Dietary fat plays a crucial role in human health, providing lipids and energy necessary for various bodily functions. However, the impact of high-fat diets (HFDs) on health is a complex issue, with varying effects depending on the type of fat consumed. This article delves into the different kinds of high-fat diets, their effects on the body, and the importance of choosing healthy fats.
Introduction to Dietary Fats
Dietary fat is an essential macronutrient that the body needs. A balanced diet should include healthful monounsaturated and polyunsaturated fats. Fat is a source of essential fatty acids, which the body cannot make itself. Fat helps the body absorb vitamin A, vitamin D and vitamin E. These vitamins are fat-soluble, which means they can only be absorbed with the help of fats.
The most typical HFD contained 45-60% of dietary fat as fat source. The fats previously used in the HFD researches include soybean oil, corn oil, canola oil, palm oil, lard, etc., and it was widely reported that the gut microbiome is a critical mediator between various diseases and HFD.
Understanding Different Types of Fats
There are primarily two main categories of dietary fats: saturated and unsaturated.
Saturated Fats
Saturated fats are typically solid at room temperature and are often found in animal products. Saturated fat tends to raise levels of cholesterol in the blood. Low-density lipoprotein (LDL) is called "bad" cholesterol. High-density lipoprotein (HDL) is called "good" cholesterol. The American Heart Association recommends people limit their daily intake of saturated fat to 5% to 6% of total daily calories.
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Unsaturated Fats
Unsaturated fats, on the other hand, are usually liquid at room temperature and are prevalent in vegetable oils, nuts, and fish. There are two main types of unsaturated fats: monounsaturated and polyunsaturated.
- Monounsaturated Fats (MUFAs): Monounsaturated fats are found in many foods, including red meats and dairy products. Many plants and plant oils are high in monounsaturated fats but low in saturated fats. Monounsaturated fats from plants may lower bad cholesterol and raise good cholesterol. Replacing saturated fats with monounsaturated fats in the diet may lower the level of bad cholesterol and triglycerides in the blood. On average, 1 tbsp of olive oil contains 14 g of fat.
- Polyunsaturated Fats (PUFAs): The two most well-known PUFAs are omega-3 and omega-6 fatty acids. These are essential fats that people must get from the food they eat because the body cannot make them. Fatty fish contain large amounts of unsaturated and omega-3 fatty acids, which play an important role in heart and brain health. The American Heart Association recommends that people eat 2 servings of fatty fish each week.
Healthy Fats and Their Benefits
Monounsaturated and polyunsaturated fatty acids, including omega-3 and omega-6 fatty acids, are both healthy fats. Including healthy fats in a meal creates a sense of fullness, slows down the digestion of carbohydrates, and adds flavor to food. Healthy fats can also aid hormone function, memory, and the absorption of specific nutrients.
MUFAs and PUFAs are healthful fats that can benefit the heart, lower LDL cholesterol, improve insulin levels, and improve blood glucose levels. As a general rule, healthful fats - such as olive oil - are liquid at room temperature.
Sources of Healthy Fats
- Fatty Fish: Fatty fish contain large amounts of unsaturated and omega-3 fatty acids, which play an important role in heart and brain health. The American Heart Association (AHA) recommends that people eat 2 servings of fatty fish each week. Around 3 ounces (oz) of cooked fish equates to one serving. Options include tuna, herring, mackerel, salmon, sardines, and anchovies. Both fresh and canned fish products are good sources of omega-3 fatty acids.
- Chia Seeds: Although they are small in size, chia seeds are rich in several nutrients. A 1 oz, or 28.35 g, serving of chia seeds contains 8.7 g of fat, much of which is polyunsaturated fat. Chia seeds are a great plant-based source of omega-3. Chia seeds also provide antioxidants, fiber, protein, iron, and calcium.
- Dark Chocolate: A 1 oz serving, or 28.35 g, of 70% to 85% dark chocolate provides 12.1 g of fat, as well as other nutrients, such as potassium and calcium. Dark chocolate also contains flavonoid antioxidants, which can have cognitive and cardiovascular benefits.
- Eggs: One 50 g hard-boiled egg offers 5.3 g of fat, 1.64 g of which are saturated, and around 78 calories (kcal). Egg yolk contains vitamin D and choline, a B vitamin that supports the function of the liver, brain, nerves, and muscles.
- Avocado: A 100 g serving of avocado contains approximately 14.7 g of fat and 160 kcal. It is high in a monounsaturated fatty acid called oleic acid, which may provide several health benefits. Oleic acid has anti-inflammatory properties, and it may play a role in cancer prevention. Avocados are also high in fiber. Avocados also contain a substance called lutein, which may be beneficial for eye health. Avocados are also a rich source of potassium.
