Dietary Fat

from UptoDate – Dietary fat Matthew W Gillman, MD, SM

• Main concerns:
  • Role of dietary fats in promoting coronary heart disease (CHD).
  • Possible links between dietary fats and obesity, cancer.
• Key insight:
  • Type of fat is more important than total fat intake for health outcomes.
  • Research has not definitively linked total fat intake to health risks like heart disease, cancer, or obesity.

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Total Fat Intake

• Guidelines:
  • Recommendations previously aimed to reduce total fat intake to less than 30-35% of total daily energy. This was based primarily on its effects on total blood cholesterol.
  • Current research suggests the type of fat (e.g., saturated, unsaturated) is more important than the total amount.
• Concerns with low-fat recommendations:
  • Heart disease:
    • Initial focus on total fat was based on cholesterol’s role in atherosclerosis. However, many other factors (antioxidants, vitamins) play roles in heart health.
  • Obesity:
    • The role of dietary fat in obesity remains unclear
    • Low-fat recommendations may have unintentionally contributed to weight gain by increasing refined carbohydrate consumption.
  • Communication issues:
    • It’s difficult for patients to understand and calculate their fat intake.
    • Low-fat advice has been translated into unhealthy food choices (e.g., fat-free products high in sugar or refined carbs).

Effects of Total Fat on Health

• Heart disease:
  • Epidemiological evidence:
    • Studies across populations show that high total fat intake does not always correlate with higher rates of coronary heart disease.
    • For instance, individuals in Crete consume high amounts of fat, mainly from olive oil, yet have low heart disease rates.
  • Cohort studies:
    • Long-term studies, like the Nurses’ Health Study, have found no significant association between total fat intake and coronary heart disease risk.
    • Meta-analyses also fail to show a clear link between total fat intake and heart disease events.
  • Randomized trials:
    • Several trials have not demonstrated significant effects of reducing total fat on cholesterol levels or cardiovascular outcomes.
    • For example, in the Women’s Health Initiative, reducing total fat intake did not significantly reduce heart disease or stroke risk.
    • (Howard BV, Van Horn L, Hsia J, et al. Low-fat dietary pattern and risk of cardiovascular disease: the Women’s Health Initiative Randomized Controlled Dietary Modification Trial. JAMA 2006; 295:655)
• Cancer:
  • Breast cancer:
    • Early case-control studies suggested a link between high fat intake and breast cancer, but cohort studies (less prone to bias) did not confirm these findings.
    • For example, the Nurses’ Health Study showed no significant association between total fat intake and breast cancer risk.
  • Colon and prostate cancers:
    • Some evidence suggests an association between dietary fat (especially from red meat) and colon or prostate cancer.
    • However, the data is not conclusive, and other factors like meat consumption may play a role.
• Obesity:
  • Rising obesity rates in the U.S. have led to scrutiny of fat’s role in weight gain.
  • However, evidence remains inconclusive regarding whether high-fat diets contribute more to obesity than other dietary factors.

Challenges with Low-Fat Diets

• Patient understanding: It’s easier for patients to grasp recommendations about specific foods (e.g., “eat more fruits and vegetables”) rather than nutrient targets (e.g., “limit fat intake to <30% of calories”).
• Unintended consequences: Low-fat diets have led to increased consumption of refined carbohydrates and low-fat, processed foods, which may contribute to obesity, lower HDL cholesterol, and increased risk of type 2 diabetes.

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Type of Fat

• Key insight: Different types of fat have varying effects on health. Evidence shows that focusing on fat type is more important than limiting total fat intake.

Trans Fats

Trans fats are artificially created through a process called hydrogenation, which solidifies liquid oils. They are commonly found in processed foods like baked goods, margarine, and fried foods.

• Sources:
  • Naturally present in small amounts in animal products.
  • Most trans fats in the diet come from industrial hydrogenation (e.g., margarine, baked goods, fried foods).
• Health effects:
  • Adverse effects on blood lipids:
    • Increase LDL cholesterol
    • decrease HDL cholesterol
    • Increase triglycerides
  • Strong link to cardiovascular disease: Studies, including the Nurses’ Health Study, show that for each 2% increase in trans fat intake, the risk of coronary heart disease increases by 93%.
  • Regulation: Many regions have restricted trans fats in foods, leading to a reduction in their use by manufacturers.

