10. Nutrition and Longevity: Calorie Restriction, Fasting and Human Research

1. What changes in the body during fasting?

After a meal, the body processes the nutrients it receives. Between meals, it increasingly draws on stores, including liver glycogen and fat. With a longer restriction of food or carbohydrates, the liver may produce more ketones. The response depends on previous meals, activity, energy stores and health; there is no universal switching hour. [2] [13]

Fasting also changes signals involved in cellular maintenance. Article 05 discusses autophagy in more detail. This recycling process also occurs under ordinary conditions; hunger and hours without food do not measure its intensity. After drawing on reserves, the body needs sufficient nutrition to repair tissues and preserve muscle. [8]

Intermittent fasting mainly controls when you eat. Calorie restriction reduces total energy intake. A ketogenic diet limits carbohydrates. These approaches can overlap, but an overnight break does not require a ketogenic diet. [3] [4] [5]

2. Who is this example plan for?

This is a gentle organisational example for an adequately nourished adult who feels well and has no relevant medical or medication restrictions. With diabetes, blood pressure medicines, diuretics or medicines that must be taken with food, discuss the routine with your treating clinician first. Do not change medication doses or timing yourself. [9]

Self-directed fasting is unsuitable during pregnancy or breastfeeding, for children, or with malnutrition, frailty, unintended weight loss or an eating disorder. Kidney or liver disease, active cancer treatment and recovery after surgery require an individual nutrition plan. Willingness and psychological well-being matter, but willpower cannot determine medical suitability. [9] [12] [15]

3. How to begin during the first month

Start by recording the time of your last and first meals for a few days. Sleep counts towards the overnight break. The following is an option, not a compulsory progression or a clinically validated timetable for activating autophagy.

The first number means hours without calories; the second means hours available for eating. Finishing dinner earlier may be more practical than delaying the first meal until late afternoon. A small controlled study found metabolic benefits of earlier eating without weight loss, but it does not establish one ideal schedule for everyone. [5]

4. Morning routine and weekly or monthly reviews

A healthy person does not need to measure GKI daily. Measurement should help understand the body's response. If it causes anxiety or an urge to keep lowering the number, focus instead on regular meals, diet quality and well-being.

5. What to eat and drink in an ordinary routine

During the eating period, have two or three complete meals according to your needs. Each main meal should contain protein, for example eggs, fish, meat, dairy or pulses. Add vegetables, appropriate portions of starchy foods, fruit, nuts and suitable fats. Long-term healthy ageing research supports the importance of overall diet quality. Preserving muscle requires adequate nutrition and regular, appropriate physical activity. [7]

Water is the basis during an ordinary overnight break. Unsweetened tea or black coffee may fit a time-restricted routine if tolerated. Milk, sugar, juice, alcohol, butter and MCT oil provide energy, so these are not part of a calorie-free break. For comparable morning measurements, wait until afterwards to have coffee too. [15]

A short overnight break does not in itself require an electrolyte supplement. Longer fasting changes water and mineral balance. Do not dose potassium, sodium or baking soda by how you feel, especially with kidney disease or certain medicines. Dizziness and palpitations are not automatically caused by mineral deficiency. [9] [12]

6. Measuring glucose and ketones and calculating GKI

You need a glucose meter and a blood ketone meter measuring beta-hydroxybutyrate. Some devices measure both but use different test strips. Follow the manufacturer's instructions. Urine ketone strips and breath acetone meters do not provide a value that can be entered directly into GKI. [2]

1. Choose a comparable time, for example before breakfast, coffee and exercise. Record the hours since your last calorie intake.
2. Wash and dry your hands well. Use compatible, in-date strips and a lancet according to the instructions. Do not share blood sampling equipment.
3. Measure glucose and ketones close together. Record both values and their units, not just the calculated ratio.
4. Note sleep, illness, exercise or unusual meals. Check an unexpected result according to the instructions; do not delay help for serious symptoms to take another measurement.

Calculating GKI

GKI = glucose in mmol/L ÷ blood ketones in mmol/L

If your meter reports glucose in mg/dL, divide it by 18 first. For example, 90 mg/dL is about 5.0 mmol/L. Ketones must be expressed in mmol/L. [2]

These are calculation examples, not targets. GKI cannot be reliably calculated with zero or unmeasurably low ketones. At very low ketone levels, a small measurement error can substantially change the ratio.

