Rectal cancer usually develops from a mix of genetic changes in rectal cells plus long‑term lifestyle and medical risk factors, rather than one single “cause.”

What actually “causes” rectal cancer?

At the most basic level, rectal cancer starts when cells in the lining of the rectum acquire DNA mutations that let them grow too fast, live too long, and ignore normal “stop” signals. Over 10–15 years, these abnormal cells can evolve from normal tissue to small polyps and eventually to invasive cancer if they aren’t detected and removed.

Scientists describe two main types of changes:

  • Inherited (germline) mutations – present from birth, passed down in families.
  • Acquired (somatic) mutations – picked up over life from aging, environment, and lifestyle.

Common genes involved include tumor‑suppressor genes such as APC and p53 and oncogenes such as KRAS, which help control cell growth, repair, and death. When enough of these controls are disrupted, cancer can form.

Major risk factors you can’t change

These factors don’t guarantee rectal cancer, but they raise the odds by altering how cells behave.

  1. Age
    • Most rectal and colorectal cancers are diagnosed after age 50, although cases are rising in younger adults.
  1. Family history
    • Having a parent, sibling, or child with colon or rectal cancer significantly increases your risk.
 * “Familial clustering” (several relatives with colorectal cancer even without a named syndrome) accounts for a noticeable share of cases.
  1. Inherited syndromes
    • Familial adenomatous polyposis (FAP): causes hundreds of colon polyps and almost always leads to colorectal cancer if not treated.
 * Lynch syndrome (hereditary non‑polyposis colorectal cancer, HNPCC): greatly increases the risk of colon, rectal, and several other cancers at younger ages.

 * These syndromes explain only a small percentage of total rectal cancers but carry very high individual risk.
  1. Personal medical history
    • Past colorectal polyps, colon cancer, or anal cancer increase the chance of a new rectal cancer later on.
 * Long‑standing inflammatory bowel disease (IBD) like ulcerative colitis or Crohn’s disease affecting the colon/rectum raises risk, especially after many years of inflammation.
  1. Prior radiation to the pelvis/abdomen
    • Previous radiation therapy to the abdomen or pelvic area (for other cancers) can increase the later risk of rectal cancer.

Lifestyle and environmental risk factors

These are often called “modifiable” because changing them can lower risk, though they don’t cancel out genetics completely.

Diet and weight

  • Diet high in red and processed meats
    • Regular intake of red meat (beef, pork, lamb) and processed meats (bacon, sausages, deli meats) is linked with higher colorectal and rectal cancer risk.
  • Diet low in fiber, fruits, and vegetables
    • Low intake of vegetables and fiber and high intake of refined foods are associated with increased risk.
  • Obesity, especially belly fat
    • Excess body weight, particularly central (waist) obesity, is a known risk factor.

Physical activity

  • Lack of exercise
    • A sedentary lifestyle is linked with a higher chance of rectal cancer; regular physical activity appears protective.

Smoking and alcohol

  • Smoking
    • Smoking increases the risk of many cancers, including colorectal; smokers are estimated to be 30–40% more likely to die of colorectal cancer than non‑smokers.
  • Alcohol
    • Heavy drinking (for example, around three or more alcoholic drinks per day) is associated with higher rectal and colorectal cancer risk.

Metabolic and other health conditions

  • Type 2 diabetes
    • Poorly controlled type 2 diabetes is linked with increased colorectal and rectal cancer risk, possibly via insulin resistance and chronic inflammation.
  • Some gynecologic cancers and pelvic conditions
    • A history of ovarian cancer and certain other pelvic conditions has been associated with higher rectal cancer risk, though mechanisms are still being studied.

