IB Biology Topic D3.1.20: Hormone Replacement Therapy and the Risk of Coronary Heart Disease Notes

HRT and Heart Disease: Why Correlation Is Not Causation

Early studies suggested HRT protected the heart, but trials showed the link was not causal. This is a classic lesson in reading evidence.

1. Picture a woman in her 50s with menopausal symptoms such as hot flushes and mood swings.
2. Her doctor suggests hormone replacement therapy (HRT) to ease them.
3. She then reads that HRT might change her risk of coronary heart disease (CHD), so she asks whether it is safe.

What HRT Is and Why It Is Prescribed

1. HRT tops up oestrogen, often with progesterone, to replace the hormones that fall at menopause.
2. It is used to ease hot flushes and night sweats.
3. It also helps with mood swings, vaginal dryness, and reduced sex drive.
4. It slows bone weakening (osteoporosis) that speeds up after menopause.

Early Studies Suggested HRT Protected the Heart

1. In the 1980s and 1990s, observational studies found that women using HRT had a lower rate of CHD.
2. This led many to believe HRT protected the heart, and it became widely accepted.

Randomised Trials Told a Different Story

1. Later randomised controlled trials (RCTs), the gold standard in research, tested HRT directly.
2. They found HRT was linked to a slight increase in CHD risk, not a decrease.

Confounding Explains the Contradiction

1. The clash between the early studies and the RCTs shows that correlation does not equal causation.
2. The early observational studies did not control for confounding factors that affected the results.
3. The main confounder was the socioeconomic status of the women who used HRT.
4. Women on HRT in those studies were more likely to be in higher socioeconomic groups, who already had lower CHD risk.
   1. They had better access to healthcare.
   2. They tended to have healthier lifestyles, such as balanced diets and regular exercise.
   3. They often had lower stress levels.
5. These advantages lowered CHD risk on their own, creating the illusion that HRT was protective.

The Real Effect of HRT on the Heart Is Genuinely Complex

1. Oestrogen can improve blood vessel function and lower LDL cholesterol, which may reduce CHD risk.
2. Timing matters, because starting HRT soon after menopause seems to differ from starting it years later.
   1. Early use may give some cardiovascular benefit, while late use may raise CHD risk.
3. Individual factors also matter, including age, health, and whether the HRT is oestrogen alone or combined with progesterone.

Why RCTs Settle Questions Observation Cannot

1. The HRT story shows why evidence-based medicine relies on the strongest study design available.
2. RCTs randomly assign people to treatment or control groups, which spreads confounders evenly and lets researchers isolate the effect of the treatment.
3. This makes their data more reliable than observational studies for judging cause and effect.
4. Long-term trials matter for HRT because its effects can change over years.

Test Yourself on the HRT Evidence