Understanding the result

A lower GKI means a lower glucose-to-ketone ratio. It may result from higher ketones, lower glucose or both. Always assess the individual readings too. GKI does not show the percentage of fat being burned, insulin sensitivity or autophagy intensity. A higher ratio does not rule out fat use and is not automatically a bad result. [2] [8]

GKI was published as a research tool for the metabolic management of brain tumours. This does not establish a universal target for healthy people or a home treatment protocol. Values below 2 or 1 do not confirm autophagy. Do not prolong fasting or try to lower glucose simply to reach such a number. [2]

7. How long to continue, and what about longer fasts?

A daily 12–16-hour break can be maintained by a suitable adult if it permits adequate nutrition and is well tolerated. End each fast at the planned time, or earlier if problems arise. Widen the eating window if sleep, strength or mood worsen or evening overeating develops.

Full-day, repeated 24-hour and multi-day fasts are separate choices with different demands. This general plan does not prescribe them every Monday or month. There is no validated calendar that guarantees more autophagy or a longer life for everyone. Medication, nutrition, illness and returning to food all need assessment. Multi-day fasting requires medical supervision. [9] [12] [13]

Ekberg's videos about 100 hours or five days without food can prompt questions about metabolic changes. A personal experiment cannot determine a suitable fasting duration for someone else. A study of a seven-day fast in 12 volunteers found extensive changes in blood proteins, not proof that all those changes benefit health. [1] [13]

8. When to stop fasting

Marked weakness, shaking, cold sweats, dizziness, palpitations or repeated vomiting are reasons to stop fasting and address the cause. Fainting, confusion, chest pain or breathing difficulty require immediate medical help. Do not interpret them as effective cleansing.

Glucose below 3.9 mmol/L is a safety signal to stop a home fasting experiment, particularly with diabetes or glucose-lowering medication. With hypoglycaemia, if conscious and able to swallow safely, take 15–20 g of fast-acting glucose or carbohydrate and recheck after 15 minutes. Repeat if still low and seek help as appropriate. Give no food or drink to an unconscious person; call emergency medical services. [10]

With diabetes, blood ketones of 1.6–3.0 mmol/L require immediate advice from the diabetes team; above 3.0 mmol/L requires emergency assessment. Do not wait with vomiting, abdominal pain, deep rapid breathing or confusion. These thresholds concern diabetic ketoacidosis risk, not fasting success. With SGLT2 inhibitors, such as dapagliflozin or empagliflozin, ketoacidosis can occur without markedly raised glucose. [11] [14]

9. Returning to food

After an ordinary 12–16-hour break, a normal, appropriate meal with protein and other nutrients is usually enough. Eat calmly rather than compensating with a feast. No special cleansing drink or supplement is required.

After several days of minimal intake, a sudden return to eating can cause dangerous shifts in minerals, known as refeeding syndrome. Risk also depends on malnutrition and health. NICE recommends cautious introduction of nutrition after more than five days with very low intake. Returning to food in this situation needs an individual medical plan. [12]

10. What human studies have found

HbA1c broadly reflects average blood glucose over the previous two to three months. Metabolic syndrome is a combination of risk factors, such as a larger waist, raised blood pressure and unfavourable glucose or blood lipid levels.

Assess results in terms of overall functioning. A worthwhile change should support metabolic health, muscle preservation and quality of life. Success does not require the longest possible fast or the lowest possible GKI.

11. Ekberg's videos and the author's experience

Author's experience — Ing. František Kokoš: In preparing this article, we used my earlier notes on Sten Ekberg's videos about 100-hour and five-day fasts. His attempt to connect meal patterns with bodily responses and laboratory findings is useful as a starting point. During my own multi-day fast, I also recorded glucose and ketones. These records describe an individual metabolic response; they do not measure autophagy or establish a routine for every reader. [1] [2]

For everyday practice, we draw on the interest in measurement, breaks between meals and regular review. Specific health conclusions are based on the studies and guidance cited.

Related article: 05 – Autophagy and cellular recycling.

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