A lot of things will happen to your body if you eat only meat, but it is determined by what you were eating before switching to only meat. Some things you can expect in the first 1 to 2 weeks when you switch from a standard American diet to just meat is withdrawal, diarrhoea, fatigue, and possibly muscle cramps.
These are all symptoms of the adaptation phase. Your body is used to getting caffeine, sugar, refined carbohydrates, and fibre, and has to adapt to function normally without those things constantly present in your diet. As most people know, caffeine withdrawal is pretty bad, but a lot of people don’t realise that there is also a withdrawal from sugar, but after a week or so, your body will get over the withdrawal from those drugs, and you will start to feel normal again.
Most people believe that your body needs fibre in order to have a bowel movement, but the opposite seems to be true, because most people experience diarrhoea during the transition period. This is caused by bile salts entering the large intestine because your digestive system is accustomed to having fibre in the small intestine, but this isn’t a problem, and your body will adapt to the lack of fibre.
Many people who are used to carbohydrates as their major fuel source, will experience a bit of fatigue while their metabolism shifts from carbohydrate-based to fat-based, but if you were following a ketogenic diet beforehand, then you will already be geared up to handle the lack of carbohydrates, and you will avoid getting fatigued.
Muscle cramps are caused by inadequate salt intake, especially during the transition period. Carbohydrates cause your body to hold on to more fluid, so when you cut out all carbohydrates, your body starts excreting water at a higher rate, and this can cause mineral loss and cramps. Increasing your salt intake will mitigate this and help prevent muscle cramps.
Once you have survived the transition period, the benefits of a meat-only diet will start to emerge like weight loss, stable energy levels, improved digestion, mental clarity, and more manageable hunger levels. I do think that eating just meat is a great short-term intervention that can drastically improve your health and give you great insights into your own personal biology, but I would not recommend that you eat only meat for the long term.
Without including things like organ meats, eggs, and bone broth in your meat-only diet you won’t be getting all the essential nutrients required for you to thrive, but comparatively, the standard American diet has bigger gaps in essential nutrients plus all the negative side effects that you could save your body from by eating meat only.