- Flaxseed: Flaxseed provides omega-3 fatty acids and a healthful dose of fiber simultaneously. Flaxseed contains 27.3 g of dietary fiber and 42.2 g of fat per 100 g. The majority of these fats are unsaturated. A single tablespoon (tbsp), or 7 g, of ground flaxseed contains 1.91 g of fiber and 2.95 g of fat.
- Nuts: Nuts can have many health benefits. They are rich in healthful fats, protein, fiber, vitamins, minerals, antioxidants, and phytosterols. They may help prevent cardiovascular disease and type 2 diabetes. There is approximately 14.1 g of fat in 1 oz of almonds, 19 g of fat in 1 oz of Brazil nuts, and 18.5 g of fat in 1 oz of walnuts. It is best to eat various unsalted nuts to reap the benefits, as each type of nut has a slightly different nutrient profile. A person can use nut butter to enjoy the benefits of nuts and seeds in a spreadable form. Each serving provides a healthful amount of monounsaturated and polyunsaturated fats.
- Olives and Olive Oil: Research reports that a compound in olives called oleuropein may help prevent diabetes. Olive oil is full of monounsaturated fats that are good for heart health. It also contains vitamin E, vitamin K, and potent antioxidants.
- Tofu: Tofu is a complete plant protein and a good source of monounsaturated and polyunsaturated fats. A 100 g serving of firm tofu provides 4.19 g of fat. This amount of tofu also provides 10.9 g of protein.
- Full-Fat Natural Yogurt: Full-fat natural yogurt contains good probiotic bacteria to support gut function. Regularly eating yogurt may reduce a person’s risk of developing cardiovascular diseases.
Unhealthy Fats to Avoid
By contrast, people often consider saturated fats and trans fats unhealthful fats. Foods rich in these substances, such as butter and lard, are often solid at room temperature.
Trans Fats
Artificial trans fats, which often appear on labels as partially hydrogenated oils, can trigger inflammation that may increase the risk of heart disease, stroke, diabetes, and many other health conditions. Even just a 2% increase of calories from trans fats daily is associated with a 23% increase in cardiovascular risk. The following foods contain trans fats: fried foods, frozen foods, such as pizzas and pies, baked goods, some margarines, and highly processed snack foods.
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High-Fat Diet (HFD) and its Impact
Dietary fat is an important part of the diet for humans to obtain lipids and energy. However, it is well known that excessive fat intake could lead to an accumulation of adipose tissue in the body and finally results in obesity and the development of a cluster of metabolic diseases, such as type2 diabetes, atherosclerosis, hypertension, and stroke. High-fat diet (HFD) was widely used in animal disease models, including obesity, diabetes, liver disease, and cardiovascular disease.
The fats previously used in the HFD researches include soybean oil, corn oil, canola oil, palm oil, lard, etc., and it was widely reported that the gut microbiome is a critical mediator between various diseases and HFD. The gut microbe has essential roles in supplying nutrients and vitamins, providing colonization resistance against pathogenic bacteria, and interacting with the host immune system and intestinal epithelium.
However, a few previous reports have shown that different sources of dietary fat have variant effects on obesity and the gut microbiome. Except for some small amounts of secondary metabolites and vitamins, the most significant difference between diet fats is the diverse content of three main types of fatty acids, which include saturated fatty acid (SFA), monounsaturated fatty acid (MUFA), polyunsaturated fatty acid (PUFA). It was proved that lard and palm oil, which contain higher SFA, are relatively unhealthy, excessive ingesting of these fats will significantly affect intestinal microbiota and body weight. Conversely, a higher proportion of unsaturated fatty acids in fish oil and olive oil is relatively healthy.
Comparative Studies of Different Dietary Fats
In the last decade, there have been some studies comparing the role of different types of fats in HFD. Studies with palm oil, olive oil, safflower oil and flaxseed/fish oil show that both the type and quantity of fat have different impact on physiology and intestinal microbiota, and the negative effects caused by palm oil was more severe. Palm oil, which is rich in saturated fatty acids compared with olive oil and safflower oil, can induce higher body weight and hepatic triglycerides, and has a greater impact on the diversity of intestinal bacteria. The different effects on gut microbes were also reported between olive oil and butter. Lipidomics-based studies have shown that HFD with fish oil and lard had significantly different effects on hepatic cholesterol metabolism, and fish oil has a lesser effect on hepatic cholesterol. Studies based on Fish oil, lard and soybean oil have shown that fish oil has a different effect on the gut microbiota compared with lard and soybean oil, and can induce the expression of more inflammatory factors, which may have stronger negative effects. On the other hand, it was reported that lard activates WAT inflammation and reduces insulin sensitivity through the TLR signalling pathway.