Saturated Fats

Saturated fats are solid at room temperature and are found in animal products (like butter, fatty cuts of meat, and cheese) and some plant-based oils (such as coconut oil and palm oil)

• Sources: Found primarily in animal products like red meat, dairy, and some plant oils (e.g., coconut oil).
• Health effects especially myristic and palmitic acids (found in dairy and meat):
  • Adverse effects on blood lipids:
    • Increase LDL cholesterol
    • Increase total cholesterol
    • Neutral or slight increase in HDL cholesterol
    • increase in triglycerides – less pronounced than with trans fats.
  • Recent meta-analyses of cohort studies have not consistently found a direct link between saturated fat intake and heart disease events.
  • Replacing saturated fats with polyunsaturated or monounsaturated fats, rather than carbohydrates, appears to reduce heart disease risk.

Monounsaturated Fats

These fats are typically liquid at room temperature

• Sources: Predominantly found in plant oils (e.g., olive oil, canola oil) and nuts.
• Health effects:
  • lower LDL cholesterol
  • maintain HDL cholesterol
  • Studies suggest that diets high in monounsaturated fats, like the Mediterranean diet, protect against heart disease.
  • In patients with type 2 diabetes, monounsaturated fats help reduce insulin resistance and improve endothelial function.

Polyunsaturated Fats

• n-6 Polyunsaturated fats:
  • Sources: Vegetable oils (e.g., safflower, sunflower, corn oils).
  • Health effects:
    • Lower LDL cholesterol
    • Lower HDL cholesterol
    • Reducing saturated fats and replacing them with n-6 polyunsaturated fats lowers the risk of coronary heart disease.
• n-3 Polyunsaturated fats (Omega-3):
  • Sources: Fish oils, flaxseed, walnuts.
  • Health effects:
    • Lower triglycerides
    • reduce platelet aggregation
    • decrease cardiovascular mortality
    • Omega-3s may protect against heart disease and have anti-inflammatory effects.

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Other Health Considerations

Stroke

• Evidence: Conflicting studies on the effect of dietary fats on stroke risk. Some studies suggest that omega-3s from fish reduce ischemic stroke risk, but more research is needed.

Diabetes Mellitus

• Polyunsaturated fats: Some evidence suggests that replacing saturated fats with polyunsaturated fats lowers the risk of type 2 diabetes.
• However, findings are inconsistent across studies.

Alzheimer’s Disease

• Fat intake and cognitive decline: Saturated and trans fats may increase the risk of Alzheimer’s disease, while monounsaturated and polyunsaturated fats may offer protective benefits.

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Dietary Cholesterol

• Sources: Eggs and animal products.
• Health effects:
  • Dietary cholesterol raises total serum cholesterol, but its contribution is less significant than that of saturated and trans fats.
  • Recent evidence suggests that moderate egg consumption does not increase the risk of heart disease or stroke in most people.

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Guidelines and Recommendations

• Total fat intake: Current guidelines recommend 25-35% of daily energy intake from fat.
  • Saturated fats: <10% of energy.
  • Trans fats: Minimize as much as possible.
  • Emphasis on replacing saturated fats with unsaturated fats (monounsaturated and polyunsaturated).

Key Recommendations for Patients

1. Focus on fat quality, not quantity:
   • Replace animal fats with healthier plant-based fats (e.g., olive oil, nuts, seeds).
   • Limit intake of processed foods high in trans fats.
2. Reduce saturated fat intake:
   • Substitute whole milk with skim or low-fat milk.
   • Use plant-based alternatives to animal fats where possible.
3. Minimize trans fat consumption:
   • Avoid foods with partially hydrogenated oils (e.g., margarine, baked goods, fried foods).
   • Check labels for trans fats.
4. Incorporate healthy fats:
   • Use oils high in monounsaturated and polyunsaturated fats for cooking (e.g., olive oil, canola oil).
   • Include omega-3-rich foods in the diet (e.g., fish, flaxseed, walnuts).