreased blood pressure, increased insulin sensitivity, and improved cholesterol. However these studies were of short duration (6 months or less) with a small number of participants (less than 40). [4-6]
One larger randomized controlled trial followed 70 post-menopausal Swedish women with obesity for two years, who were placed on either a Paleo diet or a Nordic Nutrition Recommendations (NNR) diet.  The Paleo diet provided 30% of total calories from protein, 40% fat (from mostly monounsaturated and polyunsaturated fats) and 30% carbohydrates. It included lean meats, fish, eggs, vegetables, fruits, berries, nuts, avocado, and olive oil. The NNR diet provided less protein and fat but more carbohydrate with 15% protein, 25-30% fat, and 55-60% carbohydrates, including foods similar to the Paleo diet but also low-fat dairy products and high-fiber grains. Both groups significantly decreased fat mass and weight circumference at 6 and 24 months, with the Paleo diet producing greater fat loss at 6 months but not at 24 months. Triglyceride levels decreased more significantly with the Paleo diet at 6 and 24 months than the NNR diet.
- Meal planning. Because the diet relies heavily on fresh foods, expect a time commitment to plan, purchase, prepare,and cook meals. This may be challenging for busy lifestyles or for those less experienced with cooking.
- Higher cost. Fresh meats, fish, and produce tend to be pricier than processed versions such as frozen or canned.
- Excluding foods. The exclusion of entire categories of commonly eaten foods like whole grains and dairy requires frequent label reading in the supermarket and in restaurants. It may also increase the risk of deficiencies such as calcium,vitamin D, and B vitamins, if these nutrients are not consistently eaten from the allowed foods or a vitamin supplement. For example, there are some nondairy calcium-rich foods that are absorbed well by the body such as collard and turnip greens or canned bone-in sardines and salmon, but you would have to eat five or more servings of these greens and fish bones daily to meet recommended calcium needs. (Note that some greens like spinach that are touted to be calcium-rich also contain oxalates and phytates that bind to calcium so very little is actually absorbed.) One small, short-term intervention study of healthy participants showed a 53% decrease from baseline in calcium intake after following a Paleo diet for three weeks.  Furthermore, the exclusion of whole grains can result in reduced consumption of beneficial nutrients such as fiber and thus may increase one’s risk for diabetes and heart disease.
- Health concerns of a high meat intake.Several studies have shown that a high intake of red meat is linked to a higher risk of death, cardiovascular disease, and diabetes.
- Is there potential for nutrient deficiencies, such as calcium and vitamin D, when following this diet for longer than one year that may make it inappropriate for certain at-risk groups(e.g., those with existing or at high risk of osteopenia or osteoporosis)?
- Are there long-term negative side effects of omitting entire food groups, especially if the diet is not carefully constructed to include the nutrients from the omitted foods?
- Is this diet safe and beneficial for everyone (e.g., generally healthy population, higher risk individuals with chronic diseases, elderly)?
The Paleo diet includes nutrient-dense whole fresh foods and encourages participants to steer away from highly processed foods containing added salt, sugar, and unhealthy fats. However, the omission of whole grains, dairy, and legumes could lead to suboptimal intake of important nutrients. The restrictive nature of the diet may also make it difficult for people to adhere to such a diet in the long run. More high-quality studies including randomized controlled trials with follow-up of greater than one year that compare the Paleo diet with other weight-reducing diets are needed to show a direct health benefit of the Paleo diet. Strong recommendations for the Paleo diet for weight loss cannot be made at this time.
The Source https://www.hsph.harvard.edu/nutritionsource/healthy-weight/diet-reviews/paleo-diet/