Quick Scoop: Key causes & risk factors (SEO‑friendly summary)

Below is an HTML‑formatted table as requested, highlighting the main “rectal cancer causes” in risk‑factor form.

html

<table>
  <thead>
    <tr>
      <th>Category</th>
      <th>Specific factor</th>
      <th>How it influences rectal cancer risk</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td>Genetic / inherited</td>
      <td>Familial adenomatous polyposis (FAP)</td>
      <td>Causes numerous colon polyps and almost always leads to colorectal cancer if untreated [web:3][web:5][web:9].</td>
    </tr>
    <tr>
      <td>Genetic / inherited</td>
      <td>Lynch syndrome (HNPCC)</td>
      <td>Dominant inherited syndrome that greatly increases lifetime risk of colon and rectal cancer at younger ages [web:3][web:5][web:9].</td>
    </tr>
    <tr>
      <td>Genetic / familial</td>
      <td>Family history of colorectal cancer</td>
      <td>Having a first-degree relative with colon or rectal cancer substantially raises personal risk [web:1][web:3][web:5].</td>
    </tr>
    <tr>
      <td>Age</td>
      <td>Older than 50</td>
      <td>Most colorectal and rectal cancers are diagnosed after age 50, though younger cases are increasing [web:1][web:5].</td>
    </tr>
    <tr>
      <td>Chronic inflammation</td>
      <td>Inflammatory bowel disease (ulcerative colitis, Crohn’s)</td>
      <td>Long-term inflammation of colon/rectum increases cancer risk, especially after many years of disease [web:1][web:3][web:9].</td>
    </tr>
    <tr>
      <td>Personal history</td>
      <td>Previous colorectal polyps or cancer</td>
      <td>History of adenomatous polyps, colon cancer, or anal cancer raises risk of later rectal cancer [web:1][web:9].</td>
    </tr>
    <tr>
      <td>Previous treatments</td>
      <td>Past radiation to abdomen/pelvis</td>
      <td>Radiation for other cancers in the pelvic region can increase future risk of rectal cancer [web:9].</td>
    </tr>
    <tr>
      <td>Diet</td>
      <td>High red and processed meat intake</td>
      <td>Frequently eating red and processed meats is associated with higher colorectal and rectal cancer risk [web:1][web:3][web:7][web:9].</td>
    </tr>
    <tr>
      <td>Diet</td>
      <td>Low fiber, low vegetables and fruit</td>
      <td>Diets poor in fiber, fruits, and vegetables correlate with higher colorectal cancer risk [web:1][web:3][web:9].</td>
    </tr>
    <tr>
      <td>Lifestyle</td>
      <td>Physical inactivity</td>
      <td>Sedentary lifestyle increases risk; regular exercise lowers risk [web:1][web:7][web:9].</td>
    </tr>
    <tr>
      <td>Lifestyle</td>
      <td>Smoking</td>
      <td>Smokers have higher risk of developing and dying from colorectal cancer compared with non-smokers [web:1][web:7].</td>
    </tr>
    <tr>
      <td>Lifestyle</td>
      <td>Heavy alcohol use</td>
      <td>Drinking heavily (around three or more drinks per day) increases risk of colorectal and rectal cancer [web:1][web:3][web:7][web:9].</td>
    </tr>
    <tr>
      <td>Metabolic</td>
      <td>Obesity, especially abdominal</td>
      <td>Excess body weight, particularly around the waist, is linked with higher colorectal and rectal cancer risk [web:1][web:6][web:7][web:9].</td>
    </tr>
    <tr>
      <td>Metabolic</td>
      <td>Poorly controlled type 2 diabetes</td>
      <td>Type 2 diabetes is associated with increased colorectal cancer risk, possibly via insulin resistance and chronic inflammation [web:9].</td>
    </tr>
  </tbody>
</table>

Why this matters and what to do next

From a practical standpoint, the “causes” you can act on are mostly the modifiable risks: maintaining a healthy weight, exercising regularly, not smoking, limiting alcohol, and eating more plant‑based, high‑fiber foods while limiting processed and charred meats. For people with strong family histories or known syndromes like FAP or Lynch, early and more frequent screening (and sometimes preventive surgery) is crucial.

If you or someone close to you has multiple risk factors or symptoms such as rectal bleeding, unexplained weight loss, or changes in bowel habits, it’s important to discuss this promptly with a healthcare professional who can recommend appropriate testing, usually starting with colonoscopy.

Bottom note: Information gathered from public forums or data available on the internet and portrayed here.