Most of these studies mainly focus on comparative studies of a few types of dietary fat, such as lard, olive oil, fish oil, palm oil, etc., while other fat types, including canola oil, soybean oil, sunflower oil, peanut oil etc., which are widely used in HFD or cooking, are few comparative studied, and the differences between these oils in HFD are unclear.
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Impact of Dietary Fats on Body Weight and Blood Lipid Indices
To examine whether dietary fats have different effects on the body weight gain of mice, body weight was measured in male mice with different sources of dietary fat intervention. After 8 weeks of dietary fat intervention, the body weight of all HFD diet groups were higher than the control group, and the corn oil has the highest body weight, and changes of sesame oil was the smallest. The one-way ANOVA results showed that most HFD groups significantly higher body weight (p < 0.05). However, there was no difference in peanut oil or sesame oil (p > 0.05) intervention groups compared with the control group. It implied that peanut oil and sesame oil may not contribute much to weight increase. In contrast, corn oil had the most significant effects on body weight gain. Moreover, the lard had no more weight gain than other oils, and olive oil had a higher effect on weight gain, with a mean body weight of 37.33 ± 5.56 g.
To verify whether different dietary fats have different effects on blood lipids after HFD, triglycerides and three types of cholesterol (CHOL, HDL-C, LDL-C) were surveyed in the serum of mice after the fat intervention. Compared with the control group, CHOL concentration in the serum of all HFD groups was significantly higher. The top three groups with the most significant changes of CHOL were camellia oil, olive oil, and palm oil, and the types with lower changes were soybean oil and canola oil, Among them, the CHOL of lard treatment was not very high.
The TRIG content was different between each dietary fats intervention group. The groups of lard, palm oil, corn oil, peanut oil, canola oil, and sesame oil had higher levels compared with control. Among them, palm oil and peanut oil had the largest increase. In contrast, linseed oil, walnut oil, camellia oil, olive oil, soybean oil, and sunflower oil groups showed a lower TRIG, of which linseed oil and soybean oil showed larger changes than others.
The content of HDL and LDL in all fat intervention groups was increased significantly. The camellia oil and palm oil had the most significant increase in HDL and LDL, respectively. In comparison, soybean oil and canola oil had more minor changes in both HDL and LDL after HFD treatment.
Effects of Dietary Fats on Liver Weight
Compared with the control group, the liver weight of each treatment group was increased to varying degrees. However, the olive oil intervention group was the highest among all treatments.
The liver index is an index that reflects the health of the liver, which was calculated as liver weight divided by body weight. The results showed that the top highest three groups were olive oil, linseed oil, peanut oil, while only soybean oil treatment was significant lower compared to the control group.
Effects of Fat Compositions on Physiological Parameters
To verify the effects of the three main compositions (SFA, MUFA, PUFA) of dietary fat on physiological parameters described above, we analyzed the Pearson correlation between these compositions of fat and physiological parameters. The results indicated that the effects on various physiological indicators showed different trends. Except for MUFA significantly correlates with liver weight, these three fat compositions had no significant correlation with body weight and liver weight (p > 0.05). There was a positive correlation for CHOL, SFA and MUFA, a positive correlation between TRIG and SFA, and a negative correlation for PUFA. For HDL-C and LDL-C, both SFA and MUFA showed a positive correlation.
Effects of Dietary Fats on Intestinal Microbiota
It was reported that the dietary fats intervention truly affected the gut microbiota, this prompted us to investigate whether the structure of the gut microbiota is associated with the sources of dietary fat and changes observed in blood indexes. Therefore, we analyzed the gut bacterial communities after dietary fats intervention using high-throughput sequencing of 16S rRNA gene.
The alpha-diversity and beta-diversity were analyzed. The results of Alpha-diversity analysis indicated that the Shannon index of canola oil and camellia oil was significantly higher than the control group. While, the diversity of the lard treatment group was significantly reduced, and other interventions had no difference in the richness of molecular species compare with the control group.
Practical Tips for Choosing Healthy Fats
- Use plant-based oils instead of butter or lard. For example, saute vegetables with olive oil instead of butter.
- Choose lean meat and skinless poultry. Trim visible fat from meat.
- Limit saturated fats to less than 10% of your daily calories and replace saturated fats with unsaturated fats as much as possible.
- Read nutrition labels on food packaging to help you cut down on total fat and saturated fat.
- Be mindful of portion sizes, as all types of fat are high in energy.