Organized/made more readable, see extended description for full set of changes.

Used the -As parameter with Read-Host to ensure the correct data type is obtained.
Simplified the gender check using the -eq operator.
Adjusted the formula by separating terms with multiplication in the BMR calculation.
Consolidated the nutrient intake calculation for both genders.
Removed unnecessary Exit-PSSession at the end.

Nutrition/splits.ps1

@@ -1,33 +1,30 @@
-#VARS
-[decimal]$Weight = Read-Host "Please input weight in kg"
-[decimal]$Height = Read-Host "Please input height in cm"
-[decimal]$Age = Read-Host "Please input age in years"
-[char]$Gender = Read-Host "Are you Male (M) or Female (F)".ToUpper()
+# User input
+$Weight = Read-Host "Please input weight in kg" -As [decimal]
+$Height = Read-Host "Please input height in cm" -As [decimal]
+$Age = Read-Host "Please input age in years" -As [decimal]
+$Gender = Read-Host "Are you Male (M) or Female (F)" -As [char] | ForEach-Object { $_.ToString().ToUpper() }
 
-#determining split (female)
-if ($Gender = "F") {
-    $Calories = 655.1 + (9.563 * $weight) + (1.850 + $Height) - (4.676 * $Age)
-    
-    $Protein_Cals = ($Calories/10)*2
-    $Carbs_Cals = ($Calories/10)*6
-    $Fat_Cals = ($Calories/10)*2
-    Write-Host "Your BMR is:"([math]::Round($Calories,0))
-    Write-Host "Your recommended protein intake is:" ([math]::Round($Protein_Cals,0))"kcals"
-    Write-Host "Your recommended carbohydrate intake is:"([math]::Round($Carbs_Cals,0))"kcals"
-    Write-Host "Your recommended fat intake is:"([Math]::Round($Fat_Cals,0))"kcals"
+# Calculate BMR and nutrient intake
+if ($Gender -eq "F")
+{
+    $Calories = 655.1 + (9.563 * $Weight) + (1.850 * $Height) - (4.676 * $Age)
 }
-#determing split (male)
-elseif ($Gender = "M") {
-    $Calories = 66.47 + (13.75 * $weight) + (5.003 + $Height) - (6.755 * $Age)
-    $Protein = $Calories/4
-    $Carbs = $Calories/4
-    $Fats = $Calories/9
-    Write-Host "Your BMR is: " $Calories
-    Write-Host "Your recommended protein intake is: " $Protein
-    Write-Host "Your recommended carbohydrate intake is: " $Carbs
-    Write-Host "Your recommended fat intake is: " $Fats
+elseif ($Gender -eq "M")
+{
+    $Calories = 66.47 + (13.75 * $Weight) + (5.003 * $Height) - (6.755 * $Age)
 }
-else {
-    Exit-PSSession
+else
+{
+    Write-Host "Invalid gender. Exiting."
+    Exit
 }
-Exit-PSSession
+
+$Protein_Cals = ($Calories / 10) * 2
+$Carbs_Cals = ($Calories / 10) * 6
+$Fat_Cals = ($Calories / 10) * 2
+
+# Display results
+Write-Host "Your BMR is: " ([math]::Round($Calories, 0))
+Write-Host "Your recommended protein intake is: " ([math]::Round($Protein_Cals, 0)) "kcals"
+Write-Host "Your recommended carbohydrate intake is: " ([math]::Round($Carbs_Cals, 0)) "kcals"
+Write-Host "Your recommended fat intake is: " ([Math]::Round($Fat_Cals, 0)) "